EFFECTS OF SINGLE-TREE SELECTION HARVESTING ON ROSE-BREASTED

GROSBEAKS (PHEUCTICUS LEUDOVICIANUS) IN A PREDOMINANTLY

FORESTED LANDSCAPE

by

Sonya Richmond

A thesis submitted in conformity with the requirements

for the degree of Doctor of Philosophy, Faculty of Forestry

University of Toronto

© Copyright by Sonya Richmond 2011

Effects of Single-tree Selection Harvesting on Rose-breasted Grosbeaks (Pheucticus leudovicianus) in a Predominantly Forested Landscape

Sonya Richmond

Doctor of PhilosophyFaculty of Forestry

University of Toronto

2011

ABSTRACT

Single-tree selection harvesting is frequently used in the tolerant hardwood forests

of North America but relatively little is known about how this silvicultural system affects

wildlife, including many avian species. I investigated Rose-breasted Grosbeak

(Pheucticus leudovicianus) habitat use, survival, and nestling provisioning behaviour in

twelve hardwood stands in Algonquin Provincial Park which had been harvested by

single-tree selection 0-5, 16-20, 21-25, and > 50 years previously. Density, pairing

success, and the number of fledglings per successful nest were all significantly lower in

the > 50 years post-harvest stands than in at least one other post-harvest treatment.

Density and pairing success were significantly higher in the 16-20 year post-harvest

stands than in other treatments. Neither nest nor fledgling survival differed significantly

among post-harvest treatments, but all stands were population sinks except those cut 16-

20 years previously. Nests that were initiated earlier in the season and built in areas with

higher basal area were more likely to survive, whereas fledgling survival increased with

days since fledging. Nest sites had higher cover from regenerative growth, saplings, and

understory, and lower basal area than random locations. During their first week out of

the nest, fledglings used locations with significantly higher cover from regenerative

growth, saplings, small shrubs, and raspberry and elderberry bushes than were present at

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random locations. Habitat characteristics at nest and fledgling locations were

significantly different, and estimates of nest and fledgling survival were not correlated

among harvested stands. Nests attended by after-second-year (ASY) males were initiated

significantly earlier, and territory density and productivity were significantly higher for

ASY males than for second-year (SY) males. Nestling provisioning rates, male

contribution to nestling provisioning and nest attendance, and mean nestling weights at

the time of fledging were also significantly higher at nests attended by ASY males than at

nests with SY males. This study found that single-tree selection did not have significant

negative effects on Rose-breasted Grosbeaks breeding in a predominantly forested

landscape, but like many other species of birds, experienced breeders were more

successful than less experienced breeders were.

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ACKNOWLEDGEMENTS

First and foremost I would like to thank my supervisor, Dr. Jay Malcolm for his

helpful advice and feedback, and for patiently and enthusiastically sharing his statistical

expertise during the writing phase of this project. Thank you to Dr. Erica Nol and Dr.

Dawn Burke for encouraging me to continue with my studies at the doctoral level and for

giving me the opportunity to be part of this project. I am also grateful to Dr. Sandy

Smith for her comments and direction as a committee member, and to Ken Elliott for his

role in designing and implement the forestry components of the project. The scientific

insight and direction you have all given me has made this a much stronger project on all

levels.

I am thankful to Karla Falk and Doug Tozer for their patience, enthusiasm, and

skill in coordinating the fieldwork crews, and to Nick Bartok, Alex Hood, J.-P. LeBlanc,

Rhiannon Leshyk, Meghan Noad, Andrea Orr, Eleanor Proctor, Kris Van de Sompel, and

especially Phil Wilson, Brad Woodworth, and Gillian Humphries for their help with nest

searching and monitoring, banding and radio-tracking fledglings, sitting patiently amidst

the blackflies to conduct hour-long nest watches, surveying vegetation, driving long

hours, and generally helping out with all the other aspects of fieldwork. Without the

dedication and hard work you all generously put in there would have been no data.

I would like to thank the Algonquin Forestry Authority, Bancroft-Minden Forest

Co., Canadian Forest Service, Canadian Wildlife Service, Enhanced Forest Productivity

Science Program, Mazinaw-Lanark Forest Inc., Natural Sciences and Engineering

Research Council of Canada, Ontario Ministry of Natural Resources, Ottawa Valley

Forest Inc., Tembec Inc., and Westwind Forest Stewardship for providing funding and

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support for this project. I was supported by an NSERC scholarship and grants from the

University of Toronto and Trent University.

Last but by no means least I would like to thank my family for their support,

encouragement, and unlimited interest in this project. Without your help this endeavour

would definitely not have been possible for me. Thank you especially to Sean Morton

for frequently driving “across the province” to visit me during the four month field

seasons, for generously volunteering to help in every way possible, and for always being

willing to listen. Most of all, thank you all for the endless encouragement and for

reminding me why it was important to see this through.

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TABLE OF CONTENTS

ABSTRACT...................................................................................................................iiACKNOWLEDGEMENTS.........................................................................................ivTABLE OF CONTENTS ............................................................................................viLIST OF TABLES .......................................................................................................viiiLIST OF FIGURES .....................................................................................................xLIST OF APPENDICES .............................................................................................xiGENERAL INTRODUCTION ...................................................................................1 Background................................................................................................................1 Study species .............................................................................................................5 Study rationale ..........................................................................................................7

CHAPTER 1: DEMOGRAPHY OF ROSE-BREASTED GROSBEAKS IN STANDS HARVESTED BY SINGLE-TREE SELECTION....................................11

Abstract..........................................................................................................................11Introduction ..................................................................................................................13Methods .........................................................................................................................18 Study species ............................................................................................................18 Study area .................................................................................................................19 Habitat measurements ..............................................................................................22 Nest monitoring .......................................................................................................22 Density and pairing success .....................................................................................23 Population productivity ............................................................................................24 Statistical Analysis ...................................................................................................25Results ...........................................................................................................................28 Habitat measurements ..............................................................................................28 Demographic variables ............................................................................................29 Relationship between habitat and demographic variables .......................................31 Age of male parent ...................................................................................................34Discussion ......................................................................................................................37 Demographic measures ............................................................................................37 Population productivity ............................................................................................40 Population age structure ...........................................................................................42 Conclusions ..............................................................................................................44

CHAPTER 2: SURVIVAL AND HABITAT USE BY ADULT AND FLEDGLING ROSE-BREASTED GROSBEAKS (PHEUCTICUS LEUDOVICIANUS) IN A FORESTED LANDSCAPE HARVESTED BY SINGLE-TREE SELECTION....45

Abstract .........................................................................................................................45Introduction ..................................................................................................................47Methods .........................................................................................................................51 Study species ............................................................................................................51 Study area .................................................................................................................51

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Nest searching and monitoring ................................................................................52 Radio attachment and tracking .................................................................................53 Habitat measurements ..............................................................................................54 Statistical analysis ....................................................................................................56Results ...........................................................................................................................64 Do habitat features differ between used and un-used plots? ....................................64 Do habitat features differ between nest and fledgling plots? ...................................71 Nest success and fledgling survival .........................................................................73 Are estimates of nest and fledgling survival correlated among post-harvest treatments? ...............................................................................................................74 How do habitat characteristics influence nest and fledgling survival? ....................76Discussion ......................................................................................................................79 Conclusions ..............................................................................................................86

CHAPTER 3: NESTLING PROVISIONING RATES AND FORAGING PATTERNS OF ROSE-BREASTED GROSBEAKS IN STANDS HARVESTED BY SINGLE TREE SELECTION.....................................................................................90

Abstract .........................................................................................................................90Introduction ..................................................................................................................92Methods .........................................................................................................................96 Study species ............................................................................................................96 Study area .................................................................................................................96 Nest monitoring .......................................................................................................97 Nestling provisioning ...............................................................................................97 Prey load sizes ..........................................................................................................99 Nest attendance ........................................................................................................99 Nestling weights .......................................................................................................99 Statistical analysis ....................................................................................................100Results ...........................................................................................................................102 Nestling provisioning rates ......................................................................................102 Male contribution to nestling provisioning ..............................................................103 Nest attendance ........................................................................................................104 Male contribution to nest attendance .......................................................................104 Prey load sizes...........................................................................................................108 Nestling weights at fledging ....................................................................................111Discussion ......................................................................................................................111 Single-tree selection harvesting ...............................................................................111 Nestling development ..............................................................................................114 Age of male parent ...................................................................................................117 Conclusions ..............................................................................................................120General Conclusions ....................................................................................................121Literature Cited ...........................................................................................................127

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LIST OF TABLES

Table 1.1. Locations and characteristics of the twelve tolerant hardwood stands in Algonquin Provincial Park, Ontario, Canada that were investigated in this study....................................................................................................................21

Table 1.2. Means of habitat variables (standard error in brackets) and results from two-way ANOVAs and Tukey’s tests comparing sites from four stages in the cutting cycle: 0-5, 16-20, 21-25, and > 50 years post-harvest (n = 3 per stage) and three study years (2006-2008) ....................................................................................30

Table 1.3. Means of demographic variables (standard error in brackets) and results from two-way ANOVAs and Tukey’s tests comparing sites from four stages in the cutting cycle: 0-5, 16-20, 21-25, and > 50 years post-harvest (n = 3 per stage) and three study years (2006-2008) ...........................................................................32

Table 1.4. Population growth rates for Rose-breasted Grosbeaks breeding in 12 tolerant hardwood stands in Algonquin Provincial Park harvested by single-tree selection. Stands represent four stages in the cutting cycle: 0-5, 16-20, 21-25, and > 50 years post-harvest. Models show intrinsic (λ) and exponential (r) population growth rates under best case (high adult and juvenile survival) and worst case (low adult and juvenile survival) scenarios .......................................................33

Table 1.5. Means of demographic variables (standard error in brackets) and results from two-way ANOVAs and Tukey’s tests comparing sites from four stages in the cutting cycle: 0-5, 16-20, 21-25, and > 50 years post-harvest (n = 3 per stage) and three study years (2006-2008). Results are shown for second-year (SY) and after-second-year (ASY) males...................................................................................36

Table 2.1. Number of Rose-breasted Grosbeak nests located in four height categories (low, medium, high, and very high) in 12 stands harvested by single-tree selection 0-5, 16-20, 21-25, and > 50 years prior to sampling. Number of nests in each category is shown with row percentages in brackets..........................................69

Table 2.2. Support for models predicting daily nest survival rates of Rose-breasted Grosbeaks in Algonquin Provincial Park (2006-2008) as a function of percent cover of regenerative growth, saplings, and understory, basal area (m2/ha), time-since harvest (0-5, 16-20, 21-25, or >50 years), nest concealment (%), nest height (m) and nest initiation date ................................................................................77

Table 2.3. Support for models predicting daily survival rates of Rose-breasted Grosbeak fledglings in Algonquin Provincial Park 1-21 days after fledging (2006-2008). Predictor variables include percent cover of regenerative growth, saplings, understory, canopy, basal area (m2/ha), time since harvest, concealment (%), height (m), fledging date, and days since fledging.............................................78

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Table 3.1. Mean nestling provisioning rates at Rose-breasted Grosbeak nests in 12 hardwood stands in Algonquin Provincial Park harvested by single-tree selection 0-5, 16-20, 21-25, and > 50 years prior to study (stand errors are in parentheses). Mean nestling provisioning rates are presented for nests attended by second-year (SY) and after-second-year (ASY) males, and for nests with Stage 1 (< 5 days old) and Stage 2 (> 5 days old) nestlings............................................................105

Table 3.2. Mean proportion of feeding events contributed by male Rose-breasted Grosbeaks in 12 hardwood stands in Algonquin Provincial Park harvested by single-tree selection 0-5, 16-20, 21-25, and > 50 years prior to study (stand errors are in parentheses). Mean proportion of feeding events are presented for second-year (SY) and after-second-year (ASY) males, and for nests with Stage 1 (< 5 days old) and Stage 2 (> 5 days old) nestlings....................................................106

Table 3.3. Mean nest attendance at Rose-breasted Grosbeak nests in 12 hardwood stands in Algonquin Provincial Park harvested by single-tree selection 0-5, 16-20, 21-25, and > 50 years prior to study (standard errors are in parentheses). Mean nest attendance is presented for nests attended by second-year (SY) and after-second-year (ASY) males, and for nests with Stage 1 (< 5 days old) and Stage 2 (> 5 days old) nestlings.......................................................................................................107

Table 3.4. Mean nest attendance contributed by male Rose-breasted Grosbeaks in 12 hardwood stands in Algonquin Provincial Park harvested by single-tree selection 0-5, 16-20, 21-25, and > 50 years prior to study (standard errors are in parentheses). Mean nest attendance is presented for second-year (SY) and after-second-year (ASY) males, and for nests with Stage 1 (< 5 days old) and Stage 2 (> 5 days old) nestlings.......................................................................................109

Table 3.5. Observed frequencies for prey load sizes delivered to Rose-breasted Grosbeak nests in 12 hardwood stands in Algonquin Provincial Park harvested by single-tree selection (row percents are in parentheses). Mean prey load sizes are presented for harvest treatment (0-5, 16-20, 21-25, and > 50 years post-harvest), for nests with > 5 or < 5 days old nestlings, and for nests attended by after-second-year (ASY) and second-year (SY) male parents....................................110

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LIST OF FIGURES

Figure 1.1. Map of sites used to study the effects of single-tree selection on the demography of Rose-breasted Grosbeaks in Ontario, Canada. The grey polygon indicates Algonquin Provincial Park and the numbers associated with each site indicated the number of years post-harvest........................................................20

Figure 1.2. Relationship between seven demographic variables for Rose-breasted Grosbeaks and six habitat features in four post-harvest treatments in Algonquin Provincial Park. Demographic variables are mean clutch size, territory density, number of fledglings per successful nest, pairing success, productivity, proportion of territorial ASY males, and nest survival. Habitat features are percent cover by forbs, regenerative growth, saplings, understory, and canopy, and basal area.. .35

Figure 2.1. (A) Regeneration cover, (B) sapling cover, (C) understory cover, (D) canopy cover, and (E) basal area at nest and random plots in 12 hardwood stands in Algonquin Provincial Park harvested by single-tree selection 0-5, 16-20, 21-25, and > 50 years previously. Means and standard errors are shown.....................68

Figure 2.2. (A) Regenerative cover, (B) sapling cover, (C) basal area, (D) small shrub (< 2.5 cm dbh), (E) raspberry cover, and (F) elderberry cover at locations occupied by fledglings and random plots in 12 hardwood stands in Algonquin Provincial Park harvested by single-tree selection 0-5, 16-20, 21-25 and > 50 years previously. Means and standard errors are shown..............................................70

Figure 2.3. (A) Regeneration cover, (B) understory cover, and (C) basal area at nest sites and locations occupied by fledglings 1-7, 8-14, and 15-21 days post-fledging in 12 hardwood stands harvested by single-tree selection 0-5, 16-20, 21-25, and > 50 years previously. Means and standard errors are shown....................................72

Figure 2.4. Mayfield estimates of period survival rates for Rose-breasted Grosbeak nests and fledglings at three weeks post-fledging in four stand age classes in Algonquin Provincial Park differing in time since single-tree selection logging (0-5, 16-20, 21-25, and >50 years post-harvest) (2006-2008). Means and standard errors are shown..................................................................................................75

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LIST OF APPENDICES

Appendix 2.1. List of candidate models used to explain Rose-breasted Grosbeak nest and fledgling survival in twelve tolerant hardwood stands in Algonquin Provincial Park harvested by single-tree selection 0-5, 16-20, 21-25, and > 50 years previously............................................................................................................88

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GENERAL INTRODUCTION

BACKGROUND

With 347.7 million hectares of forested land, Canada is home to 10% of the

world’s forests and 30% of the world’s boreal forest (Natural Resources Canada 2010).

Forested land in this country encompasses twelve forest regions (Rowe 1972), with the

boreal forest as the most extensive, followed by the Great Lakes – St. Lawrence. The

Great Lakes – St. Lawrence is a transitional forest zone located between the boreal forest

to the north and the deciduous Carolinian forest to the south. It stretches from

southeastern Manitoba to the Gaspé, with a gap along the northern shores of Lakes

Superior (Natural Resources Canada 2010), and together with the deciduous forest region

it contains the tolerant hardwood forest. Hardwood forests are common in the Acadian

region of northern New England and New York, the upper Midwest, and the Maritime

provinces of Canada (Seymour et al. 2002). In Ontario, tolerant hardwood forests that

consist primarily of sugar maple (Acer saccharum), American beech (Fagus grandifolia),

and yellow birch (Betula alleghaniensis) cover 3.6 million hectares and provide

approximately 3.2% of the total provincial timber harvest (OMNR 1998). Much of this

type of forest is directly affected by anthropogenic activities, such as urbanization and

logging.

Forest management in North America has recently shifted away from practices

designed solely to maximize timber yield towards a more ecosystem-based approach

(Kohm and Franklin 1997). Within this framework the focus has been on developing and

implementing economically, socially, and ecologically sustainable management practices.

Efforts to achieve ecological sustainability have mainly taken the form of management

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practices designed to emulate natural disturbance patterns (Perera et al. 2000, Seymour et

al. 2002). In the boreal forest, even-aged silvicultural systems are most often used to

emulate the large-scale disturbances characteristic of this region such as fire and wind

storms (Perera et al. 2000). In contrast, in tolerant hardwood forests of North America,

uneven-aged systems, such as shelterwood or selection cutting are the recommended

methods of harvesting (Arboglast 1957, Franklin 1989, OMNR 2000). Disturbances in

this region are typically small in scale, removing individual or small groups of trees

through insect outbreaks, ice or wind storms, or natural senescence.

The selection harvesting system involves removing individual trees or small

groups of trees once every 8-25 years (Hunter 1990, OMNR 2000, Seymour et al. 2002).

Tree marking guidelines and targets for the residual basal area to be retained in maple

dominated forests under the selection system are very similar for the Great Lakes

jurisdictions (OMNR 1998), Michigan (Tubbs 1977), and Wisconsin (WDNR 2011), but

they vary according to stand condition and type. In the maple and beech dominated

hardwood forests of the Great Lakes region, no more than one third of the basal area for a

stand is removed per entry, and a residual basal area of 16 m2/ha of > 24 cm dbh trees or

20 m2/ha of > 9 cm dbh trees is generally considered appropriate (OMNR 1998, Tubbs

1977, WDNR 2011). The removal of trees during harvesting creates many small gaps or

perforations in the canopy which encourage natural regeneration by allowing more light

to reach the understory layers (Robinson and Robinson 1999). In the first few years after

harvesting stands typically have abundant regenerative growth, low basal area, and sparse

sapling, understory, and canopy layers (Robinson and Robinson 1999, Flaspohler et al.

2002, Jobes et al. 2004). By 16-20 years after harvesting, a dense understory layer often

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forms as shade-tolerant species such as sugar maple and American beech develop

(Finegan 1984, Jobes et al. 2004). As the understory layer grows into the canopy,

managed stands develop higher basal area, abundant cover in the seedling, regenerative

growth, sapling, and canopy layers, and higher spatial complexity typical of un-harvested

hardwood stands (Robinson and Robinson 1999, Kenefic and Nyland 2000, Crow et al.

2002, Flaspohler et al 2002, Jobes et al. 2004, Angers et al. 2005, Vanderwel et al. 2006).

Single-tree selection harvesting is designed to mimic natural disturbance patterns

in tolerant hardwood forests, and therefore should have only minimal impacts on wildlife,

including migratory songbirds. However, the abundance of many forest interior birds

decreases following selection harvesting (Germaine et al. 1997, Robinson and Robinson

1999, Simon et al. 2000, Bourque and Villard 2001, Gram et al. 2003, Jobes et al. 2004,

Doyon et al. 2005, Guénette and Villard 2005, Holmes and Pitt 2007, Cooper et al. 2009,

Haché and Villard 2010, Poulin et al. 2010). Some disturbance-dependent species

become more prevalent in harvested areas (Lent and Capen 1995, Annand and Thompson

1997, Simon et al. 2000, Gram et al. 2003, Jobes et al. 2004, Holmes and Pitt 2007,

Robertson and Hutto 2007, Atwell et al. 2008), but habitats with higher densities of birds

are not necessarily more productive than other areas, or even equally beneficial (Van

Horne 1983, Vickery et al. 1992). Abundance estimates alone do not provide enough

information to assess the impacts of forest management practices on avian populations.

Long-term estimates of habitat quality and reproductive success, while more costly and

time consuming to derive, are necessary.

Studies of avian demography in harvested hardwood forests are less prevalent

than studies of community composition, but several studies have detected differences in

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reproductive success (Yahner 2000, Barber et al. 2001, Duguay et al. 2001, Robinson and

Robinson 2001, Rodewald and Yahner 2001, Moorman et al. 2002, Smith et al. 2006,

Dellinger et al. 2007, Cooper et al. 2009, Poulin et al. 2010) and density (Rodewald and

Smith 1988, Kilgo et al. 1999, Kingsley and Nol 1999, Flaspohler et al. 2002, Hagan and

Meehan 2002, Weakland et al. 2002, Jobes et al. 2004) following harvesting.

Silvicultural practices can also affect nest predator communities (Medin and Booth 1989,

Pettersson et al. 1995, Harris and Reed 2002) and prey availability (Morse and Robinson

1999, Marzluff et al. 2000, Sallabanks et al. 2000).

Although we know a little about how silviculture influences nest survival, much

less is currently known about habitat use and survival of fledglings in managed forests.

Habitat requirements of breeding adults may be different from those of family groups

during the post-fledging period (Rappole and Ballard 1987, Anders et al. 1998, Vega

Rivera et al. 1998, Cohen and Lindell 2004, King et al. 2006). For instance, dense cover

near the ground and in the sapling and understory layers may provide important cover for

the fledglings of many species regardless of the nest site characteristics selected by

adults. Since the offspring of many songbirds spend up to eight weeks on the breeding

grounds prior to migration (Anders et al. 1998), and fledgling survival strongly influences

population growth rates (Porneluzi and Faaborg 1999, Powell et al. 1999, Woodworth et

al. 1999), assessing the impact of silvicultural practices on fledglings is important.

So far, to my knowledge only one study has investigated the survival of songbird

fledglings in selection-harvested forests. This study, which was conducted in a

fragmented agricultural landscape, did not detect significant differences in juvenile

survival among harvested and un-harvested forest stands (Moore et al. 2010). No studies

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have yet been done on fledgling survival in managed hardwood forests in predominantly

forested landscapes, and since silviculture may affect avian populations differently in

agricultural and forested landscapes (Sallabanks et al. 2000, Thompson et al. 2000),

further research is necessary.

STUDY SPECIES

The Rose-breasted Grosbeak (Aves: Passeriformes: Cardinalidae: Pheucticus

leudovicianus) is a Neotropical migratory songbird that breeds across much of eastern

and central North America and over winters in central and south America (Wyatt and

Francis 2002). Although this species is still relatively abundant, it has been declining at

an annual rate of -0.8% across North America since 1966 (Sauer et al. 2008). In Ontario,

Rose-breasted Grosbeaks have been declining at a rate of approximately -2.4% per year

between 1966 and 2007 (Sauer et al. 2008), with an especially sharp decline of – 8.0%

per year between 1985 and 2005 (Cadman et al. 2007). This species provides a good

opportunity to study the effects of harvesting because, despite its declining numbers, it is

still abundant enough in the landscape to allow detection of changes in relation to

management practices.

Rose-breasted Grosbeaks nest in primary and secondary mixed and deciduous

forests, as well as in thickets, scrublands, urban parks, gardens, and other areas which

have been subject to anthropogenic disturbances (Wyatt and Francis 2002). The Rose-

breasted Grosbeak is often found in regenerating forests following harvesting, and in

some cases this species is more abundant in logged areas than in un-harvested ones

(Holmes et al. 2004, Jobes et al. 2004, Doyon et al. 2005, Guénette and Villard 2005).

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Although we know that Rose-breasted Grosbeaks tend to nest in the sapling and

understory layers, one study found that they use locations with more canopy cover but

lower sapling cover than random locations (Smith et al. 2007), whereas others suggest

they prefer areas with relatively open canopy and sub-canopy layers (Wyatt and Francis

2002). Apart from studies in fragmented agricultural landscapes (Smith et al. 2006, 2007,

Moore et al. 2010), little is known about Rose-breasted Grosbeak ecology, or how

changes in forest structure resulting from silviculture influence the availability or

selection of nest sites, the foraging behaviour, or the ability of this species to successfully

raise its young once they leave the nest.

Rose-breasted Grosbeaks typically arrive on their breeding grounds in central

Ontario during the first week of May, and nest initiation commences two weeks later.

Both males and females participate in nest building, incubating, brooding, and nestling

provisioning, with the males providing approximately one third of the incubation, half of

the brooding, and 25-30% of the nestling provisioning (Wyatt and Francis 2002).

Although bi-parental care is common in birds, in many migratory songbird species the

females typically do all or most of the incubating and brooding, as well as providing

much of the nestling provisioning (Stodola et al. 2010). Male Rose-breasted Grosbeak’s

relatively high contribution to parental care may make this species more resilient to

habitat alterations than other songbirds, because the trade-off between nest defence and

foraging may be less severe. For example, if females must spend more time away from

the nest foraging in altered habitats, Rose-breasted Grosbeak males can incubate and

defend the nest in their absence, whereas in other species the nest may be left unattended

for longer periods and be more likely to be lost to predators. However, since this species

7

is declining, and various forms of silviculture are implemented throughout its entire

breeding range in North America, it is important to assess the effectiveness of current

forest management guidelines for maintaining good quality habitat for Rose-breasted

Grosbeaks.

STUDY RATIONALE

Changes in Rose-breasted Grosbeak abundance following harvesting have

appeared in a number of silvicultural studies from across North America, with mixed

results. Rose-breasted Grosbeak abundance increased following single-tree selection

harvesting in the northern hardwood forests of Quebec (Doyon et al. 2005) and Ontario

(Jobes et al. 2004), partial harvesting in the deciduous forest fragments of southwestern

Ontario (Holmes et al. 2004), and silvicultural treatments of various intensities in New

Brunswick (Guénette and Villard 2005). However, decreases in abundance were

observed in response to selection harvesting and high-grading in fragmented Carolinian

forests (Smith et al. 2006, Thompson et al. 2009) and to partial harvesting in boreal

forests (Vanderwel et al. 2007, 2009). Other studies detected no statistical difference in

abundance following harvesting (Duguay et al. 2001, Holmes and Pitt 2007). Studies of

nest survival and productivity, which might provide insight into the inconsistent nature of

these responses and help identify silvicultural practices that maintain or improve habitat

for Rose-breasted Grosbeaks are still relatively rare.

Most research on Rose-breasted Grosbeak demography has been conducted in the

highly fragmented Carolinian forest of southwestern Ontario (Smith et al. 2006, 2007,

Moore et al. 2010). In the hardwood forest fragments of this agricultural landscape,

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single-tree selection did not significantly affect nest survival (Smith et al. 2006, Moore et

al. 2010), but density, nest success, and the ratio of after-second-year to second-year

males were slightly lower in the harvested stands than in the un-logged ones (Smith et al.

2006). Fledgling survival in these stands was not significantly affected by harvesting, but

it too was slightly lower in the harvested stands (Moore et al. 2010). In this landscape,

both harvested and un-harvested stands were classified as population sinks (Smith et al.

2006, Moore et al. 2010). Although single-tree selection harvesting did not appear to

significantly alter Rose-breasted Grosbeak survival in forest fragments, no information is

available for predominantly forested landscapes. Some songbird populations are above

replacement levels in predominantly forested landscapes and large forest fragments but

below replacement levels in smaller fragments (Burke and Nol 2000, Falk et al. 2011),

suggesting that it is important to investigate the impacts of anthropogenic activities on

reproductive success in different landscape contexts. In particular, since a considerable

portion of the Rose-breasted Grosbeak’s breeding range falls within managed forest

landscapes, it is important to assess whether harvesting affects this species differently in

predominantly forested landscapes than in fragmented agricultural ones.

To address this knowledge gap, in this thesis I investigate Rose-breasted

Grosbeak habitat use in tolerant hardwood stands harvested by single-tree selection 0-5,

16-20, 21-25, and > 50 years previously in the predominantly forested landscape of

Algonquin Provincial Park, Ontario, Canada. Algonquin Park was logged for white pine

(Pinus strobes) in the 1800’s and for hemlock (Tsuga canadensis) and yellow birch

(Betula alleghaniensis) during the 1930’s and 1940’s (Tozer and Strickland 1980). Since

1974 approximately 12% of the park has been designated as a Wilderness Zone where no

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logging activity occurs. The remaining forested area of the park (57% of its area) is

managed for timber production at a rate of approximately 2% per year (Ontario Parks

1998). Stands that consist of > 60% hardwood species have been harvested using the

selection system since the 1970’s, and some have now been regenerating long enough to

allow a second harvest entry.

I used a chronosequence or space-for-time substitution approach (reviewed in

Fukami and Wardle 2005) to study succession following single-tree selection harvesting

in tolerant hardwood stands, and to look at its long-term consequences for Rose-breasted

Grosbeaks. I chose stands that were 0-5, 16-20, and 21-25 years post-harvest to

investigate three stages in the harvest rotation. The 21-25 years post-harvest stands were

chosen to provide insight into forest conditions at the stage when a second harvest is

typically carried out, to see if forest structure and reproductive output of Rose-breasted

Grosbeaks had returned to “pre-harvest” levels. There is some concern that if

regeneration times are not long enough, cumulative changes could occur. The > 50 years

post-harvest stands were chosen to approximate “un-logged” or “pre-harvest” conditions

which could be used as a reference point against which to evaluate more recently logged

stands because completely unlogged, primary growth hardwood stands were unavailable

in my study area within the park.

Chapter one examines whether Rose-breasted Grosbeak density, pairing success,

population age structure, nest initiation date, clutch size, nest survival, and productivity

differed with time since selection harvesting. Chapter two investigates important habitat

characteristics at nest sites and locations used by fledglings, tests whether these

characteristics differ among post-harvest treatments, and investigates which habitat

10

variables have the strongest influence on nest and fledgling survival. Chapter three tests

whether nestling provisioning rates, prey load sizes, male contributions to nestling

provisioning and nest attendance, and nestling weights differed with time since single-

tree selection harvesting, stage of nestling development, or age of male parent. Finally, I

end with a general discussion on the relevance of these findings to current forest

management practices in the tolerant hardwood forests of North America.

I expected considerable changes in forest structure following single-tree selection

harvesting, and that as a result of these alterations vegetation characteristics at nest sites

and locations used by fledglings would significantly differ among post-harvest

treatments. Stands harvested by single-tree selection 16-20 years previously typically

have well developed sapling and understory layers, which appear to be preferentially

selected by Rose-breasted Grosbeaks. As a result, I predicted that these stands would

have higher densities, a greater proportion of after-second-year males, earlier nest

initiation dates, higher pairing success, larger clutch sizes, higher nest survival rates, and

greater productivity. I also expected that nestling provisioning would be more frequent

and/or larger prey loads would be provided to nestlings in these stands and that nestlings

would be heavier on average than in the other post-harvest treatments and the uncut

stands.

CHAPTER 1: DEMOGRAPHY OF ROSE-BREASTED GROSBEAKS IN STANDS HARVESTED BY SINGLE-TREE SELECTION

ABSTRACT

Single-tree selection harvesting is frequently used in the tolerant hardwood forests

of North America, but little is known about how it affects the reproductive success of

migratory songbirds. Many songbirds that breed in this forest type, including the Rose-

breasted Grosbeak (Pheucticus leudovicianus) are currently experiencing population

declines across their breeding ranges. I studied the demography of Rose-breasted

Grosbeaks in twelve tolerant hardwood stands in the predominantly forested landscape of

Algonquin Provincial Park. Nine stands had been harvested by single-tree selection 0-5,

16-20, or 21-25 years previously, and three had remained un-harvested for > 50 years.

My main objectives were to test whether density, pairing success, population age

structure, nest initiation date, clutch size, nest survival and productivity differed with time

since single-tree selection harvesting. My secondary objective was to determine whether

any of these demographic parameters varied between after-second-year (ASY) and

second-year (SY) breeders. Analysis of variance indicated that density, pairing success,

and the number of fledglings per successful nest were all significantly (P < 0.05) lower in

the > 50 years post-harvest stands than in at least one other post-harvest treatment, as

were clutch size, nest survival, and productivity, although not significantly so. The

proportion of ASY males compared to SY males was highest in the > 50 years post-

harvest stands and lowest in the 21-25 years post-harvest treatment. Density, pairing

success, and productivity were all highest in the 16-20 years post-harvest treatment, and

density and pairing success were significantly so. Nests attended by ASY males were

11

12

initiated significantly earlier than nests attended by SY males, and territory density and

productivity were significantly higher for ASY males than SY males. Habitat

characteristics did not vary significantly between nests attended by ASY and SY males.

This study found that single-tree selection did not have significant negative effects on the

reproductive success of Rose-breasted Grosbeaks in predominantly forested landscapes,

but that all post-harvest treatments were population sinks except the 16-20 years post-

harvest stands, indicating that further research into the causes of this species’ decline is

necessary.

13

INTRODUCTION

Over half (57%) of the Neotropical migratory bird species that breed in eastern

North America have been experiencing broad-scale population declines over the past

forty years (Kirby et al. 2008). Habitat loss, fragmentation, and degradation are among

the most frequently cited causes of declines in migratory songbirds (Donovan and Flather

2002, Sauer et al. 2007, Poulin et al. 2010). Changes in land-use, together with forest

management policies intended to suppress natural disturbances such as wildfire, flooding,

and insect outbreaks have also reduced the amount of habitat available for some species

(Askins 1993, Brawn et al. 2001, Trani et al. 2001, Brooks 2003, DeGraaf and Yamasaki

2003). Concern over avian population declines has led to research investigating the

potential effects of forest management practices on forest breeding birds (Duguay et al.

2001). In North America, uneven-aged harvesting practices, such as the shelterwood and

selection systems, are becoming more prevalent in mixed and tolerant hardwood forests,

because they are believed to mimic natural patterns of small-scale disturbances

characteristic of these forest types (Tubbs 1977, OMNR 2000, Seymour et al. 2002). In

Ontario, Canada these practices account for 13% of annual provincial harvests, and they

are mostly applied in the Great Lakes – St. Lawrence forest region (Cadman et al. 2007).

Single-tree and group selection harvesting produce uneven-aged stands through

the removal of individual or small groups of trees every 15-25 years (Tubbs 1977, Hunter

1990, Seymour et al. 2002). Although single-tree selection emulates natural disturbance

patterns in mature hardwood forests better than some large-scale, even-aged silvicultural

techniques (Hunter 1990, Seymour et al. 2002), considerable changes in forest structure

occur in the years following harvesting. For example, recently logged stands typically

have abundant regenerative growth, low basal area, and sparse cover in the sapling,

14

understory, and canopy layers (Robinson and Robinson 1999, Flaspohler et al. 2002,

Jobes et al. 2004). However, increased light penetration immediately following

harvesting promotes the development of shade-tolerant sugar maple and American beech

seedlings, which form a dense understory layer by about 15 years after harvesting

(Finegan 1984, Jobes et al. 2004). As the understory layer grows into the canopy,

managed stands eventually approach pre-harvest conditions, which are characterized by

high basal area, abundant cover in the seedling, regenerative growth, sapling, and canopy

layers, and high spatial complexity (Robinson and Robinson 1999, Kenefic and Nyland

2000, Crow et al. 2002, Flaspohler et al. 2002, Jobes et al. 2004, Angers et al. 2005,

Vanderwel et al. 2006).

Many studies have documented a decline in the abundance of mature-forest

species following selection harvesting (Germaine et al. 1997, Robinson and Robinson

1999, Simon et al. 2000, Bourque and Villard 2001, Gram et al. 2003, Jobes et al. 2004,

Doyon et al. 2005, Guénette and Villard 2005, Holmes and Pitt 2007, Cooper et al. 2009,

Haché and Villard 2010, Poulin et al. 2010). In contrast, the abundance of early-

successional species generally increases following selection harvesting (Lent and Capen

1995, Annand and Thompson 1997, Simon et al. 2000, Gram et al. 2003, Jobes et al.

2004, Holmes and Pitt 2007, Robertson and Hutto 2007, Atwell et al. 2008), but the gaps

created by single-tree selection may be too small to benefit some species (Flaspohler et

al. 2002, Annand and Thompson 1997, Robinson and Robinson 1999). Demographic

responses to forest management are mixed, with many species achieving comparable

rates of nest survival before and after harvesting (Bourque and Villard 2001, Gram et al.

2003, Smith et al. 2006, Cooper et al. 2009, Moore et al. 2010), but with other species

15

experiencing lower rates of nest success in harvested forests (Robertson and Hutto 2007,

Poulin et al. 2010). Differing responses to selection harvesting among avian species

suggests that a more complete understanding of the link between forest management and

avian population growth is important for identifying management practices that maintain

good quality habitat for wildlife, and are therefore ecologically sustainable.

Since reproductive success can be reduced in areas with high densities of birds

(Arcese and Smith 1988, Vickery et al. 1992), and populations can persist in areas even

when reproductive rates are low (Brawn and Robinson 1996), abundance estimates alone

are generally not considered sufficient indicators of habitat quality. The ideal free

distribution model assumes that individuals will occupy habitats in proportion to their

quality, with better habitats supporting more individuals (Fretwell and Lucas 1970,

Fretwell 1972). However, recent (Schmiegelow et al. 1997) or unpredictable (Van Horne

1983) disturbances such as logging can influence avian abundances in forested habitats.

Unreliable indicators of habitat quality (Remes 2003, Weldon and Haddad 2005, Nordby

et al. 2009), past reproductive success (Haas 1998, Citta and Lindberg 2007), perceived

predation risk (Morton 2005, Fontaine and Martin 2006), and social cues (Doliqez et al.

2004, Fletcher 2007, Seppänen et al. 2007, Ahlering et al. 2010) can also influence how

birds select territories. As a result, despite recent evidence that density may be an

adequate indicator of avian productivity in some hardwood forest regions (Underwood

and Roth 2002, Bock and Jones 2004, Perot and Villard 2009), additional measures of

demography should be included in assessments of habitat quality in managed areas

(Sallabanks et al. 2000).

16

Age-related differences in acquisition and defence of high quality territories

(Catterall et al. 1989; Donovan and Stanley 1995, Pärt 2001) may also cause a decoupling

of the predicted positive relationship between density and productivity, with older, more

experienced birds out-competing younger ones and monopolizing resources (Donazar and

Feijoo 2002). In some species, nest initiation date, clutch size (Reynolds 1997), nest

survival (Forschler and Kalko 2006), and number of fledglings per successful nest

(Préault et al. 2005, Brown and Roth 2009) are influenced by the age of the breeding

adults. Age-related differences in reproductive success may occur because: 1) inferior

phenotypes, which have lower success, experience high mortality at young ages, 2)

younger individuals postpone breeding until they can accumulate sufficient resources, 3)

reproductive output increases as reproductive value decreases with age, or 4) parental

skills improve with age (Forslund and Pärt 1995, Martin 1995, Espie et al. 2000,

Balbontin et al. 2007). Regardless of which hypothesis is supported, if older birds are

more successful at obtaining and defending territories in preferred habitats, then the ratio

of older birds to younger birds could be used to assess which habitats are perceived as

most desirable by breeding individuals. In addition, if older birds experience higher

reproductive success than younger birds, areas with higher proportions of experienced

individuals may be more productive overall. Clarifying how population age structure

influences productivity is useful for better understanding the effects of anthropogenic

activities.

The Rose-breasted Grosbeak (Pheucticus leudovicianus) is a Neotropical migrant

that breeds in both harvested and un-harvested hardwood forests across North America

(Wyatt and Francis 2002, Smith et al. 2006). This early-successional forest species

17

(Drapeau et al. 2000) uses interior and edge habitats (Freemark and Merriam 1986,

Merrill et al. 1998) as well as managed forests with high tree density, extensive sapling

and shrub cover, and well-developed understory layers (Webb et al. 1977, Possardt and

Dodge 1978). Although Rose-breasted Grosbeaks utilize features of regenerating forests,

responses to harvesting have been mixed. Estimates of abundance are typically higher in

recently harvested stands than in less recently cut (Holmes et al. 2004, Jobes et al. 2004,

Doyon et al. 2005) or unlogged stands (Jobes et al. 2004, Doyon et al. 2005), and Rose-

breasted Grosbeaks have responded positively to silvicultural treatments of various

intensities in New Brunswick (Guénette and Villard 2005). However, decreases in

abundance were observed in response to selection harvesting and high-grading in

fragmented Carolinian forests (Smith et al. 2006, Thompson et al. 2009) and to partial

harvesting in boreal forests (Vanderwel et al. 2007, 2009), whereas other studies detected

no statistical difference in abundance following harvesting (Duguay et al. 2001, Holmes

and Pitt 2007). Density of territorial males and the ratio of after-second-year to second-

year males were also lower in harvested stands than in un-harvested ones in the

fragmented landscape (Smith et al. 2006). No information on most demographic

parameters is available for Rose-breasted Grosbeaks nesting in predominantly forested

landscapes, even though approximately 40% of its range falls within relatively un-

fragmented forests. A better understanding of how silviculture influences Rose-breasted

Grosbeaks is necessary because this species is currently experiencing population declines

throughout its breeding range across central and eastern North America (Smith et al.

2006, Cadman et al. 2007, Sauer et al. 2008).

18

Here, I examined whether changes in habitat structure resulting from single-tree

selection harvesting influenced the density, population age structure, pairing success, nest

initiation dates, and productivity of Rose-breasted Grosbeaks. I predicted that density

and productivity would be higher in stands harvested 16-20 years previously than in more

recently harvested stands or un-logged ones due to the presence of relatively well-

developed sapling and understory layers that appear to be preferentially selected by

grosbeaks (Wyatt and Francis 2002). I also expected that older, after-second-year males

would out-compete second-year males in these stands and that the older males would

experience higher reproductive success. As a result, I predicted that 16-20 years post-

harvest stands would have higher densities and greater proportions of after-second-year

males, and that the older, more experienced breeders would have earlier nest initiation

dates, higher pairing success, larger clutch sizes, nest survival rates, and greater

productivity than younger, less experienced males.

METHODS

Study Species

The Rose-breasted Grosbeak is a relatively abundant Neotropical migrant that

over winters in central and South America and breeds in first- and second-growth mixed

and deciduous forests throughout much of central and eastern North America (Wyatt and

Francis 2002). Rose-breasted Grosbeaks typically arrive on their breeding grounds in

central Ontario during the first week of May, where they begin establishing territories

ranging from 0.34 – 1.3 ha (mean = 0.77 ha) (Wyatt and Francis 2004). Nesting attempts

typically commence during the last two weeks of May (Smith et al. 2006). Incubation

19

takes 12-13 days, and fledging occurs 9-12 days after the eggs hatch (Wyatt and Francis

2002). Most pairs only raise one brood per season, but double-brooding may

occasionally take place (Wyatt and Francis 2002).

Study Area

Nest monitoring and territory mapping of Rose-breasted Grosbeaks was carried

out in twelve tolerant hardwood stands in Algonquin Provincial Park, Ontario, Canada

(45° 34`N, 78°04`W; Fig. 1.1) in 2006-2008. The higher-elevation west side of this

775,000 ha park consists primarily of mixed hardwood forest, whereas the lower-

elevation east side is dominated by white and red pine (Ontario Parks 1998). The tolerant

hardwood stands I studied were located in the central and southern part of the park (mean

nearest neighbour distance of stands = 9.3 km, range = 1.0 – 34.2 km), which has been

harvested using the single-tree selection system since the early 1970’s (OMNR 1998).

This type of forest management removes approximately one-third of the over-story basal

area every 15-25 years, resulting in uneven-aged stands with a mix of canopy gaps,

regenerative growth, and mature trees (OMNR 1998). Stands used in this study were

dominated by sugar maple (Acer saccharum), American beech (Fagus grandifolia),

eastern hemlock (Tsuga canadensis), and yellow birch (Betula alleghaniensis) and they

represented four stages in the cutting cycle: 0-5, 16-20, 21-25, and > 50 years post-

harvest. Three stands (mean stand size = 34 ha) from each post-harvest treatment were

studied (Table 1.1).

20

Figure 1.1. Map of sites used to study the effects of single-tree selection on the demography of Rose-breasted Grosbeaks in Ontario, Canada. The grey polygon indicates Algonquin Provincial Park and the numbers associated with each site indicate number of years post-harvest. Solid triangles = 0-5 years post-harvest stands, solid squares = 16-20 years post-harvest stands, solid hexagons = 21-25 years post-harvest stands, and unfilled circles = > 50 years post-harvest stands.

21

TABLE 1.1. Locations and characteristics of the twelve tolerant hardwood stands in Algonquin Provincial Park, Ontario, Canada that were investigated in this study.Site

CodeCounty UTM

co-ordinatesLast

harvest year

Post-harvest treatment (years since harvest)

Searched area (ha)

Basal area (m2/ha)

Percentsugar maple (Mh)1

Percent American

beech (Be)1

Percent other

tree species(Sp)1,2

CT Nipissing 716200 5068315 2004.5 0-5 32 22.1 80.0 20.0 0.0LC Haliburton 714450 5034175 2004.5 0-5 41 19.2 41.7 20.8 37.5YO Haliburton 713676 5015892 2000.5 0-5 38 23.9 39.1 17.4 43.5LB Haiburton 714035 5018042 1989.5 16-20 25 26.7 50.0 30.0 20.0OP Nipissing 712007 5074748 1987.5 16-20 36 26.3 46.1 38.5 15.4TM Nipissing 701545 5053886 1986 16-20 34 28.3 40.9 22.7 36.4CF Haliburton 713800 5032150 1980.5 21-25 25 26.9 54.5 9.1 36.4LW Nipissing 713620 5072340 1984.5 21-25 26 29.0 30.8 15.4 53.8ZB Nipissing 701560 5056325 1984.5 21-25 43 29.0 47.1 11.8 41.2BR Nipissing 710659 5051174 1941.5 > 50 32 37.3 50.0 20.0 30.0CJ Haliburton 713860 5018886 1948.5 >50 29 32.0 29.2 29.2 41.6TR Nipissing 697000 5049750 1941.5 >50 43 30.5 37.5 25.0 37.5

1Percentage of each tree species in prism sweeps at randomly located plots within each stand2Other tree species consisted primarily of yellow birch, black cherry, striped maple, eastern hemlock, and balsam fir

22

Habitat Measurements

I measured habitat characteristics in 5 m radius, circular plots in 170 randomly

selected locations within my 12 post-harvest stands between 2006 and 2008. In each

year, the number of random plots surveyed in each stand was equal to the number of nests

monitored in that stand, providing an average of 13 plots/stand and 43 plots/treatment

over the three years. Survey plots were selected using a set of randomly generated UTM

co-ordinates in each site, and GPS units were used to locate the coordinates on the

ground. No restrictions were placed on the locations of plots, as long as they fell within

the stand boundaries. Vegetation layers were measured using a modified version of the

James and Shugart (1970) method. At each survey plot, percent cover by 1)

forbs/grasses/ferns, 2) regenerative growth (woody growth 0.5-1.3 m tall), 3) saplings

(>1.3 m tall, <2.5 cm dbh), 4) understory (≥ 2.5 cm dbh, <10.0 cm dbh), 5) sub-canopy

(>10-20 m tall), and 6) canopy (>20 m tall) were visually determined, and basal area

(m2/ha) was measured using a 2-factor cruising prism.

Nest Monitoring

I visited each stand once every 3-4 days to search for nests. When a nest was

located, a piece of coloured flagging tape was placed within 10 m of the nest and its

geographical co-ordinates were taken using a GPS unit. I then monitored nests using a

mirror attached to a telescoping pole every 3-4 days to determine nest initiation date,

clutch size, hatch date, number of nestlings, and number of fledglings per nest. Mean

values for nest initiation date, clutch size, number of fledglings per successful nest, and

number of fledglings per nest were calculated for each stand. Whenever possible, exact

23

dates for nest initiation, hatching, and fledging were confirmed by nest checks, but in

other instances (e.g., canopy nests) published lengths for nest building (4 days),

incubation (12 days), and nestling development (10 days) were used to help calculate

important dates (Scott 1998). The number of eggs present in 12 of 52 (23.1%) nests

could not be visually confirmed, so clutch size was estimated using the mean number of

eggs per nest in the stands where these twelve nests were located. I did not include nests

that were initiated on or after June 3rd in my analysis of nest initiation dates (range = May

18th – June 2nd) due to uncertainty as to whether they were first attempts or not. I

calculated nest survival for each stand using the Mayfield method (Mayfield 1975).

Density and Pairing Success

In 2007 and 2008 I estimated the density of breeding Rose-breasted Grosbeaks in

each stand using a modified version of the spot mapping technique (Bibby et al. 2000). I

marked the locations of all Rose-breasted Grosbeaks that were seen or heard during nine

visits to all stands between May 10th and June 2nd using a Global Positioning System

(GPS). I stopped mapping on June 2nd due to uncertainty in determining whether

individuals observed after this point had relocated after initial nest attempts in other

locations (Bibby et al. 2000, Dunford and Freemark 2005). Males that were observed in

the same location on three or more occasions were considered territorial. In territories

where no nest was located, a male was assumed to be paired if a female was observed in

his territory on at least one occasion, and unpaired if no female was observed after a total

of 90 minutes of observation during one or more observation periods (Bibby et al. 2000).

24

The proportion of after-second-year (ASY) males relative to second-year (SY)

males was also calculated in each stand by recording the age category of each singing

male that was mapped over the course of the season. ASY males can be visually

distinguished from SY males by their uniformly black primaries, black heads and backs,

and well-defined markings (Pyle 1997). In contrast, the head and back of SY males are

often streaked with brown, there is contrast between their all-brown or partially-brown

primaries and their darker greater coverts, and overall their markings are much less

distinct (Pyle 1997). I calculated density (no. males/ ha) and pairing success (no. paired

males / total no. males) for each stand.

Population Productivity

I calculated two different measures of productivity. The first measure was used to

assess the mean number of Rose-breasted Grosbeak fledglings produced per active nest in

each post-harvest treatment (0-5, 16-20, 21-25, and > 50 years post-harvest).

For the second measure of productivity, I estimated whether population growth

rates were above or below replacement levels in each harvest treatment. For a population

to be self-sustaining, the annual female mortality rate must not exceed the average

number of female offspring produced per female that survive to breed (Ricklefs 1973,

Pulliam 1988, Donovan et al. 1995). In other words, the long-term change in population

size (λ) must be ≥ 1. I calculated the finite rate of increase for Rose-breasted Grosbeaks

in each post-harvest treatment using a two-age-class matrix population model in the

Microsoft Excel add-in program PopTools version 3.2.3 (Hood 2010). Confidence

25

intervals for population λ were calculated using the bootstrapping technique with 100

iterations in the PopTools program.

The two-age-class model for calculating λ incorporates estimates of annual adult

and juvenile survival, which I did not directly measure. Following Moore et al. (2010), I

used estimates of annual adult survivorship ranging from 0.40 – 0.61 obtained from

literature on other Neotropical migratory songbirds. I calculated annual juvenile

survivorship for each treatment as the product of fledgling survival and annual adult

survival, based on the assumption that once juveniles reach independence their annual

survival rate is similar to that of adults (Moore et al. 2010). To calculate annual female

fecundity (number of female offspring per female per year) I assumed that Rose-breasted

Grosbeak young occur in a 1:1 sex ratio, that females who initially failed to successfully

nest would re-nest only once, and that the probability of nest success for the re-nesters

would be similar to that experienced during first nest attempts (i.e., if 30% of nests failed

during the first attempt, approximately 30% of nests in the second attempt would fail)

(Smith et al. 2006, Moore et al. 2010). Following Moore et al. (2010) I used these values

to calculate population growth rates under best case (high adult and juvenile survival) and

worst case (low adult and juvenile survival) scenarios.

Statistical Analyses

Habitat Measurements

I tested for significant (P < 0.05) correlations among habitat variables using the

Spearman rank correlation coefficient. When significant rank correlations were identified

between independent variables, the measure I judged to be of lesser ecological

26

importance to Rose-breasted Grosbeak demography was omitted from further analysis

(Burnham and Anderson 2002, Driscoll et al. 2005). During this process seedling cover

was omitted due to significant (P < 0.05) correlations with regenerative growth (+),

saplings (-), canopy (-) cover, and basal area (-). Percent cover in the canopy and sub-

canopy layers were significantly positively correlated, so these variables were combined

into one measure (hereafter referred to as canopy cover). Sapling cover was positively

correlated with understory cover (r = 0.56, P < 0.05), but both variables were retained

because they were judged to be of importance to Rose-breasted Grosbeak ecology.

I used redundancy analysis (RDA; ter Braak and Smilauer 1998) to test whether

five habitat variables differed significantly among harvesting treatments (0-5, 16-20, 21-

25, and > 50 years post-harvest) and study years (2006-2008). Habitat variables were

then compared among treatments individually using two-way analysis of variance

(ANOVA).

For each response variable a mean value was calculated for each stand in each

year. In all tests, each stand mean was weighted according to the number of survey plots

collected in that stand. I first tested the full model. Since the two-way interactions

between year and post-harvest treatment were clearly not significant (i.e., P > 0.15) for

all habitat variables tested, I removed the interaction terms and re-ran the models.

Significant differences among post-harvest treatments were investigated using two-way

analysis of variance followed by Tukey’s Studentized post-hoc range test (α = 0.05). For

all ANOVAs, model assumptions of homogeneity and normality were verified by

examination of residuals, and no transformations were necessary. Correlations and model

assumptions were tested in Statistica 7 (StatSoft Inc. 2004), redundancy analysis was

27

performed in CANOCO, and all other analyses were conducted in the PROC GLM

module of SAS v. 8.02 (SAS Institute Inc., 1999).

Demographic Variables

I used redundancy analysis to investigate nine demographic variables (territory

density, pairing success, proportion of ASY compared to SY males, nest initiation dates,

clutch size, number of fledglings per successful nest, daily nest survival, period nest

survival, and productivity) as a function of harvesting treatment (0-5, 16-20, 21-25, and >

50 years post-harvest) and study year. Demographic variables were then compared

individually among harvest treatments using two-way ANOVAs.

Estimates of nest initiation date, clutch size, and survival were available for all

three years (2006-2008), but all other demographic variables were only estimated for

2007 and 2008. For each response variable a mean value was calculated for each stand in

each year. In all tests, each stand mean was weighted according to the number of

observations collected in that stand. I first tested the full model (treatment, year, and

treatment × year interaction), but re-ran the models without the interaction term since it

was clearly not significant for any of the variables measured (i.e., P > 0.15). Significant

differences among post-harvest treatments were investigated using two-way analysis of

variance followed by Tukey’s Studentized post-hoc range test (α = 0.05). For all

ANOVAs, model assumptions of homogeneity and normality were verified by

examination of residuals, and no transformations were necessary.

28

Relationship Between Habitat Features and Demographic Variables

I used redundancy analysis (RDA; ter Braak and Smilauer 1998) to examine the

overall relationship between demographic variables and habitat characteristics in the four

post-harvest treatments. Five habitat variables were retained in the analysis as well as

nine demographic variables.

Male Age

I used a procedure similar to the one used for the demographic variables to

compare demographic measurements and habitat characteristics at nests attended by ASY

and SY males in the four post-harvest treatments. The habitat characteristics I

investigated were percent cover by regenerative growth, saplings, understory, and canopy

and basal area. Too few SY males were present in most sites to allow separate

comparisons for each study year. I pooled measurements across study years by

calculating an average for each site in each study year, then taking a weighted average of

these values, with each value weighted according to the number of observations in each

site. As in the above analysis, I investigated the differences in demographic variables and

habitat characteristics among post-harvest treatments and between nests attended by ASY

and SY males using two-way ANOVAs followed by Tukey’s Studentized post-hoc range

test (α = 0.05).

RESULTS

Habitat Measurements

Redundancy analysis indicated that vegetation variables differed significantly

with time since single-tree selection harvesting (F-ratio = 3.642, P = 0.002). Tukey’s

29

tests showed that the removal of trees resulted in significantly lower basal area and lower

cover from saplings and understory in 0-5 years post-harvest stands than in other

treatments (Table 1.2). As a result of the openings created by harvesting, these sites also

had significantly higher cover from regenerative growth than other stands. In the 16-20

years post-harvest stands, regenerative growth had developed into a denser understory

layer than was present in other post-harvest treatments. Habitat features 21-25 years after

harvesting were not significantly different from those in the 16-20 years post-harvest

stands in most study years, but cover from regenerative growth and saplings was

significantly lower than in the > 50 years post-harvest stands. The “un-logged” stands (>

50 years post-harvest) had higher basal area and higher sapling and canopy cover than

were present in the other post-harvest treatments.

Demographic Variables

Mean density of Rose-breasted Grosbeaks in this study was 0.212 (SE = 0.01)

territories / ha, mean pairing success was 0.963 (SE = 0.01), and the mean proportion of

ASY males was 0.629 (SE = 0.11), indicating that more after-second-year males

successfully held territories than second-year males. Between 2006 and 2008 I monitored

a total of 173 nests (20, 73, and 80 nests in the years 2006-2008 respectively), and I was

able to determine exact nest initiation dates for 157 first nest attempts. Mean nest

initiation date was May 26th (range = May 18th – June 2nd), with one outlying nest started

on May 15th, 2006. Nest contents could be verified for 52 nests, yielding a mean clutch

size of 3.81 eggs, and an average of 3.44 fledglings per successful nest. Mean daily

survival rate (DSR) for Rose-breasted Grosbeak nests was 0.967 (n = 173 nests), mean

30

TABLE 1.2. Means of habitat variables (standard error in brackets) and results from two-way ANOVAs and Tukey’s tests comparing sites from four stages in the cutting cycle: 0-5, 16-20, 21-25, and > 50 years post-harvest (n = 3 per stage) and three study years (2006-2008).

Time Since Harvest (years)1 P PVariable 0-5 16-20 21-25 >50 (Treatment) (Year)Forb cover (%) 2006 8.5 (3.5) 3.3 (1.4) 23.0 (6.1) 12.0 (0.1) 0.051 0.029 2007 8.6 (2.2) 2.0 (0.6) 4.4 (1.6) 5.2 (1.6) 2008 6.3 (2.0) 2.9 (0.9) 7.2 (0.4) 6.7 (1.6)Regeneration (%) 2006 14.5 (0.5)a 4.3 (0.6)bc 4.3 (0.2)b 12.0 (0.1)ac <0.001 0.061 2007 17.8 (3.2)a 7.1 (3.0)bc 4.2 (1.2)b 11.4 (1.6)ac

2008 23.4 (4.7)a 9.1 (2.9)bc 7.2 (3.3)b 18.4 (5.6)ac

Sapling cover (%) 6.3 (0.9)a 16.0 (3.9)ab 9.9 (2.5)a 25.9 (4.1)b <0.001 0.108Under-story cover (%) 20.8 (2.8)a 34.5 (4.1)b 27.1 (2.9)ab 25.3 (5.4)ab 0.066 0.516Canopy cover (%) 81.2 (6.8) 90.3 (4.8) 89.4 (3.8) 93. 8 (7.2) 0.376 0.060Basal area(m2/ha) 22.1 (1.2)a 27.2 (1.2)b 27.0 (0.8)b 28.1 (1.8)b 0.006 0.9541Superscript letters indicate significant differences between silvicultural treatments from Tukey’s HSD post-hoc test (α = 0.05).

31

period survival rate (PSR) for the entire 24 day period was 0.559, and mean productivity

was 2.02 fledglings per nest.

Density, pairing success, and the number of fledglings per successful nest were all

significantly lower in the > 50 years post-harvest stands than in at least one other post-

harvest treatment, and nest survival and productivity were also lowest in the > 50 years

post-harvest stands (Table 1.3). The proportion of ASY males compared to SY males

was highest in the > 50 years post-harvest stands and lowest in the 21-25 years post-

harvest treatment, but differences were not significant (P = 0.616; Table 1.3). Nests

were initiated significantly later in 2008 than in 2006 or 2007, and productivity estimates

were significantly higher in 2008 than in 2007.

On average, female fecundity was 1.02 female fledglings per female, with Rose-

breasted Grosbeak females producing 1.01 – 1.02 female fledglings / female / year (Table

1.4). Under the worst case scenario (low adult and juvenile survivorship) all post-harvest

treatments were population sinks. Under the best case scenario (high adult and juvenile

survival), the 16-20 years post-harvest treatment was a source, but all other treatments

remained sinks.

Relationship Between Habitat and Demographic Variables

The first two axes of the RDA analysis explained 57% of the variance in

demographic variables (41.6% and 15.4% for the first and second axes, respectively; Fig.

1.2). Sapling cover and basal area were strongly associated with the > 50 years post-

harvest stands, whereas understory and forbs were weakly associated with this treatment.

Regeneration and canopy cover were weakly associated with 0-5 years post-harvest

32

TABLE 1.3. Means of demographic variables (standard error in brackets) and results from two-way ANOVAs and Tukey’s tests comparing sites from four stages in the cutting cycle: 0-5, 16-20, 21-25, and > 50 years post-harvest (n = 3 per stage) and three study years (2006-2008). Sample sizes (n) show number of nests per treatment, but may vary slightly among demographic measures.

Time Since Harvest (years)1

Demographic Variable0-5

n = 5516-20n = 34

21-25n = 38

>50n = 29

P (Treatment)

P (Year)

Density (no. territories /ha) 0.246 (0.02)ab 0.254 (0.03)a 0.187 (0.02)ab 0.162 (0.01)b 0.031 0.727Pairing success (prop. males paired) 0.990 (0.01)a 0.972 (0.02)a 1.000 (0.0)a 0.892 (0.01)b <0.001 0.072Proportion of territorial ASY males 0.598 (0.08) 0.634 (0.08) 0.550 (0.09) 0.725 (0.11) 0.616 0.473Nest Initiation Date 2006 May 27 (5.50) May 26 (1.81) May 24 (1.28) May 24 (1.03) 0.430 <0.001 2007 May 24 (0.88) May 24 (1.03) May 22 (0.87) May 25 (1.12) 2008 May 27 (0.46) May 28 (0.56) May 27 (0.87) May 27 (0.76)Clutch size 4.000 (0.07) 3.667 (0.17) 3.625 (0.12) 3.833 (0.40) 0.477 0.283No. fledglings / successful nest 3.619 (0.13)a 3.333 (0.24)ab 3.500 (0.16)ab 2.833 (0.48)b 0.012 0.060Mayfield period nest survival rate 0.524 (0.09) 0.598 (0.09) 0.654 (0.12) 0.437 (0.09) 0.350 0.308Productivity (no. fledglings / nest) 2.305 (0.35) 1.603 (0.28) 2.282 (0.47) 1.383 (0.62) 0.593 0.113

1Superscript letters indicate significant differences between silvicultural treatments from Tukey’s HSD post-hoc test (α = 0.05).

33

TABLE 1.4. Population growth rates for Rose-breasted Grosbeaks breeding in 12 tolerant hardwood stands in Algonquin Provincial Park harvested by single-tree selection. Stands represent four stages in the cutting cycle: 0-5, 16-20, 21-25, and > 50 years post-harvest. Models show intrinsic (λ) and exponential (r) population growth rates under best case (high adult and juvenile survival) and worst case (low adult and juvenile survival) scenarios. Treatments are designated as sources if λ is ≥ 1 and a sink if λ is < 1.Time since harvest (years)

Adult survival probability a

Annual juvenile survival b

Female fecundity c λ Upper and lower

95% CI

Population status

Best Case Scenario0-5 0.61 0.337 1.019 0.966 0.864 - 1.070 Sink16-20 0.61 0.417 1.012 1.025 0.922 - 1.128 source21-25 0.61 0.249 1.022 0.893 0.790 - 0.996 Sink> 50 0.61 0.198 1.012 0.848 0.745 - 0.951 SinkWorst Case Scenario0-5 0.40 0.221 1.019 0.713 0.610 - 0.816 Sink16-20 0.40 0.274 1.012 0.760 0.657 - 0.863 Sink21-25 0.40 0.163 1.022 0.649 0.546 - 0.752 Sink> 50 0.40 0.130 1.012 0.614 0.511 - 0.717 Sinka Estimates obtained from literature on other Neotropical migratory songbirdsb Fledgling survival probability multiplied by annual juvenile survival rate, assumed to be equal to adult survival probabilityc Mean no. of female offspring / successful nest / year, assuming one attempt at re-nesting following nest failure

34

stands. All demographic variables were weakly associated with harvested stands, except

for nest initiation date, the proportion of territorial ASY males compared to SY males,

and clutch size, which were not clearly associated with any treatment.

Age of Male Parent

Nests attended by ASY males were initiated significantly earlier than nests

attended by SY males (F1,3 = 19.97, P = 0.0002) (Table 1.5). Territory density was

significantly higher for ASY males than SY males in all post-harvest treatments (F1,3 =

10.15, P = 0.005), and productivity (no. fledglings produced per active nest) was also

significantly higher for ASY males than for SY males (F1,3 = 3.94, P = 0.050).

Differences between nests attended by ASY males and those attended by SY males were

not significant for clutch size (F1,3 = 0.02, P = 0.892), mean number of fledglings

produced per successful nest (F1,3 = 0.66, P = 0.429), daily survival rate (F1,3 = 1.56, P =

0.226), or period survival rate (F1,22 = 1.32, P = 0.174), but all of these measures were

slightly higher at nests attended by ASY males than those attended by SY males.

Multivariate analysis of variance indicated that there were no significant

differences in percent cover by regenerative growth, saplings, understory, and canopy

cover or in basal area at nests attended by ASY and SY males (F5, 14 = 0.42, P = 0.827).

The interaction between post-harvest treatment and male age (ASY or SY) was also not

significant (F15, 39.05 = 0.78, P = 0.691).

35

FIGURE 1.2. Relationship between seven demographic variables for Rose-breasted Grosbeaks and six habitat features in four post-harvest treatments in Algonquin Provincial Park. Demographic variables are mean clutch size, territory density, number of fledglings per successful nest, pairing success, productivity, proportion of territorial ASY males, and nest survival. Habitat features are percent cover by forbs, regenerative growth, saplings, understory, and canopy and basal area. Filled triangles = 0-5 years post-harvest stands, filled squares = 16-20 years post-harvest stands, filled octagons = 21-25 years post-harvest stands, empty circles = > 50 years post-harvest stands. Inter-sample scaling was used, and the species scores were centered and standardized (RDA on a correlation matrix between demographic variables). The first axis explained 41.6% of the variance in demographic variables and the second axis explained 15.4%.

36

TABLE 1.5. Means of demographic variables (standard error in brackets) and results from two-way ANOVAs and Tukey’s tests comparing sites from four stages in the cutting cycle: 0-5, 16-20, 21-25, and > 50 years post-harvest (n = 3 per stage) and three study years (2006-2008). Results are shown for after-second-year (ASY) and second-year (SY) males. Bold values indicate significant differences between SY and ASY males.

Time Since Harvest (years)1

Demographic Variable Male Age 0-5 16-20 21-25 >50Density (no. territories /ha) ASY 0.122 (0.04) 0.114 (0.04) 0.081 (0.02) 0.091 (0.03)

SY 0.071 (0.00) 0.054 (0.00) 0.057 (0.01) 0.029 (0.01)Nest initiation date ASY May 24 (0.35) May 24 (0.78) May 22 (1.49) May 25 (1.65)

SY May 27 (1.24) May 27 (0.87) May 28 (0.88) May 28 (1.16)Clutch size ASY 4.000 (0.14) 3.625 (0.09) 3.833 (0.12) 4.000 (0.58)

SY 4.000 (0.00) 4.000 (-) 3.800 (0.283) 2.000 (0.58)No. fledglings / successful nest ASY 3.715 (0.17) 3.250 (0.12) 3.667 (0.236) 3.667 (0.33)

SY 3.602 (0.23) 4.000 (-) 3.800 (0.283) 2.000 (0.58)Daily nest survival ASY 0.982 (0.00) 0.983 (0.01) 0.993 (0.00) 0.980 (0.00)

SY 0.979 (0.00) 0.974 (0.00) 0.983 (0.01) 0.982 (0.00)Period nest survival ASY 0.674 (0.09) 0.699 (0.11) 0.864 (0.05) 0.632 (0.03)

SY 0.627 (0.04) 0.545 (0.03) 0.709 (0.16) 0.678 (0.14)Productivity (no. fledglings / pair) ASY 2.833 (0.47) 1.722 (0.57) 2.798 (0.27) 1.710 (0.18)

SY 1.526 (0.49) 1.638 (0.48) 2.083 (0.69) 1.000 (0.41)1Superscript letters indicate significant differences between silvicultural treatments from Tukey’s HSD post-hoc test (α = 0.05).

37

DISCUSSION

Habitat characteristics differed significantly among stands harvested by single-

tree selection 0-5, 16-20, 21-25, and > 50 years previously. As in other studies, the most

recently harvested stands had significantly higher cover from regenerative growth, lower

basal area, and lower sapling, understory, and canopy cover than other post-harvest

treatments (Robinson and Robinson 1999, Flaspohler et al. 2002, Jobes et al. 2004,

Moore et al. 2010). Increased light penetration in newly logged stands typically promotes

the establishment of shade-tolerant sugar maple and American beech seedlings (Finegan

1984, Jobes et al. 2004), and in this study resulted in significantly higher understory

cover by 16-20 years post-harvest. As previously demonstrated, the >50 years post-

harvest stands had higher basal area and higher sapling cover than other stands (Robinson

and Robinson 1999, Flaspohler et al. 2002, Jobes et al. 2004).

Single-tree selection harvesting did not appear to have a significant negative

effect on any of the demographic measures included in this study, and density, pairing

success, and the number of fledglings produced per successful nest were all significantly

lower in the > 50 years post-harvest treatment than in the harvested stands. However,

Rose-breasted Grosbeaks were below replacement levels in all stands except the 16-20

years post-harvest treatment, even using the most optimistic measures of adult and

juvenile survival.

Demographic Measures

In the predominantly forested landscape of Algonquin Provincial Park Rose-

breasted Grosbeak density ranged from 0.16-0.24 male territories per hectare, which is

38

similar to the range of densities (0.20-0.24) reported for forest fragments in south-western

Ontario (Smith et al. 2006). However, whereas density was lower in the uncut stands

than in the harvested ones in the predominantly forested landscape, the opposite was true

in forest fragments in the agricultural landscape, where Rose-breasted Grosbeaks

appeared to be attracted to the higher shrub cover, particularly in heavily cut stands

(Smith et al. 2006). Nest predation and brood parasitism by Brown-headed Cowbirds

(Molothus ater) are often higher (Paton 1994, Hoover et al. 1995, Rodewald et al. 2002,

Phillips et al. 2005, Falk et al. 2011), and pairing success is lower (Gibbs and Faaborg

1990, Villard et al. 1993, Van Horn et al. 1995, Burke and Nol 1998) in fragmented

agricultural landscapes than in continuously forested regions. As a result, openings in the

understory and canopy created by silviculture, which potentially expose songbird nests to

visual predators and brood parasites, could have a greater impact in fragmented

landscapes than in predominantly forested regions (Andrén 1995, Hanski et al. 1996).

Higher rates of nest predation and brood parasitism following harvesting could result in

lower recruitment of individuals into selection harvested stands, and therefore lower

densities relative to un-harvested fragments. Rose-breasted Grosbeak nests are

parasitized in fragmented agricultural landscapes (Smith et al. 2006, 2007), but were not

in my predominantly forested landscape, and estimates of nest survival were considerably

lower for the fragmented landscape (0.341-0.390; Smith et al. 2006, 2007, Moore et al.

2010) than in my study area (0.559). These results suggest that the effects of timber

harvesting to some extent depend on landscape context, and that adverse effects may be

fewer in forested regions than in fragmented landscapes.

39

Rose-breasted Grosbeak productivity, represented by the number of fledglings

produced per successful nest, was highest in the 0-5 years post-harvest stands and lowest

in the > 50 years post-harvest treatments. The number of fledglings produced per

successful nest ranged from 2.8-3.6, which is comparable to, but slightly higher than

estimates from southwestern Ontario, which ranged from 2.4 (Smith et al. 2006) to 3.2

(Moore et al. 2010). While more young were fledged on average from successful nests in

uncut stands (2.8) than in harvested ones (2.4) in the fragmented landscape (Smith et al.

2006), in the predominantly forested landscape significantly more young were fledged

per successful nest in 0-5 years post-harvest stands (3.6) than in uncut sites (2.8).

Estimates of the number of fledglings produced per active nest, which incorporate nest

survival rates, were also lower in the > 50 years post-harvest stands than in the more

recently harvested treatments in my study landscape, but not significantly so. Together

these results suggest that single-tree selection harvesting may have less of a negative

impact on Rose-breasted Grosbeak productivity in forested regions than in more

fragmented landscapes.

As predicted, density was highest in the 16-20 year post-harvest stands and the

population growth model indicated that these stands were at or closer to replacement

levels than the other post-harvest treatments. The 16-20 years post-harvest treatment

appeared to attract Rose-breasted Grosbeaks and to have high productivity, but in

general, most demographic measures were higher in more recently cut stands than in the

un-logged ones. Previously harvested stands were associated with higher regenerative

growth and understory cover, both of which are features previously identified as

important to Rose-breasted Grosbeak demography (Smith et al. 2006, 2007). Other

40

studies have found greater abundances of early-successional species in regenerating

forests as well (Annand and Thompson 1997, Simon et al. 2000, Gram et al. 2003,

Holmes et al. 2004, Jobes et al. 2004, Holmes and Pitt 2007, Robertson and Hutto 2007,

Atwell et al. 2008), suggesting that forest management may provide additional nesting

habitat and/or food resources for disturbance-dependent songbirds. In my study Rose-

breasted Grosbeaks built their nests at a wide range of heights (1.5 – 22 m), and they

were observed foraging in vegetation ranging from ground level to the upper layers of the

canopy. This species’ ability to demonstrate plasticity in nest site selection and foraging

substrate use may help explain why all demographic measures were not clearly higher in

one post-harvest treatment than the others.

Population Productivity

Population growth estimates indicated that even under the best case scenario,

Rose-breasted Grosbeaks in my study area were not replacing themselves except in the

16-20 years post-harvest treatments. Although population growth estimates have not

previously been determined for Rose-breasted Grosbeaks in predominantly forested

landscapes, this species is frequently found in regenerating forests 15 years after

harvesting (Pelletier and Dauphin 1996), and often increases in abundance following

logging (i.e., Jobes et al. 2004, Doyon et al. 2005). This study indicates that the dense

understory and sapling layers characteristic of hardwood stands 16-20 years post-harvest

are important for Rose-breasted Grosbeaks, and that access to regenerating forests may

provide some benefits for this species in forested areas.

41

Population growth models are highly sensitive to small changes in estimates of

annual survival rates for adults and juveniles (Porneluzi and Faaborg 1999, Powell et al.

1999, Woodworth 1999), which despite technological advancements in radio tracking are

still difficult to accurately obtain (Greenwood and Harvey 1982, Payne 1991, Paradis et

al. 1998, Anders and Marshall 2005). Common responses to this problem are to estimate

juvenile survival by dividing adult mortality by the average number of offspring fledged

(Ricklefs 1973), to assume juveniles are half as likely to survive as adults (Greenberg

1980), or to adopt 0.31 as an estimate of juvenile survival (Temple and Carey 1988).

Empirical estimates of juvenile survival from fledging to fall migration range from 0.28

to 0.75 for Juncos (Junco phaenotus), Wood Thrush (Hylocichla mustelina), and Lark

Buntings (Calamospiza melanocorys) (Sullivan 1989, Anders et al. 1997, Powell et al.

2000, Yackel Adams et al. 2001). Annual adult survival is often assumed to be 0.62

(Anders and Marshall 2005), and empirical estimates of annual survival for breeding

songbird females range from 0.40-0.55 (Cilimburg et al. 2002, Sillett and Holmes 2002,

Gardali et al. 2003, Stutchbury et al. 2009). I incorporated a range of values for adult and

juvenile survival by calculating growth rates under a worst case scenario (adult survival =

0.40, and low juvenile survival) and a best case scenario (adult survival = 0.61, and high

juvenile survival), as well as using stand specific values for female fecundity, nest

success, and fledgling survival to improve accuracy (Anders and Marshall 2005).

Although these results should be accepted with caution, this study suggests that harvested

hardwood stands in predominantly forested landscapes may be population sources for

Rose-breasted Grosbeaks by 16-20 years post-harvest.

42

Population Age Structure

The proportion of territorial ASY males compared to territorial SY males was

investigated to determine whether older, more experienced males were out-competing

younger, less-experienced ones for “high quality” habitats. Although breeding pairs with

ASY males began nest building significantly earlier than pairs with SY males, my

prediction that the 16-20 years post-harvest treatment would have the highest proportion

of ASY males was not supported. The proportion of territories held by experienced

males was highest in the un-harvested stands, although among-treatment differences were

not significant. According to the ideal-free distribution theory, a higher proportion of

older, more experienced males in the > 50 years post-harvest stands indicates that these

stands may provide better quality habitat for Rose-breasted Grosbeaks. However, since

overall density, pairing success, and productivity were lowest in this treatment, this

interpretation seems unlikely. Instead, I propose that ASY males may have been out-

competing SY males in this treatment for a more limited number of available territories

than were offered in harvested stands. Older males were less productive in the > 50 years

post-harvest stands than in other treatments, but even in these stands their productivity

was higher than that of younger males in most other treatments. The opposing patterns in

density, nest initiation dates, productivity, and proportion of older, more experienced

males among the post-harvest treatments indicates that the factors which influence

reproductive success and habitat quality are complicated, and as a result, assessments of

the effects of harvesting or other disturbances should be based on more than one

demographic measure whenever possible.

43

Rose-breasted Grosbeak nests attended by after-second-year males were started

significantly earlier, and had significantly higher productivity than nests attended by

second-year males, suggesting that more experienced breeders had a reproductive

advantage over less experienced birds. First-time breeders of other species have shown

lower nest survival than older individuals (Pärt 2001, Forschler and Kalko 2006) and they

sometimes occupy poorer quality habitats (Greenwood and Harvey 1982, Møller 1991,

Holmes et al. 1996). In some cases, second-year birds may have limited access to high-

quality territories as a result of later arrival on the breeding grounds, incomplete

knowledge of where to find good territories, and despotic distribution of more

experienced breeders (Sherry and Holmes 1989, Holmes et al. 1996, Bayne and Hobson

2001, Part 2001a). However, in my study habitat characteristics at nests attended by SY

males did not significantly differ from those at nests with ASY males, suggesting that

differences in territory quality were likely not the main cause of differences in

reproductive success. Lack of experience or differences in timing may be more likely

explanations for the lower reproductive success of first-time Rose-breasted Grosbeak

breeders.

Single-tree selection harvesting did not have a significantly negative effect on any

of the demographic measures investigated in this study. However, since fewer birds were

present in the > 50 years post-harvest stands, sample sizes for most demographic

measures were smaller in this treatment than in the others. In addition, nests in the 21-25

and > 50 years post-harvest stands tended to be higher up, making precise estimates of

nest initiation dates, brood sizes, and nest outcomes more difficult to obtain. Weighted

analyses were used in an effort to address these potential differences in sample size, but it

44

is possible they still had an effect on among-treatment comparisons. Finally, a small

proportion of territorial males (3-5%) could not be accurately aged, and although the

proportion was consistent among treatments, this source of uncertainty may have affected

the among-treatment comparisons. Increasing the number of stands studied might be one

way for future studies to increase the power of the tests and confirm that no significantly

negative effects are occurring as a result of single-tree selection cutting.

Conclusions

Density, pairing success, and productivity were higher in the more recently

harvested stands than in the > 50 years post-harvest treatment, suggesting that in tolerant

hardwood stands in predominantly forested landscapes, single-tree selection harvesting

does not appear to have negative effects on Rose-breasted Grosbeaks. However, the

proportion of older, more experienced males was highest in the > 50 years post-harvest

stands, indicating that these may have been perceived as providing higher quality habitat

by Rose-breasted Grosbeaks than more recently harvested stands. In addition, population

growth rates calculated from a wide range of realistic estimates of juvenile and adult

survival were below replacement levels in my study landscape, except in the 16-20 years

post-harvest stands, suggesting that further research is needed to help identify the

underlying causes of the decline in this species’ numbers. Selection harvesting

influenced Rose-breasted Grosbeaks differently in the predominantly forested region than

in the fragmented landscape, underscoring the importance of considering landscape and

regional contexts when designing silvicultural prescriptions (Thompson et al. 1995).

CHAPTER 2: SURVIVAL AND HABITAT USE BY ADULT AND FLEDGLING ROSE-BREASTED GROSBEAKS (PHEUCTICUS LEUDOVICIANUS) IN A FORESTED LANDSCAPE HARVESTED BY SINGLE-TREE SELECTION

ABSTRACT

Many songbirds that nest in the managed hardwood forests of North America are

currently declining, but although fledgling survival can have a considerable influence on

population growth, few studies have investigated how silvicultural practices influence

fledglings. I studied nest and fledgling survival and habitat use of Rose-breasted Grosbeaks

(Pheucticus leudovicianus) in twelve hardwood stands in a predominantly forested landscape

that had been harvested by single-tree selection 0-5, 16-20, 21-25, and > 50 years previously.

Mean nest and fledgling survival in each treatment were estimated using the Mayfield

method, and I used an information theoretic approach to assess relative support for models

explaining nest and fledgling survival as a function of habitat characteristics. Nest sites had

higher cover from regenerative growth, saplings, and understory, and lower basal area than

random locations. During their first week out of the nest, fledglings used locations with

significantly higher cover from regenerative growth, saplings, small shrubs, and raspberry

and elderberry bushes than were present at random locations. Mayfield daily survival for

Rose-breasted Grosbeak nests (n = 173) was 0.967 and period survival rate was 0.559. Daily

fledgling survival (n = 46) was 0.920 and period survival over the 21 day observation period

was 0.551. Neither nest nor fledgling survival differed significantly among post-harvest

treatments (P > 0.05). The best model for explaining nest survival included basal area, nest

initiation date, and sampling year, and suggested that nests initiated earlier in the season and

built in locations with higher basal area had a better chance of survival. The best model for

explaining fledgling survival included the number of days since fledging, indicating that as

45

46

fledglings age their probability of surviving increases. Habitat characteristics at nest and

fledgling locations were significantly different, and estimates of nest and fledgling survival

were not correlated among harvested stands, suggesting that assessments of habitat quality

for songbirds should not be limited to the nest stage. The lack of significant difference in

survival among post-harvest treatments suggests that single-tree selection harvesting is likely

not contributing to the observed declines in Rose-breasted Grosbeak populations.

47

INTRODUCTION

During the past decade there has been a shift in North America toward developing

and implementing forest management practices that mimic natural disturbance patterns

(Seymour et al. 2002). Under these guidelines, single-tree selection has been implemented in

much of the tolerant hardwood forests in northeastern North America (Tubbs 1977, OMNR

1998, Seymour et al. 2002). Although this low-intensity method of harvesting should have

minimal long-term impacts on wildlife (OMNR 2000), many migratory songbirds that breed

in this forest type are currently experiencing population declines (Newton et al. 2004,

Blancher et al. 2009). For some passerines nest survival rates remain stable before and after

single-tree selection harvesting (Bourque and Villard 2001, Duguay et al. 2001, Gram et al.

2003, Smith et al. 2006, Dellinger et al. 2007, Cooper et al. 2009, Moore et al. 2010),

whereas others experience lower rates of nest success in harvested forests (Barber et al. 2001,

Robinson and Robinson 2001, Robertson and Hutto 2007, Poulin et al. 2010). Nest survival

and habitat requirements have been well described for many North American birds (Akresh

et al. 2009), but far fewer studies have investigated fledgling survival and habitat

requirements once the young leave the nest (Anders and Marshall 2005, King et al. 2006,

Yackel Adams et al. 2006, Poulin et al. 2010). Since evaluating the long-term impacts of

forest management and other factors on declining songbird populations requires accurate

population models (Stutchbury et al. 2009) that are highly sensitive to estimates of juvenile

survival (Porneluzi and Faaborg 1999, Powell et al. 1999, Woodworth et al. 1999), studies of

fledgling survivorship are necessary.

The fledglings of many songbirds that nest in managed hardwood forests across North

America spend up to eight weeks on the breeding grounds prior to migration (Anders et al.

1998). Mortality during this period is typically high, especially during the first week out of

48

the nest when the young are unable to fly very well (Anders et al. 1997, 1998, Vega Rivera et

al. 1998, Monrós et al. 2002, Kershner et al. 2004, Yackel Adams 2006, Rush and Stutchbury

2008). Recent studies have shown that the habitat requirements of breeding adults may be

different from those of family groups during the post-fledging period (Rappole and Ballard

1987, Anders et al. 1998, Vega Rivera et al. 1998, Cohen and Lindell 2004, King et al. 2006;

but see Berkeley et al. 2007, Ciudad et al. 2009). For example, the amount and structure of

understory cover is often important during the first week post-fledging (King et al. 2006,

Mitchell et al. 2010), and the young of several species move out of the open, mature forest

used by adults for nest building into edge or early- to mid-successional forests (Anders et al.

1998, Vega Rivera et al. 1998, Pagen et al. 2000, Vitz and Rodewald 2006). These

differences suggest that conclusions based solely on studies of nest success may be

insufficient to assess the long-term impacts of forest management on songbird populations.

Recent studies have found correlations between fledgling survival and the amount of

coarse woody debris (Mitchell et al. 2010), the amount and structure of understory cover

(Mitchell et al. 2010), shrub cover (Moore et al. 2010), fledging date (Monros et al. 2002,

Yackel Adams et al. 2006, Moore et al. 2010), and weather (Yackel Adams et al. 2006). A

number of studies found that fledglings use areas with dense cover (Anders et al. 1998, Vega

Rivera et al. 1998), locations with greater than average vertical structure 0-3 m above ground

(King et al. 2006), and regenerating clear-cuts (Pagen et al. 2000, Vitz and Rodewald 2006).

One hypothesis to explain these patterns of resource use is the predator avoidance hypothesis,

which suggests that young birds seek habitats with more cover because they provide

protection from potential predators, especially during pre-migration molting (Anders et al.

1998, Vega Rivera et al. 1998). Alternatively, the optimal foraging hypothesis suggests that

juveniles’ movements are primarily determined by the need to find food (Vega Rivera et al.

49

1998). We are beginning to identify some of the habitat types and features that are important

for songbird fledglings, but we do not yet have a good understanding of the primary factors

driving resource use during the post-fledging period.

Studies of fledgling survival and habitat requirements have recently become more

prevalent, but to my knowledge only two studies have investigated fledgling survival in

managed forests (Lang et al. 2002, Moore et al. 2010). Thinning and burning in a > 30 year

old loblolly pine (Pinus taeda) forest had no effect on habitat use prior to parental

independence or on dispersal distances of Wood Thrush fledglings (Hylocichla mustelina)

(Lang et al. 2002). Similarly, the survival of Rose-breasted Grosbeak (Pheucticus

leudovicianus) fledglings in deciduous forest fragments in an agricultural landscape was not

significantly affected by single-tree or diameter-limit harvesting (Moore et al. 2010). Single-

tree selection cutting results in stands which are characterized by abundant regenerative

growth, low basal area, and sparse cover in the sapling, understory, and canopy layers a few

years after harvesting (Robinson and Robinson 1999, Flaspohler et al. 2002, Jobes et al.

2004). By 16-20 years post-harvest a dense understory layer typically forms (Finegan 1984,

Jobes et al. 2004), and eventually stands re-develop higher basal area, abundant cover in the

seedling, regenerative growth, sapling, and canopy layers, and high spatial complexity

(Robinson and Robinson 1999, Kenefic and Nyland 2000, Crow et al. 2002, Flaspohler et al.

2002, Jobes et al. 2004, Angers et al. 2005, Vanderwel et al. 2006). Since fledglings appear

to benefit from increased cover near the ground, it is reasonable to expect that single-tree

selection would not have a significant negative impact on their survival in a predominantly

forested landscape.

Here I investigated the influence of single-tree selection on survival probability and

habitat use of adult and fledgling Rose-breasted Grosbeaks (Pheucticus leudovicianus) in a

50

predominantly forested landscape. This species has been declining at an annual rate of

approximately - 2.4% on its breeding grounds in Ontario between 1966 and 2007 (Sauer et

al. 2008), with especially sharp declines (- 8% per year) between 1985 and 2005 (Cadman et

al. 2007). The Rose-breasted Grosbeak is a Neotropical migrant that nests in deciduous and

mixed forests throughout central and eastern North America (Wyatt and Francis 2002),

primarily in the sapling and understory layers in sites with relatively open canopy and sub-

canopy layers compared to random locations (Wyatt and Francis 2002). In the highly

fragmented deciduous forests of southwestern Ontario, neither nest survival (Smith et al.

2006) nor fledgling survival (Moore et al. 2010) were affected by timber harvesting, but both

harvested and un-harvested sites were population sinks (Smith et al. 2006, Moore et al.

2010). Although Rose-breasted Grosbeaks are declining, to my knowledge no studies have

investigated the demography of this species in managed stands in predominantly forested

landscapes, or investigated whether populations are self-sustaining in these areas.

I took a chronosequence approach to investigate the response of Rose-breasted

Grosbeaks to the structural changes that follow single-tree selection. Specifically, my main

objectives were to 1) identify important habitat characteristics at nest sites and locations used

by fledglings, and determine whether these differed among post-harvest treatments, 2)

estimate rates of nest and fledgling survival, and test whether these differed among post-

harvest treatments, and 3) determine which habitat features had the strongest influence on

nest success and fledgling survival in harvested stands. I expected habitat features to differ

among post-harvest treatments due to forest growth and development, and that as a result

vegetation characteristics at nest sites and fledgling locations would also differ among post-

harvest treatments. I predicted that nest survival would initially decrease following

harvesting, due to a reduction in regenerative growth and cover in the understory and canopy

51

layers, but that it would increase again by 16-20 years post-harvest with the development of a

dense understory layer. In contrast, I expected that fledgling survival would increase shortly

after harvesting due to an increase in cover near the ground from slash, raspberry bushes, and

saplings, but that it would decrease by 16-20 years post-harvest as the forest floor became

more open with the development of the understory and canopy layers.

METHODS

Study Species

Rose-breasted Grosbeaks typically arrive on their breeding grounds in central Ontario

during the first week of May. In the highly fragmented deciduous forests of southwestern

Ontario, Rose-breasted Grosbeaks nest in locations with more canopy cover but lower

sapling cover and basal area than are available at random sites (Smith et al. 2007). In these

fragmented forests nest survival is negatively associated with nest concealment, but

positively associated with canopy cover, sapling cover, and nest height (Smith et al. 2007).

Both males and females incubate and care for the young, which usually fledge 9-12 days

after hatching (Scott 1998). Little is known about the post-fledging period, but young are

unable to fly at the time of fledging and remain dependent on the adults for approximately

three weeks after leaving the nest (Wyatt and Francis 2002).

Study Area

The study was conducted in twelve tolerant hardwood stands in Algonquin Provincial

Park, Ontario, Canada (45° 34`N, 78°04`W) between 2006 and 2008 (Fig. 1.1). The west

side of this 775, 000 ha park is dominated by mixed hardwood forest, whereas the east side

consists primarily of white and red pine (Ontario Parks 1998). The stands I studied were

52

located in the central and southern part of the park (mean nearest neighbour distance among

stands = 9.3 km, range = 1.0 – 34.2 km). Tolerant hardwood stands in this part of the park

have been harvested using the single-tree selection system since the early 1970’s (OMNR

1998). This silvicultural system removes approximately one-third of the over-story basal

area every 20-25 years, resulting in uneven-aged stands with a mix of canopy gaps,

regenerative growth, and mature trees (OMNR 1998). Dominant tree species in the stands I

studied included sugar maple (Acer saccharum), American beech (Fagus grandifolia),

eastern hemlock (Tsuga canadensis), and yellow birch (Betula alleghaniensis).

In this study, three temporal stages in the cutting cycle were studied: 0-5, 16-20, and

21-25 years post-harvest. Stands that had not been logged for > 50 years were studied to

represent “un-logged” control conditions. Three sites from each temporal stage in the cutting

cycle were studied (mean stand size = 34 ha) (Table 1.1).

Nest Searching and Monitoring

Nest searching and monitoring were undertaken from May to August, 2006-2008.

Nests were monitored every 3-4 days using a mirror attached to a telescoping pole to

determine clutch size, hatching date, number of nestlings and fledglings, nest outcome, and

age of nestlings at the time of fledging. For nests that were too high to visually confirm

hatching dates, nestling age was estimated using the observed nest initiation date and clutch

size, published lengths for nest building (4 days) and incubation (12 days) (Scott 1998), and

the assumption that one egg was laid per day.

53

Radio Attachment and Tracking

For lower nests (≤ 2 m), all nestlings were removed from the nest nine days after

hatching (1-2 days prior to fledging) and given a uniquely-numbered aluminium US Fish and

Wildlife Service leg band and a unique combination of three coloured leg bands. All

nestlings were weighed using a SC-HT-S-232 digital scale with an accuracy of 0.01 g, and a

Holohil Systems (Carp, Ontario) BD-2 radio-transmitter was attached to the nestling with the

greatest mass. The transmitters weighed 0.6 g, (approximately 2.4% of a fledgling’s body

weight on day nine post-hatching) which is less than the 5% recommended by Caccamise and

Hedin (1985). Leg harnesses made from thin cotton thread were used to attach the

transmitters following the method developed by Rappole and Tipton (1991). Once all the

nestlings had been weighed, banded, and a transmitter had been attached they were returned

to the nest. For nests that were located too high to reach, fledglings were located and

captured on the ground immediately below their nest at the time of fledging, and bands and a

radio-transmitter were attached. For these nests, transmitters were attached to the heaviest

fledgling captured. Nestlings in lower nests, which were tagged prior to fledging, did not

leave their nests at a earlier age than young that were tagged post-fledging (mean age at

fledging = 10.1 and 10.3 days respectively), suggesting that radio-tag attachment likely did

not cause early fledging.

Once out of the nest, radio-tagged juveniles were located every second day for a

period of three weeks (11 possible observations per individual, mean = 7.5 observations per

individual) using programmable receivers and three-element Yagi antennas. Juveniles were

tracked on the ground, and the exact location of each radio-tagged individual was determined

visually, unless the bird was hidden by canopy foliage. The place where the bird was first

sighted was marked using a GPS unit and flagging tape, and the height and level of

54

concealment of the bird were recorded. In some instances determining the exact location of a

bird took several minutes, but this usually only happened when birds were located in the

canopy and not visible from below. I did not observe any instances when the tracker’s

movements caused the fledgling to change locations. In cases where the fledgling was first

seen flying from one location to another, the spot it was leaving from was marked. Juveniles

were considered dead if mortality was visually confirmed (i.e. a whole or partial carcass was

found) or were considered to have an unknown fate if their harness was located intact

without signs of predation, or if the signal was lost.

Habitat Measurements

Habitat characteristics were measured at nest sites, fledgling locations, and at a

corresponding number of random points. Random plots used for comparison with nests were

selected using a random number generator to produce a set of UTM co-ordinates located

within each stand, and a GPS unit was used to locate those co-ordinates on the ground.

Random surveys were not done in predominantly coniferous patches or swampy areas, but

otherwise there were no restrictions on the location of random plots as long as they fell

within the study stands.

For each fledgling that was tracked, habitat characteristics were surveyed at three

locations, one that was occupied during the first 7 days post-fledging, one that was occupied

between days 8 and 14, and one that was occupied between days 15 and 21. For each

fledgling, the location to be surveyed from each week-long period was randomly chosen

from amongst their known locations. Random points used for comparison with fledgling

locations were chosen to represent the type of habitat that would have been encountered if the

fledgling had moved the same linear distance since it was last observed, but in a different

55

direction. To determine the locations of random plots, I calculated the linear distance

between the fledgling location of interest (i.e., day 9 post-fledging) and its location when last

observed (i.e., day 7 post-fledging). I then walked this distance in a straight line from the

fledgling’s location when last seen (i.e. day 7 post-fledging) in a direction randomly chosen

by blindly spinning my compass. This random point was paired with the fledgling location

of interest (i.e. day 9 post-fledging). Due to fledgling mortality, sample sizes were smaller

for habitats surveyed during the second and third weeks post-fledging than during the first

week.

At fledgling locations, nests, and corresponding random points vegetation layers were

measured in 5 m radius circular plots using a modified version of the James and Shugart

(1970) method. Percent cover by: 1) forbs/grasses/ferns, 2) regenerative growth (woody

growth 0.5-1.3 m tall), 3) saplings (>1.3 m tall, <2.5 cm dbh), 4) understory (≥2.5 cm dbh,

<10.0 cm dbh), 5) sub-canopy (>10 m tall), and 6) canopy (>20 m tall) were visually

estimated, and basal area (m2/ha) was measured using a 2-factor cruising prism. At nest sites,

percent concealment and nest height were visually estimated, and fledgling concealment and

height were also visually estimated. At fledgling locations and corresponding plots percent

cover by small shrubs (< 2.5 cm dbh), large shrubs (2.6-8 cm dbh), wild red raspberries

(Rubus spp), and red elderberry (Sambucus spp) plants were also assessed. These variables

were selected for measurement because they have been shown to be correlated with the nest

and fledgling locations of songbirds elsewhere (Wyatt and Francis 2002, Smith et al. 2006,

Moore et al. 2010).

56

Statistical Analyses

Habitat Measurements

I used the Spearman rank correlation coefficient to test for significant (P < 0.05)

correlations among seven habitat measures, and a suite of relatively uncorrelated variables

that were judged to be of ecological importance to Rose-breasted Grosbeaks was retained for

further analysis (Burnham and Anderson 2002, Driscoll et al. 2005). Seedling cover was

significantly correlated with basal area (-), percent cover by regenerative growth (+), and

understory cover (-), and so was omitted from the analysis. Percent cover by forbs was

omitted due to significant correlations with regenerative growth (-), and understory (-),

whereas sub-canopy and canopy cover were combined into a single measure (hereafter

referred to as canopy cover) due to a significant positive correlation. Understory cover was

positively correlated with regenerative growth (r = 0.60), but both variables were retained

because they were judged to be of importance to Rose-breasted Grosbeaks.

Do habitat features differ between used and un-used plots?

With the exception of canopy cover, vegetation characteristics at nest sites and

random plots differed significantly among post-harvest treatments (F12, 135.22 = 2.42, P =

0.007), but not among study years (F8, 102 = 0.87, P = 0.542), and interactions between post-

harvest treatment and year were not significant (F30, 202 = 0.54, P = 0.975). Canopy cover

differed significantly among study years (F1,3 = 9.58, P = 0.010), so each year was analyzed

separately. All other habitat characteristics at nest sites and random plots were pooled across

study years by calculating average values for each site in each year, and then taking a

weighted average of those values. When data on vegetation characteristics at locations

occupied by fledglings was broken down by year and treatment, sample sizes were too small

57

(n < 3 in some cases) to allow meaningful analysis of habitat use. As a result, habitat use

data for fledglings were also pooled across study years, by calculating average values for

each site in each year, and then taking a weighted average of those two values.

I used weighted MANOVA to test for overall differences in habitat characteristics

among post-harvest treatments and between nest and random locations. Habitat variables

were then tested individually using split-plot analysis of variance (ANOVA). The residuals

for each habitat variable were examined, and as a result I square root transformed canopy

cover and log-transformed regenerative growth (log10 + 0.05). Tukey’s Studentized post-hoc

range test was performed on the results of split-plot ANOVAs that identified significant

among treatment differences.

I used a similar procedure to test for differences between habitat characteristics at

points used by fledglings and random locations, but separate comparisons were made for

habitat use during the first, second, and third weeks post-fledging. To obtain normality and

homoscedasticity I log-transformed regenerative growth (log10 + 0.05), and square-root

transformed sapling, understory, and canopy cover.

Do habitat features differ between nest and fledgling plots?

I used weighted MANOVA to test for overall differences in habitat characteristics at

nest sites and locations used by fledglings. Habitat variables were then tested individually

using split-plot ANOVAs, and Tukey’s Studentized post-hoc range tests were performed on

variables that significantly differed among treatments.

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Nest and Fledgling Survival

Nest and fledgling survival were determined for each post-harvest treatment using the

Mayfield estimator (Mayfield 1961, 1975). Daily survival rate (DSR) for nests was

calculated as 1 – no. of failed nests / no. of exposure days, where exposure days were equal

to the number of days the nest was under observation. Period survival rate (PSR) was

calculated as (DSR)d, where d represented the number of days between the onset of

incubation and fledging (24 days for Rose-breasted Grosbeaks). DSR estimates the

probability that a nest will survive one day, and PSR estimates the probability that a nest will

survive until the young fledge. Similar estimates were calculated for fledglings, but for DSR

the number of fledglings that died was substituted for the number of failed nests, and for

PSR, d represented the 21 day observation period for the fledglings.

Are estimates of nest and fledgling survival correlated among harvest treatments?

To test whether single-tree selection harvesting had a similar influence on nest and

fledgling survival over the course of the harvesting rotation, I calculated a Spearman rank

correlation coefficient for nest and fledgling survival.

How do habitat characteristics influence nest and fledgling survival?

I constructed a set of 42 models containing 1-3 variables to explain nest survival and

a set of 75 models containing 1-4 variables to explain fledgling survival (Appendix 2.1).

Each model represented a hypothesis to explain nest and fledgling survival based on

literature reviews, significant differences in vegetation between points occupied by nests or

fledglings and random locations, and my own observation of the study species. The

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following variables were included in the set of candidate models explaining nest and

fledgling survival:

1) REGENERATIVE GROWTH – In this study, percent cover from regenerative

growth at Rose-breasted Grosbeak nest locations was significantly higher than at

random locations. Minimum nest height was 1.5 m, suggesting that regenerating

vegetation (0.5 – 1.3 m tall) may help conceal nests from potential predators on the

ground. Similarly, this vegetation layer can provide cover for fledglings, especially

during their first week out of the nest when they are learning to fly (King et al. 2006).

Percent cover from regenerative growth was included in models for both nest and

fledgling survival.

2) SAPLING COVER – In forest fragments percent cover by saplings was positively

associated with Rose-breasted Grosbeak nest survival, even though nests were placed

in areas with fewer saplings than were present at random locations (Smith et al.

2007). In my study, Rose-breasted Grosbeaks used nest sites with significantly

higher sapling cover than was present at random locations, suggesting this variable

may be important to survival. I expected saplings to provide cover for fledglings

during the first and second weeks out of the nest when movement is limited and

mortality is high (i.e., Yackel Adams et al. 2006, Moore et al. 2010). Percent cover

by saplings was included in models explaining both nest and fledgling survival.

3) UNDERSTORY COVER – Understory cover was significantly higher at Rose-

breasted Grosbeak nest locations than at random points in my study suggesting that

the understory layer was important for nest placement and camouflage. Structure and

amount of understory vegetation have also been related to fledgling survival (King et

60

al. 2006, Mitchell et al. 2010). I included percent cover by understory in models of

nest and fledgling survival.

4) CANOPY COVER – In forest fragments Rose-breasted Grosbeaks selected nest sites

with lower canopy cover than was present at random locations, even though this

variable was positively associated with nest survival (Smith et al. 2007). In my study,

mean nest height for Rose-breasted Grosbeaks was 9.6 m, suggesting that the sub-

canopy and canopy layers would be important for nest placement, concealment, and

survival. Although canopy cover has not been directly related to fledgling survival to

my knowledge, juveniles were frequently observed feeding in the canopy once they

could fly, so I expected this variable to be important for fledgling survival as well,

and included it in models explaining both nest and fledgling survival.

5) BASAL AREA – Basal area was lower around Rose-breasted Grosbeak nests than at

random locations in forest fragments (Smith et al. 2007) and in my study. Although

the connection between fledgling survival and basal area has not been previously

established to my knowledge, basal area was included in models of both nest and

fledgling survival to determine whether habitat requirements differ before and after

fledging.

6) POST-HARVEST TREATMENT – Neither nest survival (Smith et al. 2006) nor

fledgling survival (Moore et al. 2010) of Rose-breasted Grosbeaks were significantly

influenced by selection harvesting in forest fragments. Harvest treatment was

included in the models for both nest and fledgling survival to test whether Rose-

breasted Grosbeaks responded similarly to selection cutting in a predominantly

forested landscape. Models including each variable, treatment, and an interaction

term were also tested for both nest and fledgling survival.

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7) CONCEALMENT – For some species, nests that are more camouflaged from

potential nest predators may be more likely to succeed (Martin and Roper 1988,

Filliater et al. 1994, Colombelli-Négrel and Kleindorfer 2009). In forest fragments

Rose-breasted Grosbeak nest survival was positively associated with nest

concealment, even though nests were placed in areas with less than average cover

(Smith et al. 2007). Fledglings of several species use areas with greater concealment

from dense vegetation (Anders et al. 1998, Vega Rivera et al. 1998, Marshall et al.

2003), likely to avoid predation. Percent concealment was included in models of nest

and fledgling survival.

8) HEIGHT – For Rose-breasted Grosbeaks in forest fragments in southern Ontario nest

height is positively associated with nest survival (Smith et al. 2007), and these two

variables are correlated in several other species as well (Burhans et al. 2002, Prokop

2004, Colombelli-Negrel and Kleindorfer 2009). Although I am unaware of any

studies linking fledgling height to survival, I expected that as fledglings learned to fly

and moved up into the canopy their chances of survival would increase. Height was

included in models explaining nest and fledgling survival.

9) AGE OF MALE PARENT – First time breeders of some birds have lower nest

survival than older individuals (Pärt 2001, Forschler and Kalko 2006), as well has

producing smaller clutch sizes (Reynolds 1997) and fewer fledglings (Préault et al.

2005, Brown and Roth 2009). Age of male parent (second-year or after-second-year)

was included in models of nest survival, but since only two of the fledglings I tracked

came from nests with second-year males it was not included in models explaining

fledgling survival.

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10) DATE – Nests initiated earlier in the breeding season can have higher survival rates

than nests initiated later on (i.e., Spanhove et al. 2009), so nest initiation date was

included in models for nest survival. Date of hatching (Monrós et al. 2002) and date

of fledging (Yackel Adams et al. 2006, Middleton and Green 2008, Moore et al.

2010) can be correlated with fledgling survival, so date of fledging was included in

models explaining fledgling survival.

11) DAYS SINCE FLEDGING – For many songbird species, fledgling mortality is

highest during the first 1-7 days out of the nest, when fledglings are not yet competent

fliers (Sullivan 1989, Anders et al. 1997, Naef-Daenzer et al. 2001, Cohen and

Lindell 2004, Kershner et al. 2004, King et al. 2006, Yackel Adams et al. 2006,

Moore et al. 2010). Time since fledging was included in models for fledgling

survival.

12) YEAR –There is often yearly variation in survival estimates, so year was included in

models for both nest and fledgling survival.

13) TIME SINCE HATCHING – The time between hatching and fledging ranges from 9-

12 days for Rose-breasted Grosbeaks (Wyatt and Francis 2002). If older nestlings are

larger and more fully developed at the time of fledging than younger ones, they could

be expected to have higher post-fledging survival, especially if their feathers and

wing muscles are more fully developed. Time since hatching was included in models

of fledgling survival.

14) WEIGHT AT FLEDGING – The body condition of nestlings at the time of fledging

can influence their ability to find suitable habitats for foraging and predator

avoidance, and nestlings that are in poorer condition may have lower chances of

survival (Magrath 1991, Naef-Daenzer et al. 2001, Vitz and Rodewald 2010).

63

Nestling weight at the time of fledging was included as a measure of body condition

in models explaining fledgling survival.

15) RASPBERRY COVER – Although raspberry bushes did not produce ripe fruit during

the three week period when fledglings were tracked, dense patches of raspberries

could have provided camouflage and protection for fledglings before they were

proficient fliers. Percent cover by raspberries was included in models explaining

fledgling survival.

16) ELDERBERRY COVER – Rose-breasted Grosbeaks have been observed feeding red

elderberries (Sambucus racemosa) to their fledglings (Stutchbury et al. 2005), and

elderberries appeared to be an important food source for fledglings in my study.

Percent cover by elderberry plants was included as a rough estimate of elderberry

abundance in models explaining fledgling survival.

17) SHRUB COVER – Survival of Ovenbird (Seiurus aurocapilla) (King et al. 2006),

Hooded Warbler (Wilsonia citrina) (Rush and Stutchbury 2008), and Rose-breasted

Grosbeak (Moore et al. 2010) fledglings was positively associated with the amount of

shrub cover, likely because small shrubs provide shelter from potential predators.

Percent cover by small (< 2.5 cm dbh) shrubs was included as an explanatory variable

in models of fledgling survival.

Shaffer’s logistic-exposure method (2004) was used to compare the set of models

containing variables explaining nest survival. I considered each variable separately (11

models), together with sampling year (10 models), and combined with sampling year and

date of nest initiation (9 models). I also included a set of models containing each variable

separately, post-harvest treatment, and their interaction term (10 models). A null model

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(containing the intercept only) and a global model (containing all the variables under

consideration) were also included in my set of 42 candidate models containing 1-3 variables

explaining nest survival. Shaffer’s logistic-exposure method uses Akaike’s Information

Criteria (corrected for small sample sizes: AICc) (Burnham and Anderson 2002) to compare

the relative strength of competing models and was implemented in the PROC GENMOD

module of SAS (v. 8.02).

Fledgling survival was analyzed using the nest survival model in Program MARK (v.

6.0) (White and Burnham 1999, Schmidt et al. 2008). For each fledgling only one record

was included, which corresponded to the last time that individual had been observed alive or

dead. This approach to modeling survival assumes that habitat quality determines survival,

and reduces bias resulting from decreasing sample sizes with time. Individual records were

coded with the date of fledging, the last day the bird was known to be alive, the last day the

fledgling was tracked, the fledgling’s fate (0 = survived, 1 = died), and 16 explanatory

variables. I considered each variable separately (16 models), together with sampling year (15

models), combined with sampling year and time since fledging (14 models), and in models

containing sampling year, date of fledging, and time since fledging (13 models). I also

included models containing each variable separately, post-harvest treatment, and their

interaction term (15 models), as well as a null model and global model in the set of 75

candidate models containing 1-4 variables explaining fledgling survival.

RESULTS

Do habitat features differ between used and un-used plots?

Nest Sites. – Habitat characteristics were measured at 94 Rose-breasted Grosbeak

nests and at 94 corresponding random plots. Multivariate analysis of variance showed

65

significant differences in vegetation characteristics among post-harvest treatments (F15, 149.47 =

4.54, P < 0.0001) and between nest and random plots (F5, 54 = 7.56, P < 0.0001) but the

interaction between treatment and plot type was not significant (F15, 149.47 = 0.83, P = 0.642).

Regenerative growth was significantly higher at Rose-breasted Grosbeak nest sites

(18.7 ± 2.8%) than at random locations (12.8 ± 1%; F1,3 = 6.37, P = 0.014; Fig. 2.1a).

Sapling cover was higher at nest sites (17.7 ± 1.5%) than at random locations (12.8 ± 1.8%),

except in the > 50 years post-harvest stands, but differences were not significant (F1,3 = 2.71,

P = 0.105; Fig. 2.1b). Understory cover was also significantly higher at nest sites (41.7 ±

2.9%) than at random locations (26.7 ± 2.0%; F1,3 = 18.60, P < 0.0001; Fig. 2.1c). In

contrast, Rose-breasted Grosbeaks used nest sites with significantly lower basal area (21.7 ±

0.6 m2/ha) than was available (25.6 ± 0.7 m2/ha; F1,3 = 21.21, P < 0.0001; Fig. 2.1d). Canopy

cover was higher at random locations (90.9 ± 5.0%) than at nest sites (70.0 ± 10.2%) in 2006

(F1,3 = 9.92, P = 0.010), but differences were not significant in 2007 (F1,3 = 0.35, P = 0.565)

or 2008 (F1,3 = 1.62, P = 0.221).

Nest height was higher on average in the > 50 years post-harvest treatment (mean =

11.08 m) than in the other stands (0-5: 8.60 m, 16-20: 9.19 m, 21-25: 9.89 m), but differences

were not significant (F1,3 = 1.31, P = 0.27). Low (0-4 m) nests accounted for 21.1% of the

nests in my sample, and the highest number of these nests (16) was found in the 0-5 years

post-harvest treatment (Table 2.1). Nests of medium height (4.5-9 m) comprised 33.3% of

the nests I located, and occurred in approximately equal proportions in all post-harvest

treatments except the > 50 years post-harvest stands, which had a lower proportion (18.8%).

Nests in the sub-canopy (10-20 m) and canopy (> 20 m) layers accounted for 45.5% of the

nests I monitored. High nests occurred in approximately equal proportions in all treatments

except the 0-5 years post-harvest treatment which had fewer (35.1%). The > 50 years post-

66

harvest treatment had the highest proportion of very high nests (9.4%). In general, in the 0-5

years post-harvest stands 63.1% of the nests were located below 9 m, whereas in the 16-20,

21-25, and > 50 years post-harvest treatments the proportions were 56.8, 46.8, and 46.9%

respectively (Table 2.1).

Plots in the 0-5 years post-harvest stands had significantly lower sapling (F1,3 = 5.39,

P = 0.002) and understory (F1,3 = 4.48, P = 0.007) cover and lower basal area (F1,3 = 8.57, P

< 0.001) than plots in the other post-harvest treatments (Fig. 2.1b-d). In contrast, canopy

openings created by the removal of trees resulted in significantly higher cover from

regenerative growth (F1,3 = 8.60, P < 0.001) in the 0-5 post-harvest stands than in the other

post-harvest treatments (Fig. 2.1a). By 16-20 years after harvesting, regenerative growth had

developed into a dense understory layer, giving these stands significantly lower regenerative

growth (F1,3 = 8.60, P < 0.001) and higher understory (F1,3 = 4.48, P = 0.007) cover than the

other post-harvest treatments (Fig. 2.1a, c). Habitat features 21-25 years after harvesting

were not significantly different from those in the 16-20 and > 50 years post-harvest stands.

Nest and random plots in the > 50 years post-harvest stands had significantly higher basal

area (F1,3 = 8.57, P < 0.001) than those in the 16-20 and 21-25 years post-harvest stands, and

regenerative growth (F1,3 = 8.60, P < 0.001) and sapling cover (F1,3 = 5.39, P = 0.002) were

also high (Fig. 2.1a,b,d).

Fledgling Locations. – Habitat characteristics were measured at 85 fledgling plots and

85 random locations (n = 44, 22, and 19 in weeks 1, 2, and 3 post-fledging respectively).

Weighted multivariate analysis of variance showed significant differences in vegetation

characteristics among post-harvest treatments in week one post-fledging (Week 1: F18, 201.3 =

2.95, P = 0.0001; Week 2: F18, 88.17 = 1.60, P =0.070; Week 3: F15, 61.1 = 1.40, P = 0.175) and

between fledgling and random plots in week one post-fledging (Week 1: F6, 71 = 2.09, P =

67

0.050; Week 2: F6, 31 = 0.490, P = 0.808; Week 3: F5, 22 = 1.57, P = 0.209). Interactions

between treatment and plot type were not significant in any week post-fledging (Week 1: F18,

201.3 = 0.43, P = 0.981; Week 2: F18, 88.167 = 1.38, P = 0.160; Week 3: F15, 61.13 = 0.59, P = 0.872).

During the first week post-fledging, regenerative growth was significantly higher at

locations used by Rose-breasted Grosbeak fledglings (17.4 ± 2.9%) than at random plots (9.2

± 2.8%; F1,3 = 4.78, P = 0.032; Fig. 2.2a). Percent cover by saplings was also significantly

higher at fledgling locations (17.4 ± 2.8%) than at random plots (10.7 ± 1.6%; F1,3 = 4.17, P

= 0.045; Fig. 2.2b). During their first week out of the nest, fledglings used locations with

more abundant small shrubs (11.3 ± 3.8%) than were available at random locations (3.8 ±

0.9%; F1,3 = 4.88, P = 0.030; Fig. 2.2d). Raspberry and elderberry cover were also more

abundant at fledgling locations (8.1 ± 3.7% and 1.3 ± 0.6% respectively) than at random

points (0.8 ± 0.4% and 0.1 ± 0.08% respectively) during the first week post-fledging

(raspberry: F1,3 = 4.13, P = 0.045; elderberry: F1,3 = 4.11, P = 0.042; Fig. 2.2e,f).

During the second and third weeks out of the nest, habitat use by fledglings was

highly variable and most vegetation measurements did not significantly differ between used

and un-used plots. However, during both the second and third weeks out of the nest

fledglings used locations with significantly lower basal area (22.5 ± 1.6 m2/ha and 21.2 ± 1.6

m2/ha respectively) than was present at random locations (23.9 ± 1.3 m2/ha and 26.2 ± 1.7

m2/ha respectively; F1,3 = 2.86, P = 0.020 and F1,3 = 3.65, P = 0.014 respectively).

Fledgling and random plots in the 0-5 years post-harvest stands had significantly

lower sapling cover (F1,3 = 3.84, P = 0.013) and basal area (F1,3 = 8.47, P < 0.0001) than

other post-harvest treatments, and significantly higher cover from small shrubs (F1,3 = 4.65, P

= 0.005) and raspberry bushes (F1,3 = 6.38, P = 0.0007) than were present in all other stands

68

0

5

10

1520

25

30

35

Nest Random

Plot type

Reg

ener

ativ

e co

ver (

%)

0

5

10

1520

25

30

35

Nest Random

Plot type

Sap

ling

cove

r (%

)

0

10

20

30

40

50

60

Nest Random

Plot type

Und

erst

ory

cove

r (%

)

0-516-2021-2550

0

5

10

15

20

25

30

35

Nest Random

Plot type

Bas

al a

rea

( m s

q / h

a)

Figure 2.1. (A) Regeneration cover, (B) sapling cover, (C) understory cover, (D) basal area at nest and random plots in 12 hardwood stands in Algonquin Provincial Park harvested by single-tree selection 0-5, 16-20, 21-25, and > 50 years previously. Means and standard errors are shown.

A)

B)

C)

D)

69

TABLE 2.1. Number of Rose-breasted Grosbeak nests located in four height categories (low, medium, high, and very high) in 12 stands harvested by single-tree selection 0-5, 16-20, 21-25, and > 50 years prior to sampling. Number of nests in each category is shown with row percentages in brackets. Row totals represent the number of nests in each post-harvest treatment and column totals represent the number of nests in each height category.

Nest height (m)Time since harvest

Low(0-4.0 m)

Medium(4.5 – 9 m)

High(10-20 m)

Very High(> 20 m)

Row Total

0-5 16 (28.1) 20 (35.1) 20 (35.1) 1 (1.8) 57 (100.0)16-20 5 (11.4) 20 (45.5) 19 (43.2) 0 (0.0) 44 (100.0)21-25 8 (17.0) 14 (29.8) 23 (48.9) 2 (4.3) 47 (100.0)> 50 9 (28.1) 6 (18.8) 14 (43.8) 3 (9.4) 32 (100.0)Column Total 38 (21.1) 60 (33.3) 76 (42.2) 6 (3.3) 180 (100.0)

70

A)

0

10

20

30

40

50

Random 1-7 days sincefledging

8-14 15-21

Time since fledging (days)

Reg

ener

ativ

e co

ver (

%)

D)

0

10

20

30

40

50

Random 1-7 days post-fledging

8-14 15-21

Time since fledging (days)

Smal

l shr

ub c

over

(%)

B)

0102030405060

Random 1-7 days sincefledging

8-14 15-21

Time since fledging (days)

Sapli

ng c

over

(%)

E)

0

10

20

30

40

50

Random 1-7 8-14 15-21

Time since fledging (days)

Rasp

berry

cov

er (%

)

0-516-2021-25> 50

C)

05

101520253035

Random 1-7 8-14 15-21

Time since fledging (days)

Basa

l are

a (m

/ha)

F)

0

2

4

6

8

10

Random 1-7 8-14 15-21

Time since fledging (days)

Elde

rber

ry c

over

(%)

Figure 2.2. (A) Regenerative cover, (B) sapling cover, (C) basal area, (D) small shrub (< 2.5 cm dbh), (E) raspberry cover, and (F) elderberry cover at locations occupied by fledglings and random plots in 12 hardwood stands in Algonquin Provincial Park harvested by single-tree selection 0-5, 16-20, 21-25 and > 50 years previously. Means and standard errors are shown.

71

(Fig. 2.2b-e). Habitat characteristics in the 16-20 and 21-25 years post-harvest stands were

generally intermediate between the 0-5 and > 50 years post-harvest stands, but sapling cover

was significantly higher (F1,3 = 3.84, P = 0.013) in the 16-20 years post-harvest stands than in

the 0-5 years post-harvest treatment (Fig. 2.2b). The > 50 years post-harvest treatment had

higher sapling (F1,3 = 3.84, P = 0.013) and understory cover (F1,3 = 3.18, P = 0.029) and

higher basal area (F1,3 = 8.47, P < 0.0001) than other stands.

Do habitat features differ between nest and fledgling plots?

Weighted multivariate analysis of variance indicated that habitat characteristics at

nest sites and fledgling locations differed significantly among post-harvest treatments (F15,

367.56 = 1.91, P < 0.021), and between nest and fledgling plots (F15, 367.56 = 3.06, P = 0.0001).

Interactions between treatment and plot type were not significant (F45,598.04 = 1.02, P = 0.445).

Individual split-plot ANOVAs indicated that nest plots had significantly higher

percent cover from understory than was present at locations used by fledglings during their

first (F3,3 = 4.89, P = 0.003) and second (F3,3 = 3.89, P = 0.005) weeks out of the nest (Fig.

2.3b). Nest plots also had significantly lower basal area than was present at locations used by

fledglings during their first week out of the nest (F3,3 = 9.18, P < 0.0001) (Fig. 2.3c).

Nest and fledgling plots in 0-5 years post-harvest stands had significantly higher

percent cover from regenerative growth than similar plots in the 16-20 and 21-25 years post-

harvest stands (F3,3 = 4.90, P = 0.003) (Fig. 2.3a). Nest and fledgling plots in 0-5 years post-

harvest stands also had significantly lower basal area than similar plots in 21-25 and > 50

years post-harvest stands (F3,3 = 9.18, P < 0.0001) (Fig. 2.3c).

72

A)

0

10

20

30

40

50

Nest 1-7 days 8-14 days 15-21 days

Plot Type

Rege

nera

tion

cove

r (%

)

B)

0

10

20

30

40

50

60

70

Nest 1-7 days 8-14 days 15-21 days

Plot Type

Unde

rsto

ry c

over

(%)

0-5

16-20

21-25

> 50

C)

0

10

20

30

40

Nest 1-7 days 8-14 days 15-21 days

Plot Type

Bas

al a

rea

(m s

q/ha

)

FIGURE 2.3. (A) Regeneration cover, (B) understory cover, and (C) basal area at nest sites and locations occupied by fledglings 1-7, 8-14, and 15-21 days post-fledging in 12 hardwood stands harvested by single-tree selection 0-5, 16-20, 21-25, and > 50 years previously. Means and standard errors are shown.

73

Nest Success and Fledgling Survival

Nest Success. – A total of 173 Rose-breasted Grosbeak nests were monitored over

three years (20, 73, and 80 nests respectively in the years 2006-2008), 97 (56.0%) of which

were successful. Of the 73 nests that failed, 32 (43.8%) failed during the nestling stage, 22

(30.1%) failed during the egg stage, and 19 (26%) failed either very late in the egg stage or

very early in the nestling stage. I also observed 25 nest building attempts that were

abandoned before eggs were laid. During both the egg and nestling stages, the primary cause

of nest failure was predation (68 of the 73 nests), presumably by small mammals or avian

nest predators. The secondary cause of nest failure was nest abandonment during the egg

stage (5 of the 73 nests), for unknown reasons.

Fledgling Survival. – Over three years 46 fledglings were radio-tagged of which 18

(38%) survived the three week-tracking period, 17 (40%) were confirmed dead, and 11

(23%) were lost during the tracking period, presumably due to malfunctioning radio-tags (n =

10) or harness failure (n = 1). The 10 cases where radio-tag failure was assumed all occurred

during a two week period in 2008 when signals were lost for ten individuals that were 3-6

days post-fledging and would not have been capable of flying large distances. In all years,

fledgling mortality was highest during the first week (1-7 days) post-fledging, with 12

confirmed deaths (67%) occurring during this interval. Fledgling survival to independence

was similar between 2006 (67%; n = 3, 1 confirmed death) and 2007 (65%; n = 23, 8

confirmed deaths total). Unfortunately, in 2008 only 2 fledglings were tracked for the entire

21 day period (10 were lost as a result of malfunctioning radio-tags, 9 were confirmed dead).

The primary causes of fledgling mortality appeared to be predation by hawks and

mammals. Four transmitters were recovered amid piles of feathers on top of elevated

74

perches, presumably having been left by raptors (Moore et al. 2010). One transmitter was

found on the ground directly below an active Red-tailed Hawk’s (Buteo jamaicensis) nest,

and the transmitter from another fledgling ended up in a Broad-winged Hawk’s (Buteo

platypterus) nest. One transmitter was found in the scat of a mammal, and the remains of 5

fledglings were found buried approximately 6 cm below the leaf litter, having been partially

consumed presumably by a mammal. The five remaining transmitters showed signs of

predation (partial remains of fledglings, or heavily chewed transmitter harnesses), but

predator identity was unclear.

Are estimates of nest and fledgling survival correlated among post-harvest treatments?

Mean Mayfield daily survival rate (DSR) for Rose-breasted Grosbeak nests during

the egg and nestling stages was 0.967 (n = 173 nests) and the mean period survival rate

(PSR) for the entire 24 day nesting period was 0.559. The model containing post-harvest

treatment did not have any support as an explanatory variable for nest survival (AICc weight

= 0.00), indicating that single-tree selection does not strongly influence nest success. When

nest survival estimates for each post-harvest treatment were combined across study years,

nest survival was highest in the 21-25 years post-harvest stands and lowest in the > 50 years

post-harvest treatment (Fig. 2.4).

Mean daily survival rate for Rose-breasted Grosbeak fledglings was 0.920 (n = 47),

and the probability of a fledgling surviving the entire 21 day tracking period was 0.510. The

model containing post-harvest treatment had little support as an explanatory variable for

fledgling survival (AICc weight = 0.01), suggesting that fledgling survival is not strongly

influenced by single-tree selection harvesting. Like nest survival, fledgling survival was

75

0

0.10.2

0.3

0.40.5

0.6

0.70.8

0.9

0-5 16-20 21-25 >50

Tim e s ince harvest (years )

May

field

per

iod

surv

ival

NestFledgling

FIGURE 2.4. Mayfield estimates of the probability of Rose-breasted Grosbeak nests surviving to fledge young and fledglings surviving their first three weeks out of the nest in 12 stands in Algonquin Provincial Park harvested by single-tree selection 0-5, 16-20, 21-25, and > 50 years previously. Treatment means calculated from weighted site means and standard error are shown. Samples sizes (n) are shown for the number of nests and fledglings respectively that were monitored in each treatment during 2007 and 2008.

n = 55, 8

n = 34, 8

n = 38, 16

n = 29, 8

76

lowest in the > 50 years post-harvest stands, but it was highest in the 16-20 years post-

harvest treatment (Fig. 2.4).

The Spearman rank correlation coefficient indicated that nest and fledgling survival

were not significantly correlated in harvested stands (r = 0.28, P > 0.05; n = 12).

How do habitat characteristics influence nest and fledgling survival?

Nest Survival. – The model with the best support for explaining variation in daily nest

survival included nest initiation date, sampling year, and basal area (Table 2.2). The top nine

of forty-two models tested included nest initiation date and sampling year, with weights

ranging from 0.313-0.001. The model averaged estimate (regression coefficient averaged

across models in the candidate set) for basal area was positive, indicating that nests were

more likely to survive in areas with greater basal area (Table 2.2). Nest initiation date was

negatively associated with nest survival, indicating that earlier nests had a higher probability

of survival.

Fledgling Survival. – The model with the strongest support for explaining variation in

fledgling survival included number of days since fledging (Table 2.3). The model containing

days since fledging and year also had some support, and these two variables appeared in the

top nineteen of seventy-five models, with weights ranging from 0.280-0.020. The model

averaged estimate for days since fledging was positive, suggesting that older fledglings had a

higher probability of continued survival.

77

TABLE 2.2. Support for models predicting daily nest survival rates of Rose-breasted Grosbeaks in Algonquin Provincial Park (2006-2008) as a function of percent cover of regenerative growth, saplings, and understory, basal area (m2/ha), time-since harvest (0-5, 16-20, 21-25, or >50 years), nest concealment (%), nest height (m) and nest initiation date. Shown are model averaged parameter estimates, unconditional standard error, number of model parameters (k), deviance, Akaike’s Information Criteria corrected for small sample sizes (AICc), Δi (AICc modeli – AICc model minimum), and Akaike model weights for models that fall within 2 AICc units of the best fit model. Statistics for the global model (including all variables) and the null model (intercept only) are also shown.

Model Predictors Model Averaged Estimate

Unconditional SE

K Deviance AICc Δi Wi

1 Basal areaDateYear (2006) (2007) (2008)

0.0088-0.10561.3371-0.39590.0000

0.01400.01350.58750.24280.0000

5 526.334 536.355 0.000 0.313

2 Date Year (2006) (2007) (2008)

-0.10561.3371-0.39590.0000

0.01350.58750.24280.0000

4 529.186 537.200 0.845 0.205

9 Global - - 13 520. 905 547.030 10.675 0.00110 Null (intercept only) - - 1 600.223 602.224 65.870 0.000

78

TABLE 2.3. Support for models predicting daily survival rates of Rose-breasted Grosbeak fledglings in Algonquin Provincial Park 1-21 days after fledging (2006-2008). Predictor variables include percent cover of regenerative growth, saplings, understory, canopy, small shrubs, raspberries, and elderberries, basal area (m2/ha), time since harvest, concealment (%), height (m), weight (g), fledging date, and days since fledging, days since hatching, and sampling year. Shown are model averaged parameter estimates, unconditional standard error, number of model parameters (k), deviance, Akaike’s Information Criteria corrected for small sample sizes (AICc), Δi

(AICc modeli – AICc model minimum) and Akaike model weights for models that fall within 2 AICc units of the best fit model. Statistics for the global model (including all variables), the null model (intercept only), and the model with the most support containing post-harvesting treatment are also shown. Model Predictors Model Averaged

EstimateUnconditional

SEK Deviance AICc Δi Wi

1 Days since fledging 0.555 0.286 2 22.779 24.789 0.000 0.2802 Days since fledging

Year0.555-0.003

0.2860.003

3 22.288 26.318 1.529 0.130

7 Post-harvest treatmentYearFledging dateDays since fledging

-0.677-0.0030.1290.555

0.5660.0030.1920.286

5 20.124 28.225 3.435 0.050

20 Global - - 17 38.545 39.933 5.749 0.01122 Null (intercept only) - - 1 39.339 41.346 16.557 0.000

79

DISCUSSION

Habitat characteristics at Rose-breasted Grosbeak nest sites and locations used by

fledglings differed significantly among stands harvested by single-tree selection 0-5, 16-20,

21-25, and > 50 years post-harvest. As expected, nest and fledgling plots in the most

recently harvested stands had lower basal area, lower sapling, understory and canopy cover,

and more shrubs, raspberry, and elderberry bushes than plots in other post-harvest treatments.

In general, nest and fledgling locations in the 16-20 and 21-25 years post-harvest treatments

had lower cover from regenerative growth and higher understory cover than plots in other

treatments. In the > 50 years post-harvest treatment basal area was higher than in other

stands, and cover from regenerative growth, saplings, and canopy were also high. Despite

these differences, which are generally consistent with structural changes reported in other

long-term studies of single-tree selection (Crow et al. 2002, Kenefic and Nyland 2000),

neither nest success nor fledgling survival significantly differed among post-harvest

treatments. Rose-breasted Grosbeaks also exhibited plasticity in nest site selection without

showing differences in survival rates in managed forest fragments in southern Ontario (Smith

et al. 2007), suggesting that single-tree selection harvesting likely does not have a significant

negative effect on this species.

Previous research has suggested that single-tree selection may provide suitable habitat

for many forest songbirds, and may have only limited effects on their reproductive success

(Barber et al. 2001, Duguay et al. 2001, Lang et al. 2002, Smith et al. 2006, Poulin et al.

2010). This appears true for Rose-breasted Grosbeaks as well, who showed no significant

differences in nest success (Smith et al. 2006, 2007, Moore et al. 2010) or fledgling survival

(Moore et al. 2010) in managed stands in the highly fragmented agricultural landscape of

80

southern Ontario. Although single-tree selection harvesting does not appear to negatively

influence Rose-breasted Grosbeak survival in either fragmented or predominantly forested

landscapes, there were differences in survival rates among the two landscape types.

Estimates of nest survival, which ranged from 0.341 (Moore et al. 2010) to 0.390 (Smith et

al. 2006, 2007) in the fragmented landscape, were considerably lower than my estimate

(0.559) in the predominantly forested landscape. In contrast, fledgling survival was slightly

higher (0.620) in the fragmented landscape (Moore et al. 2010) than in the predominantly

forested one (0.510). The net effect of these measures suggests that overall survival is lower

in the fragmented landscape, likely due to higher levels of nest predation. The contrasting

patterns of nest and fledgling survival indicate either that Rose-breasted Grosbeaks respond

differently to selective pressures during the nest and fledgling stages, or that nest and

fledgling survival are most strongly influenced by different variables.

Estimates of nest and fledgling survival in my study were not significantly correlated

among post-harvest treatments, supporting the idea that Rose-breasted Grosbeak responses to

environmental factors differ between reproductive phases. Seventy percent of the fledgling

deaths I observed were attributed to predation by hawks and small mammals, and 67% of

these deaths occurred in the 21-25 and > 50 years post-harvest sites. Raptors and chipmunks

have previously been identified as common predators of Wood Thrush (Schmidt et al. 2008)

and Rose-breasted Grosbeak fledglings (Moore et al. 2010). One study found that partial

harvesting in hardwood stands had relatively little impact on the density of eastern

chipmunks (Holloway and Malcolm 2006), but this species is positively associated with

many features present in un-logged stands, such as the number of declining trees, the density

of understory cover, and the amount of downed woody debris (Bennet et al. 1994, Mahan and

81

Yahner 1996, Zollner and Crane 2003, Holloway and Malcolm 2006). Mature forest may

also provide more open or better quality habitat for forest-breeding birds of prey such as

Broad-winged Hawks, which nest in tall, large diameter trees and typically remain below a

well developed canopy layer when hunting for insects, amphibians, small mammals, and

birds (Goodrich et al. 1996). Higher predation by raptors and chipmunks on fledglings, but

not necessarily nests, in less recently harvested stands may help explain differences in nest

and fledgling survival rates among post-harvest treatments.

Significant differences also existed between the habitat characteristics present at nest

sites and fledgling locations, which may help explain differences in survival patterns.

Specifically, nest plots had significantly lower basal area and higher percent cover from

understory than were present at locations used by fledglings during their first and second

weeks out of the nest. There was substantial variation associated with the habitat

measurements at locations occupied by fledglings, likely because they moved around in

search of food and shelter. The high degree of movement and variation in habitat use made it

difficult to identify important habitat features for fledglings during their second and third

weeks out of the nest. The fledglings of some species move into habitat types that contrast

significantly with those used by nesting adults (Lang et al. 2002, White et al. 2005), whereas

in other species habitat use during the nesting and post-fledging periods is similar (Berkeley

et al. 2007, Ciudad et al. 2009). Rose-breasted Grosbeak fledglings appeared to use similar

habitats to nesting adults with only a few significant differences, but this could partially be

the result of the relative similarity of habitat types available in the predominantly forested

study landscape.

82

Rose-breasted Grosbeaks tended to use nest sites with lower basal area and higher

cover from regenerative growth, saplings, and understory than was available at random

locations in all post-harvest treatments. In the fragmented woodlots of southern Ontario

Rose-breasted Grosbeaks also used nest sites with lower basal area and higher sapling cover

than was available at random points (Smith et al. 2007). Although grosbeaks consistently

build nests in locations with lower basal area, nest survival was positively associated with

this habitat feature in my study. Previous research has shown that although Rose-breasted

Grosbeak nest survival was positively associated with canopy cover, adults used nest sites

with lower canopy cover than was present at random locations (Smith et al. 2007). Together

these studies suggest that Rose-breasted Grosbeaks do not necessarily select nest sites with

habitat features that maximize reproductive success. Although there is no obvious

explanation for this, Smith et al. (2007) have suggested this species may be experiencing an

ecological trap. Locations with lower basal area and canopy cover may appear attractive

because they are typically associated with other desirable features, such as denser understory

and sapling layers, which may not be present in managed forests during all stages in the

harvest rotation.

The best models for explaining nest success included basal area, nest initiation date,

and sampling year. Nests that were started earlier in the season and built in locations with

higher basal area were more likely to be successful. Higher survival rates for nests initiated

earlier in the season have been reported for several other avian species (e.g. Middleton and

Green 2008, Spanhove et al. 2009). Older, more experienced birds often begin breeding

earlier than less experienced adults, and higher rates of nest and juvenile survival earlier in

the season are often attributed to age related differences in the timing of nest initiation

83

(Hatchwell 1991, Middleton and Green 2008). Older birds may have higher reproductive

success because they are more skilful than younger birds at selecting high quality nest sites,

foraging, defending their offspring, and working together in pairs (Forslund and Part 1995,

Martin 1995, Espie et al. 2000, Balbontin et al. 2007). In the fragmented forests of southern

Ontario Rose-breasted Grosbeak nest survival was best explained by a negative association

with nest concealment, and positive relationships with nest height and canopy and sapling

cover (Smith et al. 2007). Differences in the composition and foraging tactics of predator

communities in the two landscapes may account for the importance of different habitat

features in explaining nest survival in the two landscapes.

Nest survival was also positively associated with basal area, which was significantly

higher in the > 50 years post-harvest treatment than in the more recently harvested stands.

However, despite the positive association between nest survival and basal area, mean nest

success was slightly lower in the > 50 years post-harvest stands than in the other post-harvest

treatments, suggesting that additional factors or more complex relationships among habitat

measures may influence nest survival. For example, in the 16-20 and 21-25 years post-

harvest treatments, which had the highest mean nest survival rates, nest sites had relatively

high understory cover as well as high basal area. In comparison, the > 50 years post-harvest

treatment had the highest basal area but relatively low understory cover, suggesting

understory cover may have been important as well. Although the model containing

understory cover did not fall within 2 units of the best fit model explaining nest survival, it

was ranked third and had an AICc weight of 0.11. Alternatively, higher basal area may be

beneficial up to a certain point, after which nest survival begins to decrease. Although the

exact nature of the relationship between basal area and nest survival may still be uncertain,

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my study suggests that if basal area is further reduced in hardwood stands following a second

selection harvest, the chances of nest survival may also decrease.

In contrast to nesting adults, during their first week out of the nest, fledgling Rose-

breasted Grosbeaks used locations with higher cover from regenerative growth, saplings,

small shrubs, raspberry and elderberry bushes. A similar pattern of habitat use was observed

in Wood Thrush (Hylocichla mustelina) fledglings, which moved into areas with more dense

forest cover relative to the open, mature forest used by adults for nest building (Anders et al.

1998, Vega Rivera et al. 1998). Ovenbird fledglings (Seiurus aurocapilla) also occupied

locations with greater vertical structure 0-3 m above ground than was present at nest or

random plots (King et al. 2006). Due to fledgling movement the habitat characteristics

observed at Rose-breasted Grosbeak fledgling locations were highly variable, but this study

suggests that in predominantly forested landscapes Rose-breasted Grosbeak fledglings use

areas with higher than average cover.

The fledglings of many mature-forest bird species have been observed in regenerating

clear-cuts during the post-fledging period (Pagen et al. 2000, Vitz and Rodewald 2006),

possibly because these areas provide increased cover for thermoregulation, protection from

predators (Weathers and Sullivan 1989, Morton et al. 1991), and abundant food sources

(Anders et al. 1998, Vega Rivera et al. 1998, Vitz and Rodewald 2007). In this study, during

the first week after leaving their nests Rose-breasted Grosbeak fledglings used sites with

higher cover from raspberry bushes than was present at random locations, and although

differences were not significant, fledglings also selected areas with higher than average cover

from elderberry bushes during all three weeks post-fledging. Rose-breasted Grosbeaks have

been videotaped feeding on red elderberries in the wild (Stutchbury et al. 2005), and both

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adults and fledglings were frequently observed feeding extensively on both elderberries and

raspberries in my study area. Availability of food resources has been identified as an

important determinant of fledgling movement patterns during the post-fledging period for

several forest-breeding songbird species (Stutchbury et al. 2005, Vitz and Rodewald 2007,

Ciudad et al. 2009). Movement patterns of Rose-breasted Grosbeak families during the post-

breeding season may be partially determined by the availability of berry bushes, and since

recently harvested stands had higher cover from raspberries, this treatment may represent

preferred habitat during this period.

The best model to explain fledgling survival included the number of days since

fledging, indicating that the probability of survival increased with fledgling age. In forest

fragments, Rose-breasted Grosbeak fledgling survival was best explained by positive

associations with date of fledging, patch size, and percent shrub cover (Moore et al. 2010).

Many studies of forest songbirds have reported higher fledgling mortality during the first 1-7

days out of the nest (Sullivan 1989, Anders et al. 1997, Naef-Daenzer et al. 2001, Cohen and

Lindell 2004, Kershner et al. 2004, King et al. 2006, Yackel Adams et al. 2006, Moore et al.

2010) when fledglings are unable to fly and capable of only limited movement. Other factors

which have been commonly associated with fledgling survival prior to independence include

habitat type (Krementz et al. 1989, Anders et al. 1997, Vega Rivera et al. 1998), amount of

coarse woody debris, amount and structure of understory cover (Mitchell et al. 2010), age of

parents (Rush and Stutchbury 2008), hatching date and fledgling mass (Monrós et al. 2002),

date of fledging, and weather (Yackel Adams 2006). Although age at fledging is variable for

Rose-breasted Grosbeaks (9-12 days post-hatching), and the model containing days since

hatching was ranked as the third best-fit model explaining fledgling survival, the model

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containing days since fledging had more support. The results of my study are consistent with

the growing body of evidence that indicates that fledgling survival increases considerably

with age, and it indicates that for Rose-breasted Grosbeaks the relationship between survival

and age may be more strongly influenced by days since fledging than the length of time spent

in the nest.

Overall, single-tree selection harvesting did not appear to have a significant negative

effect on Rose-breasted Grosbeak nest or fledgling survival. However, since fewer nests and

fledglings were monitored in the > 50 years post-harvest stands, sample sizes were smaller in

this treatment than in the others. Differences in samples sizes were addressed using weighted

analyses, but it is possible they influenced among-treatment comparisons. Incorporating a

larger number of unharvested stands into future studies so as to increase sample sizes in this

treatment may help provide insight into the relationship between basal area and nest survival.

In addition, a relatively large set of candidate models was ranked to explain the survival rates

of a relatively small number of fledglings. Increasing the number of fledglings tracked might

be one way for future studies to increase the power of the tests and confirm that single-tree

selection harvesting does not negatively influence nest or fledgling survival.

Conclusions

Despite significant differences in habitat features at nest sites and fledgling locations

in stands harvested 0-5, 16-20, 21-25, and > 50 years previously, single-tree selection did not

appear to negatively influence Rose-breasted Grosbeak nest or fledgling survival.

Differences in habitat characteristics between nest sites and locations used by fledglings

indicate that creating a mosaic of forest stands in different stages of the harvest rotation

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might provide Rose-breasted Grosbeaks with access to better quality habitats during all

stages in the breeding cycle. Un-even aged silvicultural techniques applied over relatively

small areas likely provide higher quality habitat overall for Rose-breasted Grosbeaks than

even-aged practices applied over larger areas, because they provide easier access to habitats

in different stages of regeneration. Differences in nest and fledgling survival among post-

harvest treatments also suggest that future studies attempting to assess the impact of forest

management techniques or other habitat alterations on avian populations should consider

both nest and fledgling requirements.

APPENDIX 2.1. List of candidate models used to explain Rose-breasted Grosbeak nest and fledgling survival in twelve tolerant hardwood stands in Algonquin Provincial Park harvested by single-tree selection 0-5, 16-20, 21-25, and > 50 years previously.Model

no.Predictor variables included Plots for which

model was usedModels with one variable

1 Regenerative growth Nest and fledgling2 Sapling cover Nest and fledgling3 Understory cover Nest and fledgling4 Canopy cover Nest and fledgling5 Basal area Nest and fledgling6 Post-harvest treatment Nest and fledgling7 Concealment Nest and fledgling8 Height (nest, fledgling) Nest and fledgling9 Date (nest initiation, fledging) Nest and fledgling10 Year Nest and fledgling11 Age of male parent Nest12 Small shrub cover Fledgling13 Raspberry cover Fledgling14 Elderberry cover Fledgling15 Days since fledging Fledgling16 Fledgling weight Fledgling17 Time since hatching Fledgling

Models with two variables 18 Regenerative growth + year Nest and fledgling19 Sapling cover + year Nest and fledgling20 Understory cover + year Nest and fledgling21 Canopy cover + year Nest and fledgling22 Basal area + year Nest and fledgling23 Post-harvest treatment + year Nest and fledgling24 Concealment + year Nest and fledgling25 Height + year Nest and fledgling26 Date + year Nest and fledgling27 Age of male parent + year Nest28 Small shrub cover + year Fledgling29 Raspberry cover + year Fledgling30 Elderberry cover + year Fledgling31 Days since fledging + year Fledgling32 Fledgling weight + year Fledgling33 Age at fledging + year Fledgling

Models with three variables34 Regenerative growth + year + date Nest and fledgling35 Sapling cover year + year + date Nest and fledgling36 Understory cover + year + date Nest and fledgling37 Canopy cover + year + date Nest and fledgling38 Basal area + year + date Nest and fledgling39 Post-harvest treatment + year + date Nest and fledgling

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89

APPENDIX 2.1 Cont’dModel

no.Predictor variables included Plots for which

model was used40 Concealment + year + date Nest and fledgling41 Height + year + date Nest and fledgling42 Age of male parent + year + date Nest43 Small shrub cover + year Fledgling44 Raspberry cover + year + date Fledgling45 Elderberry cover + year + date Fledgling46 Days since fledging + year + date Fledgling47 Fledgling weight + year + date Fledgling48 Age at fledging + year + date Fledgling

Models with four variables49 Regenerative growth + year + date + dsfledging Fledgling50 Sapling cover year + year + date + dsfledging Fledgling51 Understory cover + year + date + dsfledging Fledgling52 Canopy cover + year + date + dsfledging Fledgling53 Basal area + year + date + dsfledging Fledgling54 Post-harvest treatment + year + date + dsfledging Fledgling55 Concealment + year + date + dsfledging Fledgling56 Height + year + date + dsfledging Fledgling57 Small shrub cover + year + dsfledging Fledgling58 Raspberry cover + year + date + dsfledging Fledgling59 Elderberry cover + year + date + dsfledging Fledgling60 Fledgling weight + year + date + dsfledging Fledgling61 Age at fledging + year + date + dsfledging Fledgling

Models with an interaction term62 Regenerative growth, treatment, treatment * regen Nest and fledgling63 Sapling cover, treatment, treatment * saplings Nest and fledgling64 Understory cover, treatment, treatment * understory Nest and fledgling65 Canopy cover, treatment, treatment * canopy Nest and fledgling66 Basal area, treatment, treatment * basal area Nest and fledgling67 Concealment, treatment, treatment * concealment Nest and fledgling68 Height, treatment, treatment * height Nest and fledgling 69 Date, treatment, treatment * date Nest and fledgling70 Year, treatment, treatment * year Nest and fledgling71 Age of male, treatment, treatment * age of male parent Nest72 Small shrubs, treatment, treatment * small shrubs Fledgling73 Raspberry cover, treatment, treatment * raspberries Fledgling74 Elderberry cover, treatment, treatment * elderberries Fledgling75 Days since fledging, treatment, treatment* dsfledging Fledgling76 Fledgling weight, treatment, treatment * fledgling weight Fledgling77 Age at fledging, treatment, treatment * age Fledgling

CHAPTER 3: NESTLING PROVISIONING RATES AND FORAGING PATTERNS OF ROSE-BREASTED GROSBEAKS IN STANDS HARVESTED BY

SINGLE TREE SELECTION

ABSTRACT

Prey availability in hardwood forests influences many aspects of songbird

reproduction, but few studies have investigated how timber harvesting in these forests

influences the ability of breeding birds to provide food for their offspring. I conducted

one hour feeding watches at Rose-breasted Grosbeak (Pheucticus leudovicianus) nests in

twelve hardwood stands in Algonquin Provincial Park which had been harvested using

single-tree selection 0-5, 16-20, 21-25, and > 50 years previously. My main objectives

were to test whether nestling provisioning rates (# trips/nestling/hr), male contribution to

nestling provisioning, nest attendance, male contribution to nest attendance, prey load

sizes, and nestling weights at the time of fledging differed with time since harvest, stage

of nestling development (< 5 or > 5 days old), or age of male parent (SY or ASY). None

of the response variables differed significantly among harvesting treatments in either

study year (P > 0.08), but they differed with stage of nestling development in 2007 (P =

0.006), and with age of male parent in both years (P < 0.04). Specifically, in 2007

provisioning was less frequent (P < 0.01) for < 5 day old nestlings than for > 5 day old

nestlings. Nest attendance was higher (P < 0.01) for < 5 day old nestlings than for > 5

day old nestlings, except in the > 50 years post-harvest treatment. In 2008, nestling

provisioning was higher at nests attended by ASY males than at those attended by SY

males (P = 0.002), and ASY males contributed more to nestling provisioning (P = 0.02)

and to nest attendance (P < 0.01) than SY males did. Mean nestling weight was higher at

nests attended by ASY males than SY males in both study years, with the difference

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significant (P = 0.04) in 2008, suggesting that younger breeders could not compensate for

lower provisioning rates with larger prey load sizes. This study suggests that the age of

the male parent is more important in determining nestling provisioning rates than time

since single-tree selection harvesting.

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INTRODUCTION

During the nesting season, most songbirds can be classified as ‘central place

foragers’ because they carry prey back to centrally located nests (Orians and Pearson

1979). According to this model, adults forage close to their nests until available

resources become depleted and then exploit food sources further away (Naef-Daenzer

2000). To maximize nestling provisioning, there should be a positive correlation between

the distance travelled while foraging and the prey load size delivered to the nestlings

(Grieco 2002). Predator avoidance (e.g., Newman and Caraco 1988) and conspecific

competition (Giraldeau et al. 1994) can influence the relationship between time spent

foraging and prey load size (Sejberg et al. 2000). Assessments of the relationship between

foraging rate and prey load size can be used to test whether changes in forest structure

resulting from silvicultural practices influence invertebrate abundance, prey availability,

or the foraging patterns of breeding songbirds.

Previous studies in hardwood forests have shown that selection harvesting alters

invertebrate communities, with some taxa increasing in abundance and others decreasing

relative to un-harvested stands (Werner and Raffa 2000, Latty et al. 2006, Nol et al. 2006,

Shields et al. 2008). Changes in the abundance of understory plants (Heliolä et al. 2001,

Heyborne et al. 2003, Latty et al. 2006), quantity of coarse woody debris (Martikainen et

al. 2000, Latty et al. 2006), availability of large diameter woody material (Hammond et

al. 2004), snag volume, and gap area (Latty et al. 2006) have been linked to changes in

arthropod abundance following harvesting. For insectivorous songbirds, prey abundance

can influence the timing of nest initiation (Marshall et al. 2002), clutch size (Zanette et al.

2006, Robertson 2009), brood size (Preston and Rotenbury 2006), number of broods per

season (Rodenhouse and Holmes 1992, Nagy and Holmes 2005), population density

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(Gordon 1997, Lyons 2005), reproductive success (Lyons 2005), nestling provisioning

rates (Nagy and Smith 1997), and nestling growth rates (Rodenhouse and Holmes 1992,

Duguay et al. 2000).

Prey availability depends on prey abundance (Holmes and Schultz 1988), prey

characteristics (Cooper and Whitmore 1990, Holmes and Schultz 1988), a bird’s

morphology and foraging strategies (Hutto 1990, Holmes and Robinson 1988, Holmes

and Schultz 1988), and vegetation structure (Robinson and Holmes 1982, Holmes and

Schultz 1988). Foraging behaviour of songbirds is strongly associated with certain

aspects of forest structure, including volume and height of trees, amount of dead wood,

tree species composition (Naka 2003), and canopy characteristics (Bohm and Kalko

2009). Large diameter trees with highly structured bark, large canopy volume, and large

amounts of dead wood typically sustain more insects than small diameter trees, smaller

volume canopies, and lesser amounts of dead wood (Mariani and Manuwal 1990). As a

result, changes in forest structure and composition resulting from silvicultural practices

such as single-tree selection could influence insectivorous passerines.

Structural changes following single-tree selection harvesting include fewer large

diameter (dbh >39.1 cm) live trees (Kenefic and Nyland 2000, Crow et al. 2002, Angers

et al. 2005), fewer snags (Angers et al. 2005, Vanderwel et al. 2006), lower basal area

(Crow et al. 2002) and canopy closure (Kenefic and Nyland 2000), and reduced mean

tree age (Angers et al. 2005, Vanderwel et al. 2006) and spatial complexity (Kenefic and

Nyland 2000). Single-tree selection also maintains (Johnson 1984, Della-Bianca and

Beck 1985, Busing and White 1997, Kenefic and Nyland 2000, Raymond et al. 2006) or

favours the regeneration of shade-tolerant species such as sugar maple (Acer saccharum)

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and American beech (Fagus grandifolia) (Poulson and Platt 1989, Kelty et al. 2003),

whereas shade-intolerant species such as yellow birch (Betula alleghaniensis) and black

cherry (Prunus serotina) are out-competed (Solomon and Grove 1999, Crow et al. 2002,

Webster and Lorimer 2002). If prey becomes less abundant or more difficult to detect as

a result of these changes, adults may spend more time searching for food, leave their

nests undefended for longer periods of time (Arcese and Smith 1988, Wolf et al. 1990,

Martin 1992), and potentially obtain less food for themselves and their nestlings. In

addition, younger, less experienced birds may not be able to respond to these structural

changes as efficiently as older birds (Ratcliffe et al. 1998).

The Rose-breasted Grosbeak (Pheucticus leudovicianus) is a Neotropical migrant

that frequently breeds in both harvested and un-harvested hardwood forests across North

America (Wyatt and Francis 2002, Smith et al. 2006). Although it is still relatively

abundant, this species is now of conservation concern in Ontario (Smith et al. 2006) due

to annual population declines of -2.4% in Ontario between 1966 and 2007 (Sauer et al.

2008) and an especially sharp decline (-8% per year) between 1985 and 2005 (Cadman et

al. 2007). Rose-breasted Grosbeaks respond inconsistently to harvesting, reacting

negatively to selection harvesting in the Carolinian forest (Smith et al. 2006, Thompson

et al. 2009) and partial harvesting in the boreal (Vanderwel et al. 2007, 2009), positively

to various silvicultural treatments in New Brunswick (Guénette and Villard 2005), but

showing no difference in abundance following selection harvesting (Holmes and Pitt

2007) and clear cutting (Duguay et al. 2001) in other regions. Although we know that

Rose-breasted Grosbeaks feed primarily on invertebrate prey during the nesting season

95

(Wyatt and Francis 2002), little information is available on how selection harvesting

influences the feeding ecology of this declining species.

Here, I tested whether changes in habitat structure resulting from single-tree

selection harvesting influence nestling provisioning rates in Rose-breasted Grosbeaks.

My main objectives were to test whether nestling provisioning rates, male contribution to

nestling provisioning and nest attendance, prey load sizes, and nestling weights differed

as a function of 1) time since single-tree selection harvesting, 2) stage in the nestling’s

development, and 3) age of the male parent. I predicted that nestling provisioning rates

would be highest, and prey load sizes would be largest by 16-20 years post-harvest,

because these stands have the well-developed sapling and understory layers (Finegan

1984, Jobes et al. 2004) preferred by this species (Wyatt and Francis 2002). Even in

unaltered habitats, the energy costs of nestling care are high for adult songbirds

(Thompson et al. 1998, Reid et al. 2002), and although both male and female Rose-

breasted Grosbeaks provision their nestlings, females of many species provide most of

the parental care (Stodola et al. 2010). If food items are easier to find and locate in the

16-20 year post-harvest stands, I would expect male contribution to nestling provisioning

and nest attendance to be lower in this treatment than in the others. I also expected that

nestling provisioning rates and prey load sizes would increase with nestling age (Norment

2003, Mitrus et al. 2010), and that nestlings cared for by older, more experienced males

would be provisioned more frequently and receive larger prey load sizes than nestlings

attended by younger males (Wunderle 1991, Limmer and Becker 2009). Finally, I

predicted that if selection harvesting negatively affected the feeding ecology of Rose-

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breasted Grosbeaks, nestling mass at the time of fledging would be higher in the 16-20

years post-harvest stands than in the other treatments.

METHODS

Study Species

During 2006 – 2008 Rose-breasted Grosbeaks arrived on their breeding grounds

during the first week of May, began nest building by the last week of May, and cared for

their nestlings throughout June and July. During the nestling provisioning period, Rose-

breasted Grosbeak diets consist primarily of insects, including Coleoptera (McAtee 1908)

and Lepidoptera larvae (Robinson and Holmes 1982, Holmes and Schultz 1988). Much

of the time, this songbird perches and gleans invertebrate prey from the periphery of

foliage and branches throughout the canopy, but it also collects invertebrates from

branches and tree trunks while hovering (Holmes and Schultz 1988) and occasionally

hawks for insects or forages at ground level (Wyatt and Francis 2002). When Rose-

breasted Grosbeaks forage in the canopy, invertebrates are obtained from the underside of

leaves during more than 65% of feeding attempts, making them lower-leaf specialists

(Holmes and Schultz 1988). Both male and female Rose-breasted Grosbeaks incubate

and provision their young, which typically fledge 9-12 days after hatching (Scott 1998).

Study Area

Fieldwork was carried out during May-August in 2007 and 2008 in twelve

tolerant hardwood forest stands in Algonquin Provincial Park, Ontario, Canada (45°

34`N, 78°04`W). The higher-elevation west side of this 775,000 ha park consists

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primarily of mixed hardwood forest, whereas the lower-elevation east side is dominated

by white and red pine (Ontario Parks 1998). The tolerant hardwood stands I studied were

located in the central and southern part of this park (mean nearest neighbour distance

among stands = 9.3 km, range = 1.0 – 34.2 km) (Fig. 1.1).

The hardwood forest stands were dominated by sugar maple (Acer saccharum),

American beech (Fagus grandifolia), eastern hemlock (Tsuga canadensis), and yellow

birch (Betula alleghaniensis), and had been harvested using single-tree selection since the

early 1970’s (OMNR 1998). This uneven-aged silvicultural system removes

approximately one-third of the over-story basal area every 21-25 years (OMNR 1998). I

studied four temporal stages in the single-tree selection cutting cycle: 0-5, 16-20, and 21-

25 years post-harvest and stands that had not been logged for >50 years. Three sites

(mean stand size = 34 ha) from each temporal stage were studied.

Nest Monitoring

Nests were monitored every 3-4 days using a mirror attached to a telescoping pole

to determine date of nest initiation, clutch size, hatching date, number of nestlings, and

fledging success.

Nestling Provisioning

Provisioning data were collected when the nestlings were 1-4 days old (n = 54;

hereafter Stage 1 nestlings) and again when they were 6-9 days old (n = 33; hereafter

Stage 2 nestlings). Altricial passerine nestlings can be aged based on whether their eyes

are open or closed, they can vocalize or not, and the developmental stage of their pin

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feathers (Jongsomjit et al. 2007). By day 5-6 after hatching Rose-breasted Grosbeak

nestlings open their eyes and are capable of making audible noises (Wyatt and Francis

2002). Nestlings were confirmed to be less than five days old if they were blind and

inaudible, and more than five days old if they could see and make sounds. To avoid

ambiguous estimates of nestling age I did not observe nestlings when they were thought

to be exactly 5 days old.

Nests were observed for 1 h periods between 06:00 and 12:00 h when nestlings

were 1-4 days old (n = 54, mean age = 2.2 d) and 6-9 days old (n = 33, mean age = 6.5 d).

Thirty-four nests were observed only during the first developmental stage, thirteen nests

were only observed when nestlings were 6-9 days old, and twenty nests were observed

during both developmental stages. Since feeding rate can vary with weather (Keller and

van Noordwijk 1994) and time of day (Cowie and Hinsley 1988), no observations were

collected if it was windy or raining, and I ensured that the proportion of observations

taken in the early (06:00-08:00), mid (08:00-10:00), and late (10:00-12:00) morning were

approximately equal among post-harvest treatments.

Observations were made using binoculars or a 20X telescope from concealed

locations at distances of approximately 15-30 m from the nest. I recorded the sex of the

bird bringing the food, the age if the bird was male (after-second-year or second-year),

the time of each feeding event, and the size of the prey delivered relative to the size of the

adult’s beak. After-second-year (hereafter ASY) males can be distinguished from

second-year (hereafter SY) males by visible differences in their plumage (Pyle 1997).

While ASY males have uniformly black primaries, black heads and backs, and well-

defined markings, SY males have less defined markings and often have brown streaking

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on their heads, backs, and primary feathers (Pyle 1997). At the end of each 1 h

observation period nest contents were checked using a telescoping pole with a mirror

attached to the end to count the number of nestlings present. Standardized feeding rates

were then calculated as the number of provisioning trips/ nestling/ hour.

Prey Load Sizes

Prey load sizes were visually assessed and categorized as small (barely visible in

the adult’s beak), medium (visible in less than one-half the length of the adult’s beak), or

large (filling more than one-half the adult’s beak) (Schaefer et al. 2004). When possible,

I visually identified prey types into broad taxonomic groups (e.g., insect order).

Nest Attendance

During the one-hour nest observations, I recorded the total amount of time that an

adult was present at the nest (either brooding the nestlings or sitting within c. 30 cm of

the nest) and the sex of the adult in attendance. I calculated the proportion of time the

nest was attended, and the proportion of the total nest attendance that was contributed by

the male.

Nestling Weights

I weighed nestlings at the time of fledging to the nearest 0.01g using a SC-HT-S-

232 digital scale. Whenever possible, multiple fledglings from each nest were weighed

and an average weight for the brood was calculated. For some higher nests (22.6%; 12 of

53 nests) only one weight measurement was obtained because I was unable to capture

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more than one fledgling once they had left the nest. These nests were distributed

approximately equally among the 0-5 (3.8%), 16-20 (5.7%), and 21-25 (5.6%) years post-

harvest stands, whereas the > 50 years post-harvest treatment had a slightly higher

proportion (7.5%).

Statistical Analyses

I used split-split plot analysis of variance to investigate four response variables as

a function of harvesting treatment (0-5, 16-20, 21-25, and > 50 years post-harvest), stage

of nestling development (< 5 or > 5 d old), and adult male age (SY or ASY). Harvesting

treatment was the whole-plot factor and the latter two were split-plot factors (Littell et al.

1996). Response variables were 1) nestling provisioning rate (number of

trips/nestling/h), 2) male contribution to nestling provisioning, 3) nest attendance, and 4)

male contribution to nest attendance. I first used multivariate analysis of variance to test

for significant relationships between the three predictor variables and each of the

response variables, and then I used univariate split-split plot analyses of variance to

investigate each response variable separately.

For each response variable a mean value was calculated for each stand from the

data collected during the 1 h nest observation periods. In all tests, each stand mean was

weighted according to the number of observations collected in that stand. Separate

analyses were conducted for 2007 and 2008 due to significant (P < 0.002) among year

differences in nestling provisioning rates, and model assumptions of homogeneity and

normality were verified by examination of residuals. I obtained homogeneity of variance

for nestling provisioning rates using a log transformation (log(x + 1)). All statistical

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analyses were conducted in the PROC GLM module of SAS (v. 8.02). I first tested the

full model. If the 3-way interaction was clearly not significant (i.e., P > 0.15) it was

removed from the analysis and the model rerun. Similarly, any 2-way interactions that

were clearly not significant in the second run were excluded from the final run.

I used contingency table analysis to investigate prey load size (small, medium, or

large) as a function of harvesting treatment (0-5, 16-20, 21-25, and > 50 years post-

harvest), stage of nestling development (< 5 or > 5 d old), and adult male age (SY or

ASY). Model significance was determined using the Mantel-Haenszel Chi-square test

statistic that is appropriate when 50% or more of the expected cell frequencies are less

than 5 (SAS 2001). Analyses were conducted in the PROC FREQ module of SAS (v.

8.02).

I could not use split-split plot analyses of variance to investigate nestling weight

at the time of fledging as a function of harvesting treatment (0-5, 16-20, 21-25, and > 50

years post-harvest) and male age (SY or ASY) due to insufficient sample size (n = 63).

Since harvesting treatment did not have a significant effect on any of the other response

variables tested, I collapsed harvesting treatment into just two levels: harvested (i.e., 0-25

years post-harvest) and unharvested (> 50 years post-harvest) for inclusion into the

analyses of nestling weights. Split-plot analysis of variance was conducted in SAS using

the procedure described above to investigate nestling weight as a function of harvest

treatment and male age.

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RESULTS

Multivariate analysis of variance indicated that the response variables differed

significantly among study years (F23.4, 48.75 = 8.89, P = 0.002), so separate analyses were

performed for 2007 and 2008. Multivariate analysis of variance indicated that there were

no significant differences in nestling provisioning rates, male contribution to nestling

provisioning, nest attendance, or male contribution to nest attendance among post-harvest

treatments in either study year (2007: F12, 26.75 = 1.48, P = 0.192; 2008: F12, 39.98 = 1.83, P =

0.076). In 2007, response variables differed significantly with stage of nestling

development (F4, 10 = 6.87, P = 0.006) but there was a significant interaction between

stage of nestling development and harvesting treatment (F12, 26.75 = 3.95, P = 0.002) for

nest attendance and male contribution to nest attendance. In 2008, response variables did

not differ significantly between stages of nestling development (F4, 15 = 1.27, P = 0.327).

Response variables differed significantly with age of male parent during both study years

(2007: F4, 10 = 3.64, P = 0.044; 2008: F4,15 = 6.44, P = 0.003) but there was a significant

interaction between harvest treatment and male age in 2007 (F12, 26.75 = 2.89, P = 0.011)

for nest attendance, and an almost significant interaction between harvest treatment and

male age in 2008 (F12, 39.98 = 1.92, P = 0.061) for nestling provisioning rates.

Nestling Provisioning Rates

In total I completed 1 h nest watches at 130 nests (90 in 2007 and 40 in 2008).

Nestlings were provisioned on average at a rate of 0.67 trips/nestling/h (SE = 0.07, range

= 0-4.33 trips/nestling/h, n = 130). Nestling provisioning rates did not differ significantly

among harvest treatments in either year (2007: F1,3 = 0.21, P = 0.887; 2008: F1,3 = 2.77, P

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= 0.078), although provisioning rates were lowest in the > 50 years post-harvest stands in

both study years (Table 3.1). Provisioning was less frequent for Stage 1 nestlings (< 5

days old) than for Stage 2 nestlings (> 5 days old) in all post-harvest treatments in 2007

(F1,3 = 9.79, P = 0.005), but in 2008 differences were not significant (F1,3 = 1.72, P =

0.209), and the mean provisioning rate in the 0-5 years post-harvest stands was higher for

Stage 1 nestlings than for Stage 2 nestlings (interaction: F1,3 = 3.67, P = 0.036). Nestling

provisioning was more frequent at nests attended by ASY males than at those attended by

SY males in all post-harvest treatments in 2007, although not significantly so (F1,3 = 3.47,

P = 0.078). Mean provisioning rates were significantly higher at nests attended by ASY

males in all stands in 2008 (F1,3 =13.45, P = 0.002) as well, except the 21-25 years post-

harvest treatment (interaction: F1,3 = 3.67, P = 0.063) (Table 3.1).

Male Contribution to Nestling Provisioning

On average, males contributed 35.2 (± 3.2 SE) % of the feeding events. Male

contribution to nestling provisioning did not differ significantly among post-harvest

treatments in either year (2007: F1,3 = 1.54, P = 0.243; 2008: F1,3 = 1.27, P = 0.31), and

among-treatment differences were not consistent between years (Table 3.2). Male

contribution to nestling provisioning did not differ significantly with nestling age in

either year (2007: F1,3 = 1.50, P = 0.238; 2008: F1,3 = 0.01, P = 0.952), but in both years it

tended to be lower for Stage 1 than Stage 2 nestlings in all stands except the > 50 years

post-harvest stands. In 2007, contributions from ASY and SY males did not consistently

differ among post-harvested treatments (F1,3 = 1.52, P = 0.236), but in 2008 ASY males

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contributed significantly more (F1,3 = 6.68, P = 0.017) than SY males in all post-harvest

treatments except the 16-20 years post-harvest stands.

Nest Attendance

Nests were attended by adults 66.7 (± 3.2) % of the time, with male attendance

accounting for only 18.8 (± 1.9) % of that time. Nest attendance did not significantly

differ among harvest treatments in either year (2007: F1,3 = 2.32, P = 0.123; 2008: F1,3 =

0.47, P = 0.709), and trends were not consistent among study years (Table 3.3). In 2007,

nest attendance was higher when nestlings were < 5 days old than when nestlings were >

5 days old (F1,3 = 8.18, P = 0.013), except in the > 50 years post-harvest treatment

(interaction: F1,3 = 4.35, P = 0.025). In 2008, nest attendance was also generally higher

for < 5 day old nestlings, but not significantly so (F1,3 = 1.23, P = 0.281). In 2007, ASY

males attended their nests less than SY males (F1,3 = 0.19, P = 0.673) except in the > 50

years post-harvest treatment (interaction: F1,3 = 3.95, P = 0.033). In 2008, nest

attendance by ASY and SY males did not differ significantly among harvest treatments

(F1,3 = 1.23, P = 0.281).

Male Contribution to Nest Attendance

Nest attendance was observed at 102 nests with ASY males and 28 nests with SY

males. The proportion of time males spent at the nest relative to females differed

significantly with time since harvest in both study years (2007: F1,3 = 3.80, P = 0.031;

2008: F1,3 = 4.53, P = 0.013). In 2007, male contribution was significantly lower in the

16-20 year post-harvest treatment than in the other treatments, but in 2008 male

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TABLE 3.1. Mean nestling provisioning rates at Rose-breasted Grosbeak nests in 12 hardwood stands in Algonquin Provincial Park harvested by single-tree selection 0-5, 16-20, 21-25, and > 50 years prior to study (standard errors and sample sizes are in parentheses). Mean nestling provisioning rates (no. trips/ nestling/ h) are presented for nests attended by second-year (SY) and after-second-year (ASY) males, and for nests with Stage 1 (< 5 days old) and Stage 2 (> 5 days old) nestlings.

Time since harvest (years)0-5 16-20 21-25 >50

2007Overall treatment mean 0.676 (0.175, 34) 0.667 (0.191, 14) 0.520 (0.142, 26) 0.459 (0.177, 16)Nestling development Stage 1 0.391 (0.081, 22) 0.002 (0.001, 6) 0.314 (0.082, 16) 0.250 (0.164, 8)

Stage 2 1.193 (0.447, 12) 1.167 (0.189, 8) 0.850 (0.330, 10) 0.667 (0.309, 8)Male Age ASY 0.766 (0.193, 30) 0.800 (0.249, 10) 0.507 (0.154, 23) 0.612 (0.220, 12)

SY 0.002 (0.001, 4) 0.335 (0.193, 4) 0.670 (0.001, 3) 0.001 (0.001, 4)2008Overall treatment mean 0.767 (0.290, 10) 0.765 (0.178, 10) 1.137 (0.252, 11) 0.667 (0.190, 9)Nestling development Stage 1 1.083 (0.293, 6) 0.570 (0.115, 7) 0.811 (0.220, 7) 0.667 (0.202, 7)

Stage 2 0.436 (0.172, 4) 1.220 (0.485, 3) 1.707 (0.493, 4) 0.665 (0.665, 2)Male Age ASY 1.767 (0.290, 8) 0.582 (0.145, 4) 1.585 (0.338, 6) 1.000 (0.001, 6)

SY 0.500 (0.001, 2) 0.887 (0.281, 6) 0.600 (0.214, 5) 0.141 (0.001, 3)

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TABLE 3.2. Mean proportion of feeding events contributed by male Rose-breasted Grosbeaks in 12 hardwood stands in Algonquin Provincial Park harvested by single-tree selection 0-5, 16-20, 21-25, and > 50 years prior to study (stand errors and sample sizes are in parentheses). Mean proportion of feeding events are presented for second-year (SY) and after-second-year (ASY) males, and for nests with Stage 1 (< 5 days old) and Stage 2 (> 5 days old) nestlings.

Time since harvest (years)0-5 16-20 21-25 >50

2007Overall treatment mean 0.466 (0.068, 34) 0.338 (0.081, 14) 0.286 (0.101, 26) 0.125 (0.060, 16)Nestling development Stage 1 0.405 (0.082, 22) 0.154 (0.067, 6) 0.222 (0.088, 16) 0.167 (0.110, 8)

Stage 2 0.580 (0.118, 12) 0.522 (0.131, 8) 0.400 (0.245, 10) 0.082 (0.054, 8)Male Age ASY 0.496 (0.069, 30) 0.309 (0.082, 10) 0.250 (0.115, 23) 0.167 (0.077, 12)

SY 0.001 (0.000, 4) 0.500 (0.289, 4) 0.500 (0.001, 3) 0.001 (0.000, 4)2008Overall treatment mean 0.285 (0.099, 10) 0.300 (0.113, 10) 0.497 (0.068, 11) 0.472 (0.138, 9)Nestling development Stage 1 0.274 (0.107, 6) 0.357 (0.148, 7) 0.486 (0.110, 7) 0.500 (0.159, 7)

Stage 2 0.300 (0.200, 4) 0.167 (0.167, 3) 0.518 (0.017, 4) 0.375 (0.375, 2)Male Age ASY 0.342 (0.110, 8) 0.292 (0.172, 4) 0.578 (0.087, 6) 0.708 (0.109, 6)

SY 0.001 (0.000, 2) 0.305 (0.163, 6) 0.400 (0.100, 5) 0.001 (0.000, 3)

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TABLE 3.3. Mean nest attendance at Rose-breasted Grosbeak nests in 12 hardwood stands in Algonquin Provincial Park harvested by single-tree selection 0-5, 16-20, 21-25, and > 50 years prior to study (standard errors and sample sizes are in parentheses). Mean nest attendance is presented for nests attended by second-year (SY) and after-second-year (ASY) males, and for nests with Stage 1 (< 5 days old) and Stage 2 (> 5 days old) nestlings.

Time since harvest (years)0-5 16-20 21-25 >50

2007Overall treatment mean 0.694 (0.053, 34) 0.581 (0.084, 14) 0.713 (0.081, 26) 0.595 (0.120, 16)Nestling development Stage 1 0.719 (0.059, 22) 0.793 (0.065, 6) 0.998 (0.002, 16) 0.500 (0.189, 8)

Stage 2 0.645 (0.108, 12) 0.422 (0.110, 8) 0.257 (0.101, 10) 0.690 (0.152, 8)Male Age ASY 0.673 (0.055, 30) 0.464 (0.091, 10) 0.689 (0.086, 23) 0.793 (0.108, 12)

SY 1.000 (0.001, 4) 0.875 (0.072, 4) 1.000 (0.001, 3) 0.002 (0.001, 4)2008Overall treatment mean 0.715 (0.099, 10) 0.727 (0.102, 10) 0.663 (0.092, 11) 0.576 (0.152, 9)Nestling development Stage 1 0.647 (0.131, 6) 0.860 (0.578, 7) 0.721 (0.115, 7) 0.593 (0.167, 7)

Stage 2 0.810 (0.155, 4) 0.417 (0.252, 3) 0.560 (0.162, 4) 0.515 (0.485, 2)Male Age ASY 0.658 (0.110, 8) 0.820 (0.148, 4) 0.705 (0.090, 6) 0.385 (0.182, 6)

SY 1.000 (0.001, 2) 0.665 (0.143, 6) 0.612 (0.183, 5) 0.957 (0.043, 3)

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contribution was significantly higher in this treatment than in the others (Table 3.4). In 2007,

males contributed significantly more to nest attendance when nestlings were < 5 days old

than when they were > 5 days old (F1,3 = 8.54, P = 0.010) except in the > 50 years post-

harvest treatment (interaction: F1,3 = 5.33, P = 0.010). In 2008 nest attendance by males was

also higher when nestling were < 5 days old, but not significantly so (F1,3 = 3.36, P = 0.081).

ASY males contributed significantly more to nest attendance than SY males in both study

years (2007: F1,3 = 13.45, P = 0.002; 2008: F1,3 = 17.31, P < 0.001).

Prey Load Sizes

During 37 of 73 (51%) feeding events it was possible for observers to accurately

identify the type of food item being delivered to the nestlings. Insect matter which could not

be identified to family was delivered to nestlings during 16 feeding events (22%), adult

Lepidoptera were brought during 11 visits (15%), and larval Lepidoptera were brought

during 8 feeding events (11%). Rose-breasted Grosbeak adults were also observed feeding

dragonflies (1%) and leaf buds (1%) to their nestlings.

Small prey loads were the most frequently delivered in all post-harvest treatments

except the 16-20 years post-harvest stands, where large loads were the most frequently

provided, although differences were not significant (X2 = 3.098, df = 1, P = 0.078) (Table

3.5).

The age of the male parent was not significantly associated with the prey load sizes

delivered (X2 = 1.576, df = 1, P = 0.209). Small prey loads were most frequently delivered at

nests attended by ASY males, followed by large and then medium loads. At nests attended

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TABLE 3.4. Mean nest attendance contributed by male Rose-breasted Grosbeaks in 12 hardwood stands in Algonquin Provincial Park harvested by single-tree selection 0-5, 16-20, 21-25, and > 50 years prior to study (standard errors and sample sizes are in parentheses). Mean nest attendance is presented for second-year (SY) and after-second-year (ASY) males, and for nests with Stage 1 (< 5 days old) and Stage 2 (> 5 days old) nestlings.

Time since harvest (years)0-5 16-20 21-25 >50

2007Overall treatment mean 0.282 (0.045, 34) 0.056 (0.182, 14) 0.220 (0.037, 26) 0.162 (0.045, 16)Nestling development Stage 1 0.388 (0.053, 22) 0.280 (0.103, 6) 0.298 (0.025, 16) 0.132 (0.053, 8)

Stage 2 0.082 (0.040, 12) 0.085 (0.033, 8) 0.080 (0.049, 10) 0.192 (0.075, 8)Male Age ASY 0.301 (0.046. 30) 0.211 (0.065, 10) 0.215 (0.043, 23) 0.217 (0.051, 12)

SY 0.001 (0.000, 4) 0.025 (0.014, 4) 0.250 (0.000, 3) 0.001 (0.000, 4)2008Overall treatment mean 0.094 (0.043, 10) 0.820 (0.053, 10) 0.125 (0.043, 11) 0.070 (0.042, 9)Nestling development Stage 1 0.137 (0.068, 6) 0.204 (0.070, 7) 0.160 (0.062, 7) 0.900 (0.052, 7)

Stage 2 0.034 (0.034, 4) 0.130 (0.098, 3) 0.065 (0.009, 4) 0.001 (0.000, 2)Male Age ASY 0.113 (0.050, 8) 0.360 (0.037, 4) 0.138 (0.045, 6) 0.105 (0.059, 6)

SY 0.001 (0.000, 2) 0.063 (0.031, 6) 0.110 (0.079, 5) 0.001 (0.000, 3)

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TABLE 3.5. Observed frequencies for prey load sizes delivered to Rose-breasted Grosbeak nests in 12 hardwood stands in Algonquin Provincial Park harvested by single-tree selection (row percents are in parentheses). Mean prey load sizes are presented for harvest treatment (0-5, 16-20, 21-25, and > 50 years post-harvest), for nests with < 5 or > 5 days old nestlings, and for nests attended by after-second-year (ASY) and second-year (SY) male parents.

Prey Load Size Small Medium LargeYears Since Harvest 0-5 35 (51.5) 19 (27.9) 14 (20.6)

16 – 20 13 (35.1) 9 (24.3) 15 (40.5)21 – 25 29 (46.8) 17 (27.4) 16 (25.8)

> 50 14 (63.6) 2 (9.1) 6 (27.3)Nestling Age < 5 days 45 (60.0) 23 (30.7) 7 (9.3) > 5 days 45 (39.8) 24 (21.2) 44 (38.9)Male Age ASY 81 (50.0) 38 (23.4) 43 (26.5) SY 14 (48.3) 9 (31.0) 6 (20.7)

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by SY males small prey loads were also most frequently delivered, followed by medium and

then large loads.

Nestling Weights at Fledging

Mean nestling weight at the time of fledging was 31.1 g (± 0.4 g) (range = 22.5 – 37.5

g, n = 63). Mean nestling weight did not significantly differ with harvest treatment in 2007

(harvested: 30.6g; unharvested: 30.3g; F1,1 = 0.19, P = 0.665) or in 2008 (harvested: 31.5g;

unharvested 32.3g; F1,1 = 0.19, P = 0.672). Mean nestling weight was higher at nests

attended by ASY males than SY males in both 2007 (ASY: 30.8g; SY: 29.6g) and 2008

(ASY: 32.0g; SY: 30.3g) but only significantly so in 2008 (2007: F1,1 = 1.87, P = 0.180;

2008: F1,1 = 4.80, P = 0.043).

DISCUSSION

Single-tree Selection Harvesting

Nestling provisioning rates, nest attendance, the size of prey loads being delivered to

nestlings, and nestling weights did not significantly differ among post-harvest treatments in

either study year. Previous research has found that harvesting can alter prey availability for

insectivorous songbirds (Pettersson et al. 1995, Duguay et al. 2000), but most studies have

focused on the effects of clear-cutting on ground-dwelling arthropods (Niemelä et al. 1993,

Greenberg and McGrane 1996, Marra and Edmonds 1998, Heliӧlä et al. 2001, Koivula et al.

2002). Studies of invertebrate responses to less-intensive silvicultural methods are less

frequent (Koivula 2002, Moore et al. 2002, Siira-Pietikainen et al. 2003, Pihlaja et al. 2006),

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and most focus on changes in diversity and species richness (i.e., Werner and Raffa 2000,

Ulyshen et al. 2004, Latty et al. 2006, Shields et al. 2008), making potential changes in prey

availability for songbirds difficult to assess (Halaj et al. 2008). Two studies concluded that

there was no difference in overall abundance of ground invertebrates following selection

harvesting (Atlegrim and Sjoberg 1996, Moore et al. 2002), but one study found that in the

first fifteen years after single-tree selection harvesting there was an increase in the abundance

of generalist and open habitat ground beetles, but similar species richness compared to

unharvested hardwood stands (Vance and Nol 2003). I am unaware of any studies which

have investigated the consequences of single-tree selection for nestling provisioning rates or

nestling condition at the time of fledging.

Nestling growth rates have been linked to variations in food abundance resulting from

natural causes (Price 1985, Dias and Blondel 1996, Holmes et al. 1996) and from clear-cut

harvesting (Duguay et al. 2000). My study suggests that if there were any changes in

arthropod abundance or prey availability resulting from single-tree selection harvesting they

did not appear to have a significantly negative influence on the Rose-breasted Grosbeak’s

ability to find food during the nestling phase.

Studies investigating changes in arthropod diversity and species richness following

selection harvesting show that responses are largely taxon specific, with groups that favour

more open habitat increasing whereas mature forest groups decrease in abundance (Werner

and Raffa 2000, Latty et al. 2006, Shields et al. 2008). In particular, spiders (Arachnida)

decreased following harvesting (Moore et al. 2002), millipedes (Polydesmida), snails

(Stylommatophora) (Strayer et al. 1986, Moore et al. 2002), hoverflies (Diptera), bees

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(Hymenoptera), and click beetles (Coleoptera) (Nol et al. 2006) were more abundant, and

there was no difference in the abundance of Carabid beetles (Coleoptera) (Moore et al. 2002)

and crickets (Orthoptera) (Moore et al. 2002) before and after harvesting. In addition, wood-

boring insects (Ulyshen et al. 2004, Shields et al. 2008), grasshoppers (Orthoperta) and wolf-

spiders (Araneae) (Shields et al. 2008) were more abundant in canopy gaps than in

surrounding areas with higher canopy cover, but weevils (Coleoptera) (Shields et al. 2007)

and rove beetles (Coleoptera) (Yu et al. 2006) were less abundant in these areas. If any

taxon-specific changes in invertebrate abundance occurred after single-tree selection, Rose-

breasted Grosbeaks were able to compensate sufficiently to maintain nestling provisioning

rates and nestling masses at the time of fledging.

The absence of significant differences in nestling provisioning rates, nest attendance,

and nestling weights among post-harvest treatments could be the result of minimal changes

in arthropod abundance or prey availability following single-tree selection harvesting

(Atlegrim and Sjoberg 1996, Moore et al. 2002). Alternatively, while there is evidence to

suggest that food is limiting for songbirds during the breeding season (Martin 1986), it is

possible that Rose-breasted Grosbeaks were able to compensate for any changes in food

availability by altering their foraging behaviour. For example, Coquerel’s Coua (Coua

coquereli) were able to maintain their rate of prey capture despite structural changes

following logging by adjusting their feeding activities (Chouteau 2009). Feeding rate can

also vary with weather (Keller and van Noordwijk 1994), time of day (Cowie and Hinsley

1988), and intensity of nestling begging (Ottosson et al. 1997, Burford et al. 1998, Greico

2001), and although I do not think it is likely given the relatively large sample sizes here,

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variations in feeding rates caused by these factors may have obscured differences among

post-harvest treatments.

A lack of significant differences in the proportion of time Rose-breasted Grosbeaks

allocated to nest attendance, or in the proportion of nest attendance provided by males in the

varying post-harvest treatments also supports the conclusion that single-tree selection had

minimal impacts on Rose-breasted Grosbeak feeding ecology. Fedy and Martin (2009)

demonstrated that time spent foraging was negatively correlated with time spent brooding by

females of Red-faced Warblers (Cardellina rubrifrons) and Gray-headed Juncos (Junco

hyemalis caniceps). When males were not in attendance females spent more time being

vigilant, their foraging efficiency decreased, and they spent less time on the nest (Fedy and

Martin 2009). In my study nest attendance and nestling provisioning rates were both slightly

higher in more recently harvested stands than in un-harvested ones, which suggests that prey

were more abundant or more easily located and captured after selection cutting. Since male

contribution to nest attendance was also slightly higher in more recently harvested stands, an

alternative hypothesis is that predation pressure, and the need for nest defence, were also

higher following selection harvesting. However, no aspect of Rose-breasted Grosbeak

feeding ecology that I studied showed consistently significant differences among post-harvest

treatments.

Nestling Development

Nestling provisioning was less frequent and nest attendance was higher for < 5 day

old nestlings than for > 5 day old nestlings, but differences were only significant in 2007,

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possibly as a result of larger sample sizes during that year. Larger nestlings have greater

energy requirements, and in many species both males and females increase the frequency of

feeding as nestlings mature (e.g. Haggerty 1992, Norment 2003; Mitrus et al. 2010). In

addition, parents adjust their provisioning rates and the amount of food they bring according

to the frequency and intensity of nestling begging, and older nestlings are capable of more

vigorous and sustained demands (Haggerty 1992). Also as expected, Rose-breasted

Grosbeak nest attendance was higher when nestlings were <5 days old than when nestlings

were >5 days old during both study years. In many passerines the amount of time spent

brooding decreases with the age of the nestlings, partially because more time is spent

foraging and thermoregulatory requirements of nestlings are lower (Moreno 1987, Sanz and

Moreno 1995, Sanz and Tinbergern 1999, Chastel and Kersten 2002).

In both study years male Rose-breasted Grosbeaks contributed less to nestling

provisioning and nest attendance than females did during both stages of nestling

development. According to Evolutionary Stable Strategy (ESS) models, males and females

of monogamous species with biparental care should make approximately equal contributions

to parental care (Maynard-Smith 1977, Winkler 1987). However, in situations where

paternity is uncertain or the opportunity for additional fertilizations is high, males may

provide less parental care than females (Maynard-Smith 1977, Breitwisch et al. 1986, Smith

et al. 1988, Westneat 1988). Males contribute slightly less to nestling provisioning than

females in Northern Mockingbirds (Mimus polyglottos) (Breitwisch et al. 1986), Tree

Swallows (Tachycineta bicolour) (Leffelaar and Robertson 1986), Pied Flycatchers,

(Ficedula hypoleuca) (Alatalo et al. 1988), Great Tits (Parus Major) (Smith et al. 1988) and

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Eastern Phoebes (Sayornis phoebe) (Conrad and Robertson 1993), and research has shown

that up to 40% of nests contain nestlings sired through extra-pair copulations in some species

of songbirds (Stutchbury et al. 1997). Future assessments of paternity in Rose-breasted

Grosbeak nestlings are needed to determine if high rates of extra-pair copulations can

account for the lower contribution to parental care by the males relative to the females of this

species.

Rose-breasted Grosbeak males’ contribution to provisioning almost doubled for > 5

day old nestlings (39.5%) relative to < 5 day old nestlings (23%), but differences were not

significant in either study year, possibly as a result of small sample sizes and high variation

among post-harvest treatments. No significant differences in the proportion of nestling

provisioning provided by males between the two stages suggests that male and female Rose-

breasted Grosbeaks increase their feeding effort at the same rate during nestling

development. In some species, including Great Tits (Parus major) (Sanz and Tinbergen

1999), Eastern Kingbirds (Tyrannus tyrannu) (Morehouse and Brewer 1968), Gray Catbirds

(Dumatella carolinensis) (Johnson and Best 1982), and Eastern Phoebes (Sayornis phoebe)

(Conrad and Robertson 1993) males contribute more to nestling provisioning during the early

stages, while the females brood. Rose-breasted Grosbeak males contributed significantly

more to nest attendance during the early stages (when nestlings were < 5 days old) than later

on, but did not alter their relative contribution to nestling provisioning. This suggests that

when females cannot adequately meet the conflicting demands of brooding and foraging,

Rose-breasted Grosbeak males will contribute proportionally more to nest attendance rather

than to nestling provisioning.

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Age of Male Parent

Nestling provisioning rates were lower at nests attended by younger, less experienced

males than at nests with older males, and the younger adults were unable to compensate by

providing larger prey load sizes. As a result, nestlings raised by younger males weighed less

at the time of fledging than nestlings reared by older males. Other studies have found that

the age of both male and female parents can influence the date of clutch initiation, clutch size

(Reynolds 1997), number of fledglings produced (Préault et al. 2005, Brown and Roth 2009),

nestling condition (Préault et al. 2005, but see Wheelwright and Schultz 1994), and quality of

parental care (Préault et al. 2005). The body condition of nestlings at the time of fledging

can influence their ability to find suitable habitats for foraging and predator avoidance, and

as a result, fledglings in poorer condition may have lower chances of survival (Magrath 1991,

Naef-Daenzer et al. 2001, Vitz and Rodewald 2010; but see Anders et al. 1997). The

correlation between body condition and fledgling survival suggests that for Rose-breasted

Grosbeaks, the offspring of first-time breeders, which weigh less at the time of fledging, may

have a fitness disadvantage relative to offspring raised by more experienced adults.

Several hypotheses have been proposed to explain differences in reproductive success

between SY and ASY adults (reviewed in Curio 1983, Forslund and Pärt 1995, Martin 1995).

The selection hypothesis suggests that reproductive success appears higher in older

individuals because inferior phenotypes, which would have lower success, experience high

mortality at young ages. The recruitment hypothesis proposes that younger individuals may

postpone breeding in order to accumulate sufficient resources to successfully breed later in

life. According to the restraint hypothesis, older individuals should have higher reproductive

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output because survival and reproductive value decrease with age. Finally, the constraint

hypothesis suggests that older individuals have higher reproductive success because parental

skills such as nest-site selection, foraging ability, coordination between partners, and

offspring defence improve with age (Forslund and Part 1995, Martin 1995, Espie et al. 2000,

Balbontin et al. 2007). These hypotheses are not mutually exclusive (Curio 1983, Forslund

and Pärt 1995, Martin 1995, Brown and Roth 2009), and I did not specifically test any of

them. The lack of difference I observed between habitat characteristics at nests with older

and younger Rose-breasted Grosbeaks suggests that lack of experience (constraint

hypothesis) was more likely a cause of differences in nestling provisioning rates than unequal

habitat quality, but further research is required to clarify the causes of lower reproductive

success in less experienced Rose-breasted Grosbeaks.

Previous studies have found that younger birds of many species cannot forage as

efficiently as older, more experienced adults (e.g. Janson 1990, Wunderle 1991, Daunt et al.

1999, Limmer and Becker 2009; but see Heise and Moore 2003). Differences in foraging

ability (Daunt et al. 1999), diet choice (Rutz et al. 2006), the ability to capture large (Catterall

et al. 1989, Desrochers 1992) or energy rich (Limmer and Becker 2009) prey items, timing of

nest initiation (Stutchbury and Roberston 1988, Hatch and Westneat 2007; but see Daunt et

al. 1999) and access to high quality foraging territories (Catterall et al. 1989, Pärt 2001) may

contribute to the positive relationship between age and the amount or quality of food

obtained. Evidence that the foraging skills and reproductive success of young birds improve

with age (Stuchbury and Robertson 1988, Desrochers 1992, de Forest and Gaston 1996,

Catry and Furness 1999) supports the hypothesis that age-related differences in reproductive

119

performance can be attributed to a lack of previous experience in younger birds (Yasukawa

1981, Nol and Smith 1987, Wheelwright and Schultz 1994).

As a result of age-related differences in foraging ability, in environments and years

when food resources are particularly scarce, younger birds may be more strongly affected

than older birds (Hannon and Smith 1984, Bedard and LePointe 1985, Hussell and Quinney

1987, Stutchbury and Robertson 1988) or they may be unable to provide enough food to

successfully fledge young (Sydeman et al. 1991). In my study nestlings raised by younger

male parents weighed less than nestlings raised by older parents, but differences were only

significant in 2008. Most Rose-breasted Grosbeaks have nestlings during June, and there

was considerably more rain in 2008 than in 2007 during this month, which suggests that

younger Rose-breasted Grosbeak parents may have more difficulty adjusting to adverse

weather conditions than more experienced adults.

In addition to age-related differences in nestling provisioning rates, nest attendance

and male contribution to nest attendance also varied with the age of the male parent. Overall

nest attendance was higher at nests attended by younger, less experienced males than at nests

with older, more experienced males. Higher nest attendance could be expected to increase

nest survival, but nests with younger males were not more successful on average than nests

with older males (Table 1.5, Chapter 1). In addition, while nest attendance was higher at

nests with younger males, the SY males contributed less to nestling provisioning than older

males did during both stages in the nestlings’ development. This suggests that females who

breed with second-year males may provide more parental care than females paired with

older, more experienced males. The body condition of female songbirds decreases rapidly at

120

the onset of brooding and remains low until the young fledge (Curlee and Beissinger 1995,

Sanz and Moreno 1995, Chastel and Kersten 2002). Incubation and nestling provisioning are

both energetically costly for females (Thomson et al. 1998, Reid et al. 2002), which suggests

that female Rose-breasted Grosbeaks who are mated with SY males may have much lower

body condition than females mated with more experienced males who provide more parental

care.

Conclusions

This study suggests that single-tree selection does not negatively influence Rose-

breasted Grosbeak’s ability to find food, provision their nestlings, or attend their nests in

hardwood forests. None of the response variables differed significantly among post-harvest

treatments in either study year, but they differed with stage of nestling development and with

age of male parent. Provisioning was less frequent and nest attendance was higher for < 5

day old nestlings than for > 5 day old nestlings. Fledglings from nests attended by younger

(SY) males weighed less than offspring from nests attended by older (ASY) males, and may

have been at a fitness disadvantage. Further research is needed to determine why younger

breeders were less effective at nestling provisioning.

GENERAL CONCLUSIONS

In the predominantly forested landscape of Algonquin Provincial Park, single-tree

selection did not appear to have a significant negative effect on the reproductive success

of Rose-breasted Grosbeaks. Density, pairing success, and the mean number of

fledglings per successful nest were significantly higher in more recently harvested stands

than in the > 50 years post-harvest stands, and nest success, fledgling survival, and

productivity were also higher in the harvested stands, although not significantly so.

These demographic trends partially support my prediction that single-tree selection would

benefit Rose-breasted Grosbeaks. However, the proportion of ASY males was slightly

higher in the > 50 years post-harvest treatment than in the other stands, which could

indicate that the “unharvested” stands provided higher quality territories, assuming that

older, more experienced birds select and successfully defend better quality territories than

younger, less experienced ones. Since overall density was significantly lower and the

proportion of ASY males was not significantly higher in the > 50 years post-harvest

treatment, it seems unlikely that the unharvested stands provided significantly better

habitat. Rather, ASY males may have been out-competing SY ones for a more limited

number of high quality territories in these stands than were available in other treatments.

The lack of agreement among demographic measures indicates that the factors

influencing Rose-breasted Grosbeak reproductive success are complex, and as a result,

relying on any single demographic measure to assess the effects of habitat management

may result in misleading conclusions.

Single-tree selection harvesting may not have a significant negative effect on

Rose-breasted Grosbeaks in predominantly forested landscapes, but population

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122

productivity was below replacement levels in all stands except in the 16-20 years post-

harvest stands. This suggests that forest management policies should be implemented

with caution in areas where the welfare of this species is of concern, at least until the

causes of the Rose-breasted Grosbeak’s decline can be identified. My study suggests that

the dense understory and sapling layers characteristic of forests regenerating after single-

tree selection harvesting may slightly increase Rose-breasted Grosbeak productivity, but

some other forest management practices may have more negative consequences. For

example, forest management policies intended to suppress natural disturbances such as

wildfires, flooding, and insect outbreaks have reduced the amount of early-seral forest

across North America (Askins 1993, Brawn et al. 2001, Trani et al. 2001, Brooks 2003,

DeGraaf and Yamasaki 2003). Creating forested landscapes which provide access to a

range of forest types and successional stages may be beneficial for mitigating the

population declines of the Rose-breasted Grosbeak and other disturbance-dependent

species, but silvicultural applications should be selected and implemented with caution.

This study was conducted in stands that were regenerating after their first harvest,

and some of them were reaching the stage when a second harvest could be carried out.

Silvicultural guidelines for tolerant hardwood forests in the states just south of the Great

Lakes suggest that a second harvest can be made using single-tree selection 8-15 years

after the first (Tubbs 1977), while guidelines for Ontario suggest waiting 15-20 years

(OMNR 1998). Although basal area was not significantly lower in the 16-20 or 21-25

years post-harvest stands than in the > 50 years post-harvest treatment, other measures of

forest structure were significantly different. Rose-breasted Grosbeak density and pairing

success were significantly higher in the 16-20 years post-harvest treatment than in the >

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50 years post-harvest stands, and pairing success was significantly higher in the 21-25

years post-harvest stands than in the “unharvested” stands. This suggests that neither

forest structure nor Rose-breasted Grosbeak demography had completely returned to

“pre-harvest” conditions even by 25 years after harvesting, and that shorter harvest

rotations may lead to cumulative changes in forest structure and avian ecology.

Specifically, basal area, which was positively associated with nest survival, was still

slightly lower in the 21-25 years post-harvest stands than in the > 50 years post-harvest

treatment. However, further study of forest structure and Rose-breasted Grosbeak

demography following the second cutting would be necessary to test for cumulative

changes.

Although the first single-tree selection harvest rotation did not appear to have a

significant effect on nest survival, my study suggests that basal area, sampling year, and

timing of nest initiation were important variables explaining nest survival, with nests built

in areas with higher basal area and started earlier in the season having a greater chance of

survival. The tendency for older, more experienced birds, which typically have higher

reproductive success, to begin nesting earlier may partially explain the positive

association between early breeding and nest survival. However, it is possible that the

hatching dates of young from nests initiated earlier in the season are more closely

synchronized with the emergence of insect prey, and parents at these nests may be able to

devote more time to nest defence rather than foraging. Many studies have observed a

positive correlation between earlier egg-laying dates and increasing temperatures (i.e.,

Crick et al. 1997, Crick and Sparks 1999, Dunn 2004, Torti et al. 2005). If nests that are

initiated earlier allow birds to take advantage of earlier insect peak abundances, this could

124

indicate that Rose-breasted Grosbeaks may be flexible enough to adapt to climate change

as well as to changes in forest structure resulting from forest management practices.

However, as this species is currently declining across its breeding range, potentially

negative effects of climate change, such as more variable weather conditions (IPCC

2002) or changes in the emergence time of insect prey should not be ignored.

When assessing the influence of climate change, habitat loss or degradation, forest

management practices, or other factors it is important to study both nest and fledgling

survival and habitat requirements. Although single-tree selection harvesting did not

significantly alter nest or fledgling survival, habitat characteristics at nests sites and

locations used by fledglings were significantly different, and patterns of nest and

fledgling survival among the post-harvest treatments were not strongly correlated. This

suggests that conclusions regarding the influence of various factors on the growth rates of

avian populations should not be based solely on estimates of nest survival.

My study also suggests that population age structure should be taken into

consideration in future studies of avian demography and population growth rates. Overall

productivity of after-second-year males was significantly higher than that of second-year

males, nestling provisioning was more frequent at nests with older, more experienced

males, and nestlings weighed more on average at the time of fledging in nests with older

males. After-second-year males also contributed more to nestling provisioning and nest

attendance than second-year males did. The density of ASY males was significantly

higher than the density of SY males in all post-harvest treatments, but if management

practices or other factors altered these proportions to favour more SY adults at the

expense of ASY breeders, there could be negative consequences for population

125

productivity. Variation in reproductive success between more and less experienced

parents should be taken into consideration when investigating the consequences of

management practices because all species may not exhibit the same patterns or responses.

There are several areas for improvement in this study. First, the population

models I used relied on estimates of annual adult mortality from the literature, which may

not have been appropriate for this particular species. Although accurate estimates of

annual survival are extremely difficult to obtain, future research should focus on

improving these estimates to allow more reliable assessments of population growth rates.

Second, increased sample sizes and a larger number of study sites would have improved

the statistical power of my results, and may have provided additional insight into

suggested results. Finally, investigations of nestling provisioning rates and

considerations of prey availability would have been complimented by direct measurement

of insect and fruit abundance.

Future research should focus on areas outside of Ontario, and should aim to

identify habitat types and areas where Rose-breasted Grosbeak populations are above

replacement levels. Although the population estimates derived in this study suggested

Rose-breasted Grosbeak populations were sinks in most post-harvest treatments, and

similar results were reported for forest fragments (Smith et al. 2006, Moore et al. 2010),

single-tree selection harvesting did not appear to be having a negative effect on Rose-

breasted demography. Future studies could also consider factors that might affect this

species during migration and on the wintering grounds, such as weather, climate change,

land-use change, and habitat loss or degradation. Long-term studies should also be a

priority for future research, to allow a better understanding of temporal variation in nest

126

survival and productivity, and determine how these parameters respond to selection

harvesting over the long-term.

This study suggests that single-tree selection harvesting can maintain and even

improve habitat for Rose-breasted Grosbeaks in predominantly forested landscapes if it is

strictly implemented according the guidelines set out by the Ontario Ministry of Natural

Resources. It should be emphasized that single-tree selection is not equally beneficial for

all migratory songbirds, and may even be detrimental to some forest interior species.

Nonetheless, for species such as the Rose-breasted Grosbeak, which benefit from habitat

features characteristic of regenerating forests, this system can provide an economically

and ecologically sustainable system of timber harvesting. These findings should be of

value to municipalities, conservation authorities, landowners, land managers, and

consultants involved in assessing or developing forest management plans, silvicultural

prescriptions, or stewardship plans.

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