LRH: Winter et al.

RRH: Does density response vary due to body size?

DOES BODY SIZE AFFECT A BIRD’S SENSITIVITY TO PATCH SIZE AND

LANDSCAPE STRUCTURE?

MAIKEN WINTER1,4, DOUGLAS H. JOHNSON2, AND JILL A. SHAFFER3

1 State University of New York, College of Environmental Sciences and Forestry, 1 Forestry

Drive, Syracuse, NY 13210, USA.

2 U.S. Geological Survey; Department of Fisheries, Wildlife, and Conservation,Biology 204;

Hodson Hall, 1980 Folwell Ave., University of Minnesota, St. Paul, MN 55108,

USA.

3 U.S. Geological Survey, Northern Prairie Wildlife Research Center, 8711 37th Street SE,

Jamestown, ND 58401, USA

4 Current address and address of the corresponding author: Cornell University, Laboratory of

Ornithology, 159 Sapsucker Woods Rd., Ithaca, NY 14850, USA. email:

mw267@cornell.edu; phone: (607) 254-2476

Winter

Abstract. Larger birds are generally more strongly affected by habitat loss and fragmentation

than are smaller ones because they require more resources and thus larger habitat patches.

Consequently, conservation actions often favor the creation or protection of larger over

smaller patches. However, in grassland systems the boundaries between a patch and the

surrounding landscape, and thus the perceived size of a patch, can be indistinct. Using eight

grassland bird species with different body sizes, we investigated whether these species

perceived variation in patch size and in landscape structure in a consistent manner. Data

were collected from surveys conducted on 44 patches of northern tallgrass prairie during

1998-2001. The response to patch size was very similar among species regardless of body

size (density gradually increased with patch size), except for Greater Prairie-Chicken

(Tympanuchus cupido), which showed a threshold effect and was not found in patches

smaller than 140 ha. In landscapes containing 0-30% woody vegetation, smaller species

responded more negatively to increases in woody vegetation than larger species, but above an

apparent threshold of 30% woody vegetation, larger species were not detected. Further

analyses revealed that the observed variation in the response to patch size and landscape

structure among species was not solely due to body size per se, but to other differences

among species. These results indicate that a stringent application of concepts that require

larger habitat patches appears to limit the number of grassland habitats that can be protected

and may not always be the most effective conservation strategy.

Keywords: Body size, conservation, grassland bird, landscape, patch size, umbrella

species.

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INTRODUCTION

Larger-bodied animals are known to have lower densities and larger home ranges than

smaller-bodied animals – a pattern that is consistent across many animal taxa (Schmid et al.

2000, Haskell et al. 2002). The main reasons for this pattern are greater energy needs of and

lower density of food resources for larger species (Jetz et al. 2004). It is thus not surprising

that larger species are generally more sensitive to habitat loss and fragmentation than are

smaller species (Haskell et al. 2002). Consequently, creating or maintaining larger habitat

patches is of high priority in many conservation plans. Because conservation plans for larger

species can also protect other species inhabiting the same habitat, larger species are often

used as umbrella species (Poiani et al. 2001, Suter et al. 2002). However, using larger-

bodied species to determine optimal patch size and landscape structure is complicated by

several factors:

1) A species’ patch-size requirement can vary with resource availability within a habitat

patch (Haskell et al. 2002) and with the structure of the surrounding landscape (Bakker et

al. 2002, Fletcher and Koford 2002, Cunningham and Johnson in press, Winter et al. in

press). It is therefore not possible to generalize patch-size requirements across even a

single species’ breeding range.

2) It is more difficult to collect reliable distribution data on rare species than on abundant

species (Seoane et al. 2005). Data on the distribution of smaller species – if they are

more abundant – might therefore be a more reliable indicator of habitat needs of a

community..

3) The size of a habitat patch as defined by humans might not reflect an animal’s perception

of a patch. This is especially true for species that inhabit open habitats such as native

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grasslands within agricultural areas, where the edge of a patch is less clearly defined than

in patches of other habitat types.

Grassland birds have been the focus of intensive research and conservation because of

their consistent and steep population declines in many parts of the world (Peterjohn and

Sauer 1999, Donald et al. 2000, Newton 2004). In North America, the Bird Conservation

Area (BCA) concept (Henderson and Sample 1995; <http://www.iowadnr.com/wildlife/files/

BCA_index.html> [20 November 2005]) involves guidelines for grassland bird conservation:

ideally, managers should establish large core areas (> 700 ha) of high-quality grassland

habitat surrounded by larger expanses of non-forested habitats, such as smaller-grain

cropland or hayland. This habitat configuration is based on the requirements of Greater

Prairie-Chicken (Tympanuchus cupido), a species that is mostly absent from smaller

grassland patches and from patches that are surrounded by forested habitat (Ryan et al. 1998,

Merrill et al. 1999, Winter and Faaborg 1999), and therefore serves as an umbrella species

for the protection of many other grassland birds. However, even though it is desirable to

protect larger habitat patches in the form of BCAs, such protection might not be the most

cost-effective practice with which to protect grassland birds throughout their range (Davis

2004, Winter et al. in press).

Recently, we demonstrated that two passerine species nesting in northern tallgrass

prairies did not respond consistently to the size of grassland patches, whereas an increasing

percentage of woody vegetation surrounding a grassland patch consistently resulted in lower

species’ densities (Winter et al. in press, also see Grant et al. 2004). The lack of a consistent

patch-size effect may have been caused by the smaller home-range sizes of the focal species.

In contrast, larger-bodied non-passerine species might respond more strongly to patch size as

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well as to the structure (e.g., percentage of trees and tall shrubs) of the surrounding

landscape. If larger species were more sensitive to patch size and the percentage of woody

vegetation structure, then a conservation focus on larger prairie patches would be justified.

However, if such a pattern were not generally true, and larger species would also use smaller

habitat patches, then the conservation of smaller prairie patches could benefit both smaller

and larger species. If in certain settings smaller and larger species responded similarly to

variation in patch size, then it would be more efficient to monitor smaller species (found at

higher densities) to make management decisions regarding larger-bodied species. In this

paper we determined whether eight species of tallgrass prairie birds, four passerines and four

non-passerines, showed similar responses to variation in patch size and percent woody

vegetation in the landscape, and whether differences among species were largely the result of

variation in body size.

METHODS

Between 1998 and 2001 we conducted bird surveys in 44 study plots located in three regions:

within Sheyenne National Grassland in southeastern North Dakota and two regions in

northwestern Minnesota close to the cities of Crookston and Glyndon (for a detailed

description of study sites and methods see Winter et al. 2005a, b). Study plots were located

within prairie patches that ranged between 2.4 and 1245.6 ha ( x = 214.0 ha), and the

percentage of woody vegetation within a 200-m buffer of the study plots ranged between 1

and 46% ( x = 13.1 %). The sizes of prairie patches were not strongly correlated with the

percentage of woody vegetation within a 200-m buffer (r = -0.20, P = 0.18, n = 44). The size

of study plots within the fragments varied between 1.5 and 20 ha ( x = 10.5 ha), depending on

the size of the prairie patch within which the study plot was embedded. Study plots were

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marked with flags or wooden laths at 50-m intervals along transects that were 100 m apart.

Bird censuses were conducted twice a year between the end of May and early July between

05:00 and 10:00 CDT, and under conditions of low-to-moderate wind speeds (≤ 35 km/h), no

precipitation, and good visibility. The annual maximum count of a species on a plot was

used to determine its annual density (number of males/100 ha). A single observer conducted

all counts within a year, with two observers (MW and JAS) alternating in successive years.

As an index of body size, we used a species’ mass (g) that was derived either from

our own study or from studies closest to our sites (Table 1). Species are: Le Conte’s Sparrow

(Ammodramus leconteii), Savannah Sparrow (Passerculus sandwichensis), Bobolink

(Dolichonyx oryzivorus), Western Meadowlark (Sturnella neglecta), Northern Harrier

(Circus cyaneus), Upland Sandpiper (Bartramia longicauda), Marbled Godwit (Limosa

fedoa), and Greater Prairie-Chicken. For each species, we used male body size for analysis.

A contiguous grassland patch was defined following Winter et al. (2005a). The size

of the grassland patch in which study plots were located was determined from digital

orthophoto quads (DOQs) that were imported into Map and Image Processing System,

versions 6.6 and 6.7 (<http://www.microimages.com> [20 November 2005]). The DOQs

were from 1991 for Minnesota and from 1997 for North Dakota. We also used these images

to determine the percentage of shrubs and trees within 200-m and 1-km buffer zones, where

buffer zones included the study plot. Within a 200-m buffer zone the percentage of shrubs

and trees was highly correlated with the percentage of shrubs and trees within a 1-km buffer

zone (r = 0.78, P < 0.001, n = 44 study plots). Assuming that the percentage of shrubs and

trees closest to the study plots had a larger effect on the distribution of birds within the study

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plots than the percentage of shrubs and trees at farther distances, we used only the percentage

of shrubs and trees within a 200-m buffer zone in all analyses and refer to it as “landscape.”

STATISTICAL ANALYSES

Because models included both fixed effects (body size, patch size, and landscape) and

random effects (prairie), we analyzed data with PROC MIXED in SAS 8.2 (SAS 1999,

Littell et al. 1996). Density data were log-transformed in all analyses with logcount =

ln(count +1). Maximum likelihood was used to fit models, in order to appropriately penalize

AIC values for increasing numbers of both random and fixed effects. Previous analyses

indicated that grassland bird density varied greatly among prairies (Winter et al. 2005b). We

therefore included prairie as random effect in each of the statistical models.

To reduce the number of models tested, we conducted our analyses in two steps

(Winter et al. 2005a, b). In a first step, we determined if body size (continuous variable)

influenced the effect of patch size on bird density by comparing five models with the

following sets of explanatory variables: (1) patch size; (2) body size; (3) patch size and body

size; (4) patch size × body size; and (5) patch size, body size, and patch size × body size. We

then used the body-size model that best fit the data, based on Akaike’s Information Criterion

corrected for smaller sample sizes (AICC: Burnham and Anderson 2002), as the base model

for a second set of analyses. If more than one model was supported (∆AICC < 4), then we

used the most inclusive model as the base model, i.e., the model that included the largest

number of variables. In the second step we determined if the density response to the

percentage of woody vegetation in the surrounding landscape was influenced by body size –

given the previously determined effect of patch size – by comparing four models: (1) base

model (from step 1); (2) base model with landscape variable; (3) base model with landscape

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× body size interaction; and (4) base model with landscape and landscape × body size

interaction. From these, we determined the best-supported model as the final model.

Variation in the response of density to patch size and the percentage of woody

vegetation structure among birds with different body sizes might not be due to body size per

se, but possibly due to differences in ecology and behavior. We investigated this possibility

using a similar two-step analysis as described above, but included species as a categorical

variable instead of body size as a continuous variable. We compared AICC between the body

size and species models, and graphically illustrated the outcome of the two sets of analyses

(body size and species models) by plotting the estimated densities derived from each base

and final model (Fig. 1). The magnitude of the response to patch size and landscape structure

seems much greater in species that have high densities than in rare species, because absolute

changes in numbers and thus slopes are greater in abundant species. These absolute changes

in density do not give a clear impression of the magnitude of patch size and landscape effects

relative to other species. We therefore also plotted the relative changes in density between

maximum and minimum patch size or percentage woody cover by dividing the estimated

maximum by the estimated minimum densities as derived from the base or final models,

separately for each body size and species model (Fig. 2).

RESULTS

Models that included species as a categorical variable fit the data better than models that

included body size (∆AICC > 600). Thus, changes in density with patch size and landscape

structure cannot be predicted based simply on body size, but vary with the species in

question. For example, the body-size model predicted that the three smallest species would

respond similarly to changes in landscape structure (Fig. 1b); in reality, Savannah Sparrows

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and Bobolinks were more strongly affected by landscape structure than were Le Conte’s

Sparrows (Fig. 1d, 2).

Patch size had a negligible effect on bird density (slope: 0.0006 ± 0.0002), especially

at patch sizes below 500 ha (Fig. 1a). A 1000-ha increase in patch size only roughly doubled

the density of most species (Fig. 1a, c). Even larger-bodied species (Northern Harrier,

Marbled Godwit, and Upland Sandpiper) occurred in relatively small patches (<20 ha) with

the exception of Greater Prairie-Chicken: this species was absent from prairie patches smaller

than 140 ha. Neither body size nor inter-specific variation had much influence on this

response to patch size: the best-supported body size and species models did not include an

interactive effect with patch size (Table 2).

The addition of landscape to patch-size models greatly improved the fit of both the

body size and species models (∆AICC = 26 and 27, respectively). Density decreased with

increasing percentage of woody vegetation within a 200-m buffer (Fig. 1b, d). The estimated

rate of this decrease did not vary with body size, but did vary by species (Table 2, Fig. 2).

Smaller species appeared more sensitive to changes in the percentage of woody vegetation,

because a 50% decrease in woody cover within a 200-m buffer caused an increase in density

that varied by 3-25% in smaller species, whereas the same decrease changed the density of

any larger species by less than 2.2% (Fig. 2). However, if woody cover exceeded 30%

within a 200-m buffer, larger species disappeared. Thus, an apparent threshold in woody

cover exists for larger species, above which these species did not occur.

DISCUSSION

Our results showed that we cannot assume that larger species always require larger habitat

patches and more open landscapes than smaller species, because models that included body

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size received less support than models that included species. In fact, within a certain range of

percentage woody cover, smaller species were more strongly affected by changes in

landscape than larger species. Thus, in some circumstances, providing suitable habitat for

smaller-bodied species can also ensure the protection of larger-bodied species. A single,

larger-bodied umbrella species, therefore, might not always be the ideal subject to guide

community-wide conservation decisions (Andelman and Fagan 2000, Roberge and

Angelstam 2004), but management recommendations need to be adapted to the specific

settings of the area to be managed.

Our findings have several important implications to the conservation of grassland

birds in northern tallgrass prairies, and might also be applicable to other systems:

(1) In open landscapes, patch size variation is of relatively little importance even for larger-

bodied species. All species but Greater Prairie-Chicken were found in prairie patches smaller

than 20 ha. Thus, the protection and management of even smaller prairie patches can be

worthwhile not only for passerine birds (Davis 2004, Winter et al. in press), but also for

several larger-bodied, non-passerine species. However, to ensure the occurrence of Greater

Prairie-Chicken, some grassland patches need to be at least 140 ha in size even if the patch is

located in relatively open landscape.

(2) The response to patch size can be very similar among species. Thus, using larger-bodied

species as umbrella species might not always be necessary. Relying on the response of

smaller species instead would have the benefit of more accurate density estimation, because

larger animals are estimated with lower accuracy (Seoane et al. 2005).

(3) Even a seemingly small amount of woody vegetation can greatly reduce the number of

individuals of a species present in a grassland patch. Any reduction of woody vegetation can

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thus be beneficial to grassland birds. This observation seems to constitute a general pattern

for grassland birds in the northern prairies, since studies elsewhere reported similar results

(Bakker et al. 2002, Fletcher and Koford 2002, Grant et al. 2004, Cunningham and Johnson

in press).

Related to this last point, a reduction of woody vegetation surrounding prairie patches

would also ensure safer nesting sites. Nesting success may be lower for nests placed close to

woody vegetation (Temple and Johnson 1990, Winter et al. 2000). In addition, Herkert et al.

(2003) showed that grassland bird nesting success is higher in larger fragments. In the

current study, we were not able to compare nesting success between passerine and non-

passerine species because of small sample sizes for non-passerine nests. However, analyses

of the effect of patch size and landscape on nesting success of several passerines did not

indicate any influence of these factors on nesting success in northern tallgrass prairie (Winter

et al. in press), indicating that in open landscapes nesting success may not be as severely

influenced by patch size as in more forested regions.

Effective grassland bird conservation requires a balance between the time and money

required and the degree to which high-quality refuges for grassland species can be obtained.

In an open agricultural landscape such as our study area, we suggest that a mix of both

smaller and larger (>140 ha) prairie patches will likely support all grassland-nesting species.

Certainly, the protection of larger continuous areas will always be optimal. However, a

stringent application of concepts that involve only larger habitat patches appears to limit the

number of grassland habitats that can be protected and may not always be the most effective

conservation strategy during times of limited financial support for conservation.

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ACKNOWLEDGMENTS

We thank the more than 40 field assistants whose efforts we greatly appreciate. W. D.

Svedarsky of the University of Minnesota in Crookston, R. Julian of the U.S. Fish and

Wildlife Service, B. M. and S. Winter and the late G. Yalch of The Nature Conservancy, and

B. Stotts of the U.S. Forest Service provided logistical help. The Nature Conservancy, U.S.

Fish and Wildlife Service, USDA Forest Service, and the Minnesota Department of Natural

Resources allowed us access to the study sites. J. E. Austin, the late D. P. Fellows, P. J.

Pietz, and M. A. Sovada of the Northern Prairie Wildlife Research Center loaned equipment

and supplies. B. R. Euliss provided the patch size and data on the percentage of woody

vegetation. The Cornell Laboratory of Ornithology provided working facilities for the first

author. The manuscript was greatly improved by comments from Todd Grant, Wesley

Hochachka, and … The study was funded by the U.S. Geological Survey; the U.S. Fish and

Wildlife Service, Regions 3 and 6; the American Bird Conservancy; and the Northwest

Research and Outreach Center at the University of Minnesota, Crookston.

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Table 1. Male body size of focal species, ordered by increasing body size.

Species Abbreviation Body size (g) n Source

Le Conte’s Sparrow LCSP 12.8 14 Winter et al., unpubl. dataSavannah Sparrow SAVS 18.0 101 Winter et al., unpubl. dataBobolink BOBO 33.7 19 Winter et al., unpubl. dataWestern Meadowlark WEME 115.3 53 Maher 1979Upland Sandpiper UPSA 151.0 38 Dorio 1977Marbled Godwit MAGO 326.0 43 Gratto-Trevor 2000Northern Harrier NOHA 336.0 93 Bildstein 1988Greater Prairie-Chicken GPCH 933.0 318 Horak 1985

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Table 2. Models that best describe variation in density of eight grassland-nesting birds in the

northern tallgrass prairies of Minnesota and North Dakota, 1998-2001. Prairie was included

in each model as random effect because previous analyses indicated that density varies

greatly among prairies (Winter et al. 2004). The most inclusive patch-size model with a

ΔAICc < 4 was used as the base (BASE) for the landscape model, and the most inclusive

landscape model was designated as the final model (FINAL).

Variables included ΔAICc

Akaike

weight

Body size and Patch sizeBody size: BASE 0.0 0.99Body size, Patch size 8.7 0.01Body size, Patch size, Body size × Patch size 34.9 0.00Body size × Patch size 278.9 0.00Null 336.8 0.00Patch size 345.5 0.00Body size and LandscapeBody size, Landscape: FINAL 0.0 0.98Body size, Landscape, Body size × Landscape 8.2 0.02Body size 26.5 0.00Body size × Landscape 265.4 0.00Null 363.2 0.00

Species and Patch sizeSpecies: BASE 0.0 0.99Species, Patch size 8.7 0.01Species, Patch size, Species × Patch size 98.7 0.00Species×Patch size 98.7 0.00Null 1091.1 0.00Patch size 1099.9 0.00Species and LandscapeSpecies, Landscape, Species × Landscape: FINAL 0.0 1.00Species, Landscape 23.9 0.00Species 50.8 0.00Species × Landscape 966.8 0.00

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FIGURE LEGENDS

Figure 1. Estimated densities of bird species nesting in northern tallgrass prairies between

1998 and 2001 increase (males/100 ha ± SE) with patch size (a and c) and decrease with

increasing percentage of woody vegetation within a 200-m buffer zone (b and d). Variations

in these patch size and landscape effects are better explained by inherent differences among

species (c and d) than by body size per se (a and b). Estimated values were derived from the

base and final models shown in Table 2. For species’ name abbreviations see Table 1.

Figure 2. Relative change in species’ density with percentage of woody vegetation

(estimated density at lowest woody percentage / estimated density at highest woody

percentage) in northern tallgrass prairies, 1998-2001. For species’ name abbreviations see

Table 1.

10 100 1000

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