Breeding Ecology of American Kestrels on the Llano Estacado ...

Breeding Ecology of American Kestrels on the Llano Estacado of Texas

by

Shea D’Lynne Mullican, B.S.

A Thesis

in

Wildlife, Aquatic, and Wildlands Science and Management

Submitted to the Graduate Faculty

of Texas Tech University in

Partial Fulfillment of

the Requirements for

the Degree of

MASTER OF SCIENCE

Approved

Dr. Clint Boal

Chair of Committee

Dr. Warren Conway

Dr. Richard Stevens

Mark Sheridan

Dean of the Graduate School

December, 2018

Texas Tech University, Shea Mullican, December 2018

ii

ACKNOWLEDGMENTS

There are many wonderful people, without whom, none of this research would

have been possible. To my advisor, Dr. Clint Boal: thank you for your expertise,

guidance, constructive criticism, incredible patience, and life lessons. I would like to

thank my committee members, Dr. Warren Conway and Dr. Richard Stevens, for their

reassurance, direction, and sincerity when working with me. I would like to thank the

research technicians who assisted me in the field including Grant Kilcrease, Sarah

Whitson, Jovaan Rue, Madeleine Thornley, and Jonathan McClellan. Many thanks to the

volunteers who graciously spent countless hours reviewing kestrel video footage: Stephen

Kielius, Maggie Meier, Jennifer Long, and Casper Mackenzie. I would also like to thank

Byron Buckley for his friendship over the years and always coming to my aid when I

needed it. Most of all, I’d like to thank my loving husband, Tyrel Mullican, for his

tolerance, hugs, daily encouragement, and electrical know-how.

Sandia National Labs and the Texas Tech University National Wind Institute

provided the funding for this research. The Association of Natural Resource Scientists at

Texas Tech University (hereafter TTU), the Department of Natural Resources

Management at TTU, and the TTU Graduate School provided travel funds for

conferences.


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TABLE OF CONTENTS

ACKNOWLEDGMENTS... ............................................................................................... ii

LIST OF TABLES ............................................................................................................. iv

LIST OF FIGURES .............................................................................................................v

I. AMERICAN KESTREL NESTING ECOLOGY ON THE LLANO

ESTACADO

Introduction ..................................................................................................................... 1

Methods........................................................................................................................... 3

Study Area ................................................................................................................... 3

Field Methods .............................................................................................................. 7

Results ............................................................................................................................. 9

Discussion ..................................................................................................................... 14

Literature Cited ............................................................................................................. 19

II. ADULT SURVIVAL AND FIDELITY ESTIMATES OF AMERICAN KESTRELS

NESTING IN ARTIFICIAL NEST BOXES IN LUBBOCK COUNTY TEXAS

Introduction ................................................................................................................... 32

Methods ......................................................................................................................... 34

Study Area ................................................................................................................. 34

Field Methods ............................................................................................................ 38

Analysis ..................................................................................................................... 38

Results ........................................................................................................................... 40

Adult Survival ........................................................................................................... 40

Mate Fidelity.............................................................................................................. 40

Site Fidelity................................................................................................................ 40

Discussion ..................................................................................................................... 41

Literature Cited ............................................................................................................. 44


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LIST OF TABLES

1.1. Occupancy of nest boxes and number of nest attempts by American kestrels in

Lubbock Co., TX from 2013-2018. ........................................................................... 22

1.2. Comparison of nest attempts, clutch size, brood size, fledglings/brood, nest

success, and sex ratio among early and late nesting attempts by American

kestrels nesting in nest boxes in Lubbock County, TX from 2013–2018. ............... 23

1.3. Comparison of nest initiation dates between years of American kestrels nesting

in nest boxes in Lubbock County, TX from 2013–2018. .......................................... 24

1.4. Examination of nest initiation phenology of marked adult kestrels that were

captured in consecutive years in Lubbock, County, TX from 2015 to 2017. ............ 25

1.5. S(clutch) was chosen as the best fit model from 15 originally created to assess

American kestrel nest survival from nest data in Lubbock County, TX from

2013−2018. S(clutch) had the lowest AICc value while the next best model

S(constant) had an AICc value of 114.592. ............................................................... 26

1.6. Comparison of American kestrel productivity among published studies. ................. 27

2.1. Adult kestrel age classes based on capture-recapture/resight data in Lubbock

County, TX from 2013−2018. The majority of banded birds were only caught

once. The oldest birds breeding within the nest box program are at least 5 years

old. ............................................................................................................................. 47

2.2. Status of American Kestrels nesting in successive years with marked mates in

nest boxes in Lubbock County, TX from 2013–2018. ............................................. 48

2.3. Number of years that male and female American kestrels demonstrated site

fidelity by using nest boxes within 20 active territories in consecutive years in

Lubbock County, TX from 2013−2018. .................................................................... 49


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LIST OF FIGURES

1.1. Location of the study area in relation to the Llano Estacado. .................................... 28

1.2. Location of three study sites in Lubbock County, TX used to assess American

kestrel breeding ecology. The green dots represent individual nest box

locations. Site "A" is the Lubbock Lake Landmark site, "B" is the Reese

Technology Center site, and “C” is the Texas Tech University Native

Rangeland site. The average distance of the nearest nest box at site “A” is

241.2 m, 608.3 m at site “B”, and 84 m at site “C”. .................................................. 29

1.3. Daily Survival Rates (DSR) modelled in Program MARK from American

kestrel nest data in Lubbock County, TX from 2013−2018. The model,

S(clutch), demonstrated that larger clutch sizes have higher daily survival

rates. The error bars represent the standard error associated with each estimate. ..... 30

1.4. Nest survival modelled in Program MARK from American kestrel nest data in

Lubbock County, TX from 2013−2018. The model, S(clutch), demonstrated

that larger clutch sizes are more likely to survive the nesting period. The error

bars represent the 95% confidence intervals associated with each nest survival

estimate. ..................................................................................................................... 31

2.1. Location of the study area in relation to the Llano Estacado. .................................... 50

2.2. Location of three study sites in Lubbock County, TX used to assess American

kestrel breeding ecology. The green dots represent individual nest box

locations. Site "A" is the Lubbock Lake Landmark site, "B" is the Reese

Technology Center site, and “C” is the Texas Tech University Native

Rangeland site. The average distance of the nearest nest box at site “A” is

241.2 m, 608.3 m at site “B”, and 84 m at site “C”. .................................................. 51


1

CHAPTER I

AMERICAN KESTREL NESTING ECOLOGY IN THE LLANO ESTACADO

INTRODUCTION

As a colorful and charismatic raptor, American kestrels (Falco sparverius) easily

catch the interest of both scientists and the birding community. The American kestrel

(hereafter, kestrel) is North America’s smallest falcon and is considered the most

numerous and widespread falcon on the continent (Smallwood and Bird 2002). Kestrels

have a wide geographic distribution and can also be found in Central and South America.

This falcon, like many raptors, displays reversed sexual size dimorphism, with the female

being approximately 10% heavier than males (Smallwood and Bird 2002). As a

secondary cavity nester, kestrels will nest in cavities excavated by woodpeckers or take

advantage of natural cavities within trees (Smith et al. 1972). Suitable nesting cavities

limit the breeding range of kestrels, but the expansion of woody species has allowed

kestrels to expand their breeding range into the Great Plains which provided exceptional

hunting opportunities (Smallwood and Bird 2002). The placement of artificial nest boxes

also provide kestrels breeding season access to habitat that they may otherwise not

occupy (Smallwood and Bird 2002).

Populations of kestrels have been declining over the last few decades in North

America. It is estimated that kestrel populations in North America are declining by 47%

based on collective data from nest box monitoring programs, USGS Breeding Bird

Surveys, Christmas Bird Counts, and raptor migration counts from 1966 to 2010

(American Kestrel Partnership, hereafter AKP, 2018; Farmer and Smith 2009;

Smallwood et al. 2009). Research has yet to identify a definite cause for the declines, but


2

it is most likely a combination of biological and ecological factors such as landscape

changes leading to increased human-caused stress in disturbed areas (Strasser and Heath

2013), climate change (Steenhof and Peterson 2009), chemical exposure (Mitra et al.

2011), loss of suitable nesting habitat, competition for nesting cavities, and disease

(Smallwood and Bird 2002). The shortgrass prairie on the High Plains, once experiencing

kestrel population growth, now has an uncertain population trend (Bird Conservation

region 18; AKP 2015; AKP 2018; North American Bird Conservation Initiative 2016).

Despite the conservation concern for the species, few studies have been conducted on the

kestrel population in the High Plains.

Aspects of kestrel ecology can easily be studied through nest box monitoring

programs as nest boxes enable researchers to closely examine changes in reproduction

and population size over time. Thus, nest box monitoring programs can be an important

component in understanding overall population trends that have been observed in the past

decades across North America. Linner (2014) reported a year-round average kestrel

density of 2.5 birds/km2 in Lubbock and Hockley counties, Texas. Kestrel populations in

areas with nest box programs, however, are likely artificially elevated above what would

be normal for the area. For example, 32 kestrel nest boxes were erected in 2013, centered

on the Reese Technology Center in Lubbock County, Texas, as part of a long-term

monitoring project (Linner 2014). In May 2017, the breeding kestrel density on the Reese

Technology Center was approximately 5 birds/km2 (unpub. data).

Despite nest box programs artificially increasing local populations, they still

allow assessment of population parameters, such as reproductive success and survival

within the area. Productivity and survival are contributing factors to understanding


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overall population trends. According to the American Kestrel Partnership (2016), the next

closest nest box program to that in Lubbock TX is outside of Dallas, >480 km east, which

suggests that nest boxes are not driving regional population trends. More likely, the

presence of kestrels in the region are influenced by anthropogenic structures and

agroforestry practices that provide nesting and or roosting locations and hunting perches.

Therefore, a study of kestrels using a relatively small number of nest boxes is likely an

accurate representation of kestrel populations trends in the Southern High Plains of

Texas.

I conducted research to better understand the nesting ecology of American

kestrels in the Llano Estacado region of the Southern High Plains of Texas. My

objectives were to determine reproductive parameters of nesting success, productivity,

and nesting phenology of the local population, and compare the results with those

reported in the literature for other populations.

METHODS

Study Area

The Llano Estacado, also referred to as the Southern High Plains, is considered

the southern tip of the larger Great Plains (Figure 1.1). The Llano Estacado is in

Northeast New Mexico and the panhandle of Texas, which lies within the Bird

Conservation Region 18 (North American Bird Conservation Initiative 2016). The area is

characterized by high flat land that lies between the Caprock Canyon to the east in Texas

and Mescalero Ridge to the west in New Mexico (Leatherwood 2010). While the

southern portion lacks a distinct physical boundary, the Johnson Creek branch of the

Colorado River is considered the lower extent of the Llano Estacado (Leatherwood


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2010). The southern ridge of the Canadian River valley delineates the north border

(Leatherwood 2010).

My study area is Lubbock County, TX which is centrally located within the Llano

Estacado; it is an area of relatively level land with 0-3% of gentle slopes at an elevation

varying around 975 m (Soil Survey Staff 2015). Lubbock County has a semiarid climate

with an average annual precipitation of 467.6 mm, the majority of which occurs between

May and October (Graves 2010). The maximum summer temperature in July averages

33.3° C and the minimum winter temperatures average -3.9° C in January (Graves 2010).

Lubbock County is part of the southern shortgrass prairie in the High Plains,

characterized as a short and mixed grass prairie composed primarily of blue grama

(Bouteloua gracilis) and buffalograss (Bouteloua dactyloides), and dotted with numerous

playas and few shrubs (Diamond 2010; Graves 2010).

Compared to its historic prairie state, the Llano Estacado is now a very different

ecosystem (Leatherwood 2010). Anthropogenic development of the landscape, both

urbanization and agricultural development, altered the vegetative community from

historic continuous prairie to the fragmented landscape of primarily irrigated cropland

and urban housing (Diamond 2010). Today, the majority of land in Lubbock County is

cropland, dispersed farmhouses, and small towns and cities. Cropland accounted for

approximately 73% of landcover in Lubbock County (1715 km2 of 2333 km2 total)

(USDA 2012), which is known for producing cotton, despite occasional droughts (Graves

2010). Of the total cropland acreage in 2012, 48.5% were cotton fields (205,581 acres of

423,830 total cropland acres; USDA 2012). Additionally, fire suppression allowed woody

species, such as oak (Quercus spp.), cedar (Juniperus spp.), elm (Ulmus spp.), and


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mesquite (Prosopis spp.), to expand their distributions and invade parts of the High

Plains (Leatherwood 2010). The expansion of these woody species allowed secondary

cavity nesters, such as the American kestrel, to also expand their breeding range

(Smallwood and Bird 2002).

My study area consists of three sites, the Lubbock Lake Historic Landmark, the

Reese Technology Center, and the Texas Tech Native Rangeland (Figure 1.2). All of the

sites are classified as rangeland ecological sites of differing transitional stages (Soil

Survey Staff 2015). Transitional stages refer to the seral stages of succession which is

described as a gradient for the three sites. Within these three sites, nest boxes were placed

in remnant native prairie communities that are bordered by asphalt roads/highways,

runways, farmlands, golf courses, or urban neighborhoods. All three sites have different

amounts of prairie coneflower (Ratibida columnifera) and other annual forbs, cacti

(Opuntia spp.), honey mesquite (Prosopis glandulosa), elm, yucca (Yucca spp.), and

native grasses. The study sites support numerous wildlife species of interest and concern

such as the Texas horned lizard (Phrynosoma cornutum), black-tailed prairie dog

(Cynomys ludovicianus), burrowing owl (Athene cunicularia), prairie falcon (Falco

mexicanus), and ferruginous hawk (Buteo regalis) (Texas Parks and Wildlife Department

2016).

The Lubbock Lake Historic Landmark (Figure 1.2; hereafter, Lubbock Lake) is a

136 ha area in central Lubbock County. The site is primarily managed as an

archaeological and educational center. Of the three sites, Lubbock Lake most closely

resembles a pre-European native prairie ecosystem. Lubbock Lake has several hiking

trails scattered throughout the property with varying topography and is similar to the


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Reese Technology Center site with a primarily shortgrass/blue grama dominant

vegetation community with some woody species present. Lubbock Lake is open to the

public so this site experiences low to moderate levels of human activity on the trails

throughout the year.

The largest study site is the Reese Technology Center (Figure 1.2; hereafter

RTC), a decommissioned United States Air Force base on the west side of Lubbock

County, TX. The RTC has >700 ha with the majority of the vegetation classified as a

shortgrass/blue grama dominant vegetation community (Soil Survey Staff 2015). The

RTC falls in between Lubbock Lake and the Texas Tech Native Rangeland in terms of

seral stages of succession. Concrete runways, one industrial sized turbine and three scaled

wind turbines, playas, and black-tailed prairie dog colonies are present within the

landscape. The RTC experiences daily amounts of manufacturing and machinery activity

mostly due to Zachary and Forterra, an electrical manufacturing company and a water-

related infrastructure manufacturer, respectively. Most of this activity occurs on the

perimeter of RTC.

The Texas Tech Native Rangeland (Figure 1.2; hereafter, Rangeland) is 55 ha in

central Lubbock County. The site encompasses a playa and is comprised of dense honey

mesquite shrubs, broom snakeweed (Gutierrezia sarothrae), threeawn (Aristida spp.),

Russian thistle (Salsola kali), ragweed (Ambrosia spp.), and gramas (Bouteloua spp.)

(Soil Survey Staff 2015; S. Mullican pers. obs.). The Rangeland study site can be

described as a remnant native prairie that has been suppressed of fire and intensely

invaded by mesquite. The density of mesquite and abundance of threeawn, can be used as

an indicator of poor quality rangelands (Stubbendieck 2011). The Rangeland is the most


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degraded ecosystem of the three sites. Graduate and undergraduate students at Texas

Tech University conduct research at the Rangeland site, but it is not open to the public.

Field Methods

I used a total of 29 nest boxes that were distributed among the three study sites:

the RTC site has 17 nest boxes, there are 8 are on the Rangeland site, and 4 are on

Lubbock Lake. I considered a nest attempt as initiated when one egg had been laid. I

recorded initiation date, clutch size, brood size, sex ratio, and number of fledglings for all

nests, when possible. Nest boxes were checked every two weeks for possible initiation of

nests starting in March of each year from 2013–2018. A 10-foot ladder was used to

access nest boxes. Once the breeding season was underway, I increased nest check

frequency as more eggs were laid and began to hatch. I defined brood size as the number

of chicks that successfully hatched per nest attempt. Nests were then checked every 5-7

days until the oldest chick reached 14 days old. I used a photographic key to age chicks

(Klucsarits and Rusbuldt 2007). Once the oldest chick reached 14 days of age, I increased

nest check frequency to every 3-5 days. The increased frequency of nest checks was

needed to ensure that the chicks were aged accurately. Once the youngest chick reached

18 days old, the entire brood (18 – 22 days old) was hand-captured from the nest box and

each nestling was banded on its right leg with a USGS numeric aluminum leg band.

Procedures followed the AICUC protocol #16034-04 and TTU AICUC SOP 043. I

recorded sex ratio of broods during banding as kestrels are also dimorphic in plumage as

nestlings. I considered a nesting attempt as successful when at least one chick in a brood

reached 80% of fledging age (Steenhof and Newton 2007); this is approximately 24 days

old for American kestrels. This method of determining nest success is an acceptable


8

standard for raptor ecologists, as it is the age after which mortality is lowest for nestlings

prior to leaving the nest (Steenhof and Newton 2007). Nest failures occurred when eggs

did not hatch or if chicks did not survive to fledging age. The number of fledglings was

defined as the number of nestlings per nest attempt that reached 24 days old (80% of

fledging age).

I determined box occupancy rate as the proportion of all nest boxes available in a

given year to the number of boxes that were used (counting any box with 2 nest attempts

in a single season only once). Incubation periods average about 30 days (Bird and Palmer

1988), and female kestrels lay a single egg every other day and incubation usually starts

when the penultimate egg is laid (Bird and Palmer 1988). Thus, I estimated initiation as

30 days prior to the hatching date, plus 3, 5, 7, 9, or 11 days for clutch sizes of 2, 3, 4, 5,

or 6, respectively. I calculated nest initiations by back-dating of the oldest chick, either at

hatching day (if observed) or at the age when the oldest chick was banded, in addition to

the incubation and egg laying period. I calculated the range, median, and average nest

initiation dates across and within each year. Nest initiation, clutch size, brood size,

number of nests, and number of fledglings were averaged over the entire year (including

first and second nesting attempts) for known-fate nests only, unless otherwise stated.

Means were reported with one standard deviation, unless otherwise stated. I defined early

nesting attempts as those that were initiated between March 5th and April 30th. I defined

late nesting attempts as those that were initiated between May 1st and June 30th,

regardless of whether it was an individual’s “first” or “second” nest. Second broods

(including true double-broods) were rarely observed and were not specified in my

analyses as it wasn’t possible to verify every “second” nest as a true second nesting


9

attempt. Although some adult kestrels were marked, it was difficult in subsequent years

to recapture or resight previously captured adults.

I used JMP Pro (SAS Institute 2015) for Microsoft Windows to examine means of

clutch size, brood size, number of fledglings per successful nest, and initiation date.

Analysis of Variance (ANOVA) was initially used to test for significance in reported

means. I used Tukey-Kramer HSD to test where these differences occurred in the

reported means.

I modeled daily nest survival rates (DSR) in Program MARK with clutch size,

precipitation, maximum temperature, nest initiation date, year, and study site as possible

covariates. The average maximum temperature for the two months a nest existed, along

with sum of precipitation, were gathered online from the National Oceanic and

Atmospheric Administration (hereafter, NOAA). Kestrels are altricial, meaning that the

chicks require parental care before they can leave the nest. Nest success for altricial

species occurs when at least one offspring from the brood fledges. Therefore, I

considered the “nestling period” for analysis in Program MARK to include both the egg

and nestling stage: which is approximately 60 days for kestrels. I calculated the

probability of a nest surviving (initiation to completion) by raising the daily survival rate

(hereafter, DSR) to the nestling period.

RESULTS

Kestrel nest box occupancy rates were assessed from 2013 to 2018, during which

there were between 29 and 32 nest boxes available to kestrels depending on year (Table

1.1). Occupancy steadily increased over the first five breeding seasons, with a maximum


10

of 73% of 30 nest boxes occupied in 2017. Nest box occupancy in 2018 decreased to

48% (Table 1.1).

The number of nesting attempts increased over the first five years as well, with 2

attempts in 2013, 9 in 2014, 18 in 2015, 19 in 2016, 25 in 2017, and 14 in 2018 (Table

1.2). Eighty-seven of 98 nesting attempts recorded from 2013 to 2018 have known-fates

(Table 1.2). Reproductive estimates varied among years (Table 1.2). Additionally, clutch

size, brood size, and number of fledglings per brood were typically greater for early

nesting attempts than late nesting attempts in 2014 to 2017 (Table 1.2); there was only

one late nest attempt in 2018 and it failed during incubation. Hatching success varied

among years, but was lowest in 2015 at 65% and greatest in 2016 at 89%. Hatching

success from 2013 through 2018 averaged 75.7% with 342 of 452 eggs hatching. Clutch

size averaged 4.8 eggs ± 0.8 (mode = 5) and brood size averaged 4.1 ± 1.2 chicks (mode

= 5). There were no differences in mean clutch size (F5,78 = 0.47, P = 0.792) or brood size

(F5,78 = 1.18, P = 0.325) of all known-fate nests among years.

When examining reproduction on the basis of early and late nest attempts, I found

the average clutch sizes were larger (F1, 80 = 8.69, P = 0.004) among the 64 early nesting

attempts (4.95 ± 0.09 SE) than the 18 late nesting attempts (4.39 ± 0.17 SE). Average

brood sizes were not larger (F1, 80 = 2.30, P = 0.133) among the 64 early nesting attempts

(4.14 ± 0.15 SE) than the 18 late nesting attempts (3.67 ± 0.28 SE). However, average

number of fledglings per successful nest was greater (F1, 71 = 4.95, P = 0.029) among the

58 early nesting attempts (3.48 ± 0.17 SE) than the 15 late nesting attempts (2.67 ± 0.33

SE) for which data were available.


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Of the 342 nestlings, 244 reached 80% of the fledging age (24 days of age or

older) with successful nests averaging 3.3 ± 1.3 fledglings (mode = 3; Table 1.2). There

were differences in mean fledglings per successful nest among years (F5,69 = 4.7798, P =

0.0008). Post hoc Tukey-Kramer HSD tests indicated that the mean number of fledglings

per successful nest was less in 2013 (1.0 ± 0.84 SE) than in 2017 (4.0 ± 0.26 SE, P =

0.0138) and 2018 (3.82 ± 0.36 SE, P = 0.0344). The mean number of fledglings per

successful nest was greater in 2017 (4.0 ± 0.26 SE) than in 2015 (2.6 ± 0.31 SE, P =

0.0111). Observed nest success slightly varied among nesting attempts and between year

(Table 1.2); 75 of 87 (86.2%) total nest attempts were successful (Table 1.2).

I was unable to backdate 11 of the 95 nests due to nest failure occurring in the egg

stage. Across the six breeding seasons, the median nest initiation date was March 26th

while the nest initiation date averaged April 10th (Table 1.3). There were two main peaks

each breeding year that corresponded with early and late season nests: March 16−March

31st and May 16−May 31st, respectively. Only two nests were initiated between April

16−May 15th. With the exception of 2018 breeding season, mean nest initiation dates

occurred earlier each year by a few days up to two weeks earlier (Table 1.3). There were

differences in initiation dates of all nests among years (F5,78 = 2.63, P = 0.029), but post

hoc tests failed to indicate where those differences occurred.

When assessing nest initiation on the basis of early and late attempts, there were

differences in the means of initiation dates for early nests among years (F4,59 = 3.27, P =

0.017), but post hoc tests failed to determine where those differences occurred. There

were no differences in the means of late nests between years (F4,15 = 0.46, P = 0.761).

Although my data are qualitative, it also appears age and/or experience may influence


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initiation of nesting. Of the 4 adult kestrels I was able to identify or capture in

consecutive years, all birds initiated nests earlier than the original year the individual was

captured (Table 1.4).

I modelled daily survival for 87 known-fate nest attempts. There were 11 nests

that failed during the egg stage and, therefore, initiation dates were unattainable. Instead,

a value was substituted for initiation dates for the eleven nests. The value entered was the

mean initiation date of the early or late nests for the year that the failure occurred. This

method was altered for a late attempt that occurred between June 11-July 15 in 2018. The

only late nest of 2018 was initiated months after the other nests, as a result, the mean of

initiation dates was not an acceptable substitute. For the single late attempt of 2018, June

28th was chosen because it is the equal difference between the date the nest was found

(July 15) and the previous nest box check (June 11). I created 15 original models for

analysis, but only 1 model fit the data well (Table 1.5). The covariates that did not fit the

data included: initiation date, year, and study site. I was unable to test precipitation and

temperature in any model, due to convergence issues and questionable beta values. I was

unable to pinpoint an acceptable range of when the nests failed. For example, many nests

that failed during the egg stage were not recorded as failed until after 30 days of the

initial nest discovery. This created obvious, large dips in DSR that likely did not match

reality.

The best fit model was time constrained with clutch size as a covariate. The next

best fit model had a time constraint with no covariates. However, prior to adding the

2018 nest data, the best fit model included clutch size and precipitation as covariates

indicating that high precipitation amounts resulted in lower survival. I was unable to test


13

this further upon adding the 2018 nest data due to convergence issues. DSR for the

average clutch size (4.8) was estimated at 0.99767. For a 60-day nesting period, this

translated to a nest survival estimate of 86.9% (0.9976760). The modelled DSR estimates

of the best fit model, S(.)+clutch, demonstrated that larger clutch sizes resulted in greater

nest survival (Figure 1.3). Thus, modelled nest survival (DSR raised to the nesting period

of 60 days) was greater in nests with larger clutches (Figure 1.4).

Additionally, I examined DSR and nest survival in Program MARK by using

2014−2018 early attempts only. This allowed for a time model to be created without

convergence issues or nonsensical standard errors. The time model demonstrated a near-

constant DSR of 1.0. The only changes in DSR in the time model were from day 47−52

and day 57 in the study; DSR on these days were estimated, depending on the day,

between 0.9910171−0.9743362. These days coordinated with nest failures that were

observed in the field for early nest attempt in 2014−2018. When using early attempts

only, the competing models became S(T+clutch+precipitation+temp) and

S(T+clutch+precipitation) and S(T+clutch). After model averaging the competing

models, DSR was lowest on day 47 (0.98533 ± 0.01483 SE), day 48 (0.99258 ± 0.00068

SE), day 52 (0.93576 ± 0.03750 SE), and day 57 (0.98060 ± 0.01989 SE). All other days,

DSR was estimated at 1.0. Nest survival was modelled at approximately 89.7% but

confidence intervals estimated it between 54.8%−100%. There is model uncertainty

associated with the competing models due to each model carrying 25-50% of the AICc

weight and model likelihood estimated at 44% for the least competing model,

S(T+clutch). In addition to model uncertainty, there is also variation associated with

model averaging to produce daily DSR estimates. Each model contributes variation to the


14

total variation associated with each DSR estimate. Model variation ranged from 0-88%

which means the three models, after averaging, accounted for 0-88% of the variation

associated with DSR and nest survival.

DISCUSSION

There is a general lack of information about American kestrel populations

breeding, migrating, and wintering in the Southern High Plains of Texas. Early survey

work was primarily based on road surveys conducted across the Southern Great Plains

(Allan and Sime 1943). More recently, the model-estimated average abundance of

American kestrels counted per BBS route per year, indicated an uncertain trend for the

kestrel population within the shortgrass prairie (AKP 2018). Elsewhere, kestrel

populations across the nation appear to be declining (Smallwood et al. 2009; AKP 2018).

It is challenging to evaluate why a population is stable, increasing, or decreasing without

first examining the complex processes that drive demographics. The factors involved in

population dynamics are births, deaths, immigration, and emigration. Although I was

unable to study immigration and emigration of kestrels within my study area, I was able

to examine reproductive efforts.

Nest box occupancy and number of nesting attempts increased every year from

2013−2017, but decreased in 2018. An abrupt increase in occupancy rates initially

follows the introduction of nest boxes to an area (Hamerstrom et al. 1973; Smallwood

and Collopy 2009). This is indicative of otherwise suitable habitat being limited by a

critical resource. In this case, nest sites were limiting and once that resource limitation is

alleviated (e.g., nest boxes), the kestrel population responds positively. The reduced

number of nesting attempts made in 2018 is likely a reflection of local environmental


15

conditions. During the three months prior to the 2018 breeding season, Lubbock, TX

received only 0.5 mm of precipitation while in previous years winter precipitation ranged

from 19–86.6 mm (NOAA 2018). The average occupancy rate over the six breeding

seasons (47%) is similar to other nest box studies conducted in North America (Table

1.6). As expected, early nesting attempts had greater clutch sizes, brood sizes, and

number of fledglings compared to late nesting attempts; this was consistent with the

pattern of seasonal decline in clutch sizes that has been observed elsewhere (Sockman

and Schwabl 2001). Mean clutch size, brood size, and number of fledglings per

successful nest were greater than means reported for resident kestrel populations in

Florida (Smallwood and Collopy 2009; Gault et al. 2004; Table 1.6). Several kestrels

breeding in nest boxes within my study area are residents as they are re-sighted multiple

times on their territory throughout the winter (S. Mullican pers. obs.). Thus, I

hypothesized that productivity parameters would be more similar to other resident kestrel

populations. This was not the case as productivity parameters of mean clutch size, brood

size, and number of fledglings per successful nest were more similar to those of kestrels

breeding in higher latitudes such as Missouri, Idaho, and Saskatchewan (Toland and

Elder 1987; Steenhof and Peterson 2009; Bortolotti 1994). However, my observed nest

success (87%) was highest in this study (Table 1.6). The best fit DSR model indicated

nest survival was 86.9% which is consistent with the observed nest success (86.2%). This

would suggest that the model fits the data well.

The high nest success in my study area is likely due to a combination of several

factors: climate, individual age and experience, and residency status. A relatively mild

winter climate on the Llano Estacado may benefit adult kestrels by allowing females to


16

build greater fat reserves to efficiently produce eggs and initiate nesting earlier. Bortolotti

and Wiebe (1993) found that small-bodied females had higher hatching failures

indicating that body size relative to egg size may contribute to greater nest success. Age

and previous nesting experience of adult pairs likely influences productivity as well.

Young birds lack experience in reproduction (e.g., incubation of eggs), survival, and

foraging efficiently. In comparison, older birds may have the advantage of experience,

but may lack the capability of producing viable eggs/sperm. Birds resident to the area

likely have an advantage over migrant birds due to familiarity with potential predators or

risk areas, mates, territory, and local climate. Resident kestrels in Oregon nested almost a

month earlier than most of the migrant kestrels (Henny and Brady 1994).

Chicks that fledge earlier than others can take advantage of learning to be a young

kestrel without competition from other juveniles. The post-fledging dependence period is

a period in which recent fledglings are dependent upon their parents for food while

attempting to learn how to fly, land, navigate, hunt, avoid risks, etc. Fledglings within a

brood will compete with each other for food deliveries from adults and will likely

compete against fledglings from outside their brood as well, given that prey availability

and abundance is limited (S. Mullican pers. obs.).

Nest initiations in my study ranged from March 9th to June 14th and averaged

April 10th. These dates are similar to a kestrel population in southwestern Idaho which

initiated nests between March 14th through July 7th and averaged April 28th (Steenhof and

Peterson 2009). Steenhof and Peterson (2009) hypothesized that progressively warmer

winters allowed kestrels to forego migration which possibly led them to nest earlier.

Kestrels in northwest Florida initiated nests from March 22nd through June 23rd (Gault et


17

al. 2004). In central Missouri, average nest initiation (in 1982) was April 1st (Toland and

Elder 1987). Nest initiations in my study area appear to be very similar to kestrels

breeding elsewhere in North America. And like Steenhof and Peterson (2009), we did not

find winter temperatures or spring precipitation to significantly influence nest initiation

dates. However, individual kestrels may respond differently to their environment and

experiences. I found that two kestrels caught in 2016 initiated nests 7-16 days earlier than

the initial year (2015) they were first caught. However, the two individuals did not

continue to nest earlier in 2017, but nested 2-3 days later than initiation in 2016. This

could be due to random variation or individual response to mate availability, prey

abundance or availability, or local weather patterns. A much larger sample size and

higher capture rate would be needed to examine this theory further.

Early nest attempts averaged 25 March, 15 days earlier than the overall average.

This difference could be a result of having partial migrants within the local breeding

population. It is possible that birds returning from more southern latitudes, such as South

TX or Mexico, stopped migration in my study area for favorable habitat and joined the

“local” breeding population. This would support the theory that resident birds in my

study area may initiate breeding earlier in comparison to migrants.

Nest survival, based on the average clutch size of 4.8, was 86.9% for the best fit

model created in Program MARK. The modelled nest survival matches the observed nest

success (86.2%). Surprisingly, the best fit model only included clutch size as a covariate

while weather covariates did not fit the data. Food availability often influences the

reproduction of avian species (Stearns 1992). Prey availability for American kestrels may

be directly affected by weather conditions such as extreme drought, flooding, or


18

temperatures. I was unable to account for such events in my models, but many prey

species of kestrels will become inactive and seek shelter during unfavorable conditions,

thereby becoming unavailable to the kestrels. Likewise, a raptor that becomes drenched

will have a hard time thermoregulating and difficulty hunting. Dawson and Bortolotti

(2000) also found that nest success was affected by inclement weather (either cold, rainy

days or hot, dry days).

Improvements to this line of research can be made as more data become available.

First, temperature and precipitation at each study site should be taken for better precision

of local conditions. It is common for one side of Lubbock Co. to get heavy rain while the

other side receives none. Additionally, an ibutton could be placed inside of the nest box

to provide insight to the conditions that eggs are exposed to while inside of the nest box.

Second, I believe it is important to determine whether eggs are fertile, infertile, or

experienced embryonic death. Wiebe (1996) reported infertile egg and embryo death as

the two main causes of partial hatching failure in American kestrel clutches. Determining

egg fertility and local environmental conditions may better explain nest failures and

abandonments. It will also help to create better models in Program MARK that could

potentially allow productivity predictions for climate change scenarios.


19

LITERATURE CITED

Allan, P. F., and P. R. Sime. 1943. A hawk census on Texas panhandle highways. The

Wilson Bulletin 55:29-39.

American Kestrel Partnership [AKP]. 2015. Declines of American kestrel populations.

http://kestrel.peregrinefund.org/kestrel-decline. Accessed 12 Sept 2015.

American Kestrel Partnership [AKP]. 2016. By the numbers.

http://kestrel.peregrinefund.org/index.php?action=map. Accessed 21 Aug 2016.


http://kestrel.peregrinefund.org/index.php?action=map. Accessed 9 Sept 2018.

Bird, D. M. and R. S. Palmer. 1988. American kestrel. Pages 253−290 in R. S. Palmer,

editor. Handbook of North American birds II, vol. 5. Yale University Press, New

Haven, Connecticut, USA.

Bortolotti, G. R. 1994. Effect of nest-box size on nest-site preference and reproduction in

American Kestrels. Journal of Raptor Research 28:127−133.

Bortolotti, G. R., and K. L. Wiebe. 1993. Incubation behaviour and hatching patterns in

the American kestrel Falco sparverius. Ornis Scandinavica 24:41−47.

Dawson, R. D., and G. R. Bortolotti. 2000. Reproductive success of American kestrels:

the role of prey abundance and weather. Condor 102:814−822.

Diamond, D. D. 2010. Grasslands. Handbook of Texas Online. Texas State Historical

Association. https://www.tshaonline.org/handbook/online/articles/ryl02.

Accessed 16 Aug 2016.

Farmer, C. J., and J. P. Smith. 2009. Migration monitoring indicates widespread declines

of American kestrels (Falco sparverius) in North America. Journal of Raptor

Research 43:263–273.

Gault, K. E., J. R. Walters, J. Tomcho Jr., L. F. Phillips Jr., and A. Butler. 2004. Nest

success of southeastern American Kestrels associated with Red-cockaded

Woodpeckers in old-growth longleaf pine habitat in northwest Florida.

Southeastern Naturalist 3:191-204.

Graves, L. L. 2010. Lubbock County. Handbook of Texas Online. Texas State Historical

Association. https://www.tshaonline.org/handbook/online/articles/ryl02.


Hamerstrom, F., F. N. Hamerstrom, and J. Hart. 1973. Nest boxes: an effective

management tool for kestrels. Journal of Wildlife Management 37:400−403.


20

Henny, C. J., & G. L. Brady. 1994. Partial migration and wintering localities of American

Kestrels nesting in the Pacific Northwest. Northwestern Naturalist 75:37−43.

Klucsarits, J. R., and J. J. Rusbuldt. 2007. A photographic timeline of Hawk Mountain

Sanctuary’s American kestrel nestlings. Zip publishing. Columbus, Ohio, U.S.A.

Leatherwood, A. 2010. Llano Estacado. Handbook of Texas Online. Texas State

Historical Association.

https://www.tshaonline.org/handbook/online/articles/ryl02. Accessed 11 Aug

2016.

Linner, K. M. 2014. Assessing the influence of wind turbines and land-use on mortality

risk of avian species on the Llano Estacado with emphasis on birds of prey.

Thesis, Texas Tech University, Lubbock, USA.

Mitra, A., C. Chatterjee, and F. B. Mandal. 2011. Synthetic chemical pesticides and their

effects on birds. Research Journal of Environmental Toxicology 5:81–96.

National Oceanic and Atmospheric Administration [NOAA]. 2018. National Centers for

Environmental information, Climate at a Glance: City Time Series.

https://www.ncdc.noaa.gov/cag/. Accessed 17 Sept 2018

North American Bird Conservation Initiative. “Bird Conservation Regions Map.” 2016.

http://nabci-us.org/resources/bird-conservation-regions-map/#bcr18. Accessed 13

May 2017.

SAS. 2015. JMP®, Version 12. SAS Institute Inc., Cary, NC, 1989-2007.

Smallwood, J. A., and D. M. Bird. 2002. American kestrel (Falco sparverius). Account

602 in A. Poole, editor. The birds of North America. Cornell Lab of Ornithology,

Ithaca, New York, USA.

Smallwood, J. A. and M. W. Collopy. 2009. Southeastern American Kestrels respond to

an increase in the availability of nest cavities in north-central Florida. Journal of

Raptor Research 43:291-300.

Smallwood, J. A., M. F. Causey, D. H. Mossop, J. R. Klucsarits, B. Robertson, S.

Robertson, J. Mason, M. J. Maurer, R. J. Melvin, R. D. and Dawson. 2009. Why

are American kestrel (Falco sparverius) populations declining in North America?

Evidence from nest-box programs. Journal of Raptor Research 43:274–282.

Smith, D. G., C. R. Wilson, and H. H. Frost. 1972. The Biology of the American Kestrel

in Central Utah. Southwestern Naturalist 17:73–83.


21

Sockman, K. W., and H. Schwabl. 2001. Covariation of clutch size, laying date, and

incubation tendency in the American Kestrel. Condor 103:570−578.

Soil Survey Staff, Natural Resources Conservation Service, United States Department of

Agriculture. 2015. Web Soil Survey. http://websoilsurvey.nrcs.usda.gov/.


Stearns, S. C. 1992. The evolution of life histories. Oxford University Press, Oxford,

England.

Steenhof, K. and I. Newton. 2007. Assessing nesting success and productivity. Pages

181–191 in D. M. Bird and K. L. Bildstein, editors. Raptor research and

management techniques. Hancock House Publishers, Surrey, Canada.

Steenhof, K., and B. E. Peterson. 2009. American kestrel reproduction in southwestern

Idaho: annual variation and long-term trends. Journal of Raptor Research 43:283–

290.

Strasser, E. H., and J. A. Heath. 2013. Reproductive failure of a human-tolerant species,

the American kestrel, is associated with stress and human disturbance. Journal of

Applied Ecology 50:912–919.

Stubbendieck, J., S. L. Hatch, and N. M. Bryan. 2011. North American wildland plants: a

field guide. University of Nebraska Press.

Texas Parks and Wildlife Department [TPWD], Wildlife Division, Diversity and Habitat

Assessment Programs. 2016. TPWD county lists of protected species and species

of greatest conservation need. Lubbock County. Revised January 5, 2016.

Accessed 16 Aug 2016. http://tpwd.texas.gov/gis/rtest/.

Toland, B. R., and W. H. Elder. 1987. Influence of nest-box placement and density on

abundance and productivity of American kestrels in central Missouri. Wilson

Bulletin 99:712–717.

USDA. 2012. Census of Agriculture – County Data. National Agriculture Statistics

Service 252–284. Accessed 21 Aug 2016.

http://www.agcensus.usda.gov/Publications/2012/Full_Report/Volume_1,_Chapte

r_2_County_Level/Texas/st48_2_001_001.pdf.

Wiebe, K. L. 1996. The insurance-egg hypothesis and extra reproductive value of the

last-laid eggs in Clutches of American kestrels. The Auk 113:258−261.


22

Table 1.1. Occupancy of nest boxes and number of nest attempts by American kestrels in Lubbock Co., TX from 2013–2018.

*3 of the 97 nests were included in occupancy rates but were censored from further analyze due to

human error and honeybee interference.

Year No. available

boxes

No. occupied

boxes Occupancy rate No. nest attempts*

2013 32 4 13% 4

2014 30 9 30% 11

2015 30 19 63% 22

2016 30 17 57% 22

2017 30 22 73% 25

2018 29 14 48% 14


23

Table 1.2. Comparison of nest attempts, clutch size, brood size, fledglings/brood, nest success, and sex ratio among early and late

nesting attempts by American kestrels nesting in nest boxes in Lubbock County, TX from 2013–2018.

Year Attempt Total nest

attempts

Clutch size

Mean ± SD (n)

Brood size

Mean ± SD (n)

Fledglings/ brood

Mean ± SD (n)

Nest

success

Sex ratio at

banding M:F

2013 late 2 4.5 ± 0.7 (2) 4.5 ± 0.7 (2) 1.0 ± 0.0 (2) 100% 3:5

2014 early 8 5.0 ± 0.5 (8) 3.7 ± 1.0 (7) 3.3 ± 0.5 (6) 86% 18:8

late 3 3.7 ± 0.6 (3) 3.3 ± 0.6 (3) 2.0 ± 1.5 (2) 100% 5:5

both 11 4.6 ± 0.8 (11) 3.6 ± 0.8 (10) 2.8 ± 1.0 (8) 89% 23:13

2015 early 14 5.1 ± 0.6 (14) 3.5 ± 1.5 (11) 2.7 ± 1.2 (10) 77% 28:11

late 7 4.9 ± 0.7 (7) 4.1 ± 0.9 (7) 2.4 ± 1.1 (5) 100% 15:12

both 21 5.0 ± 0.7 (21) 3.8 ± 1.3 (18) 2.6 ± 1.0 (15) 83% 43:23

2016 early 17 4.9 ± 1.0 (17) 4.6 ± 1.3 (17) 3.2 ± 1.5 (13) 93% 29:39

late 5 4.4 ± 0.9 (5) 3.0 ± 1.6 (5) 2.4 ± 1.1 (5) 100% 9:6

both 22 4.8 ± 1.0 (22) 4.2 ± 1.5 (22) 3.0 ± 1.5 (18) 95% 38:45

2017 early 19 4.8 ± 0.7 (19) 4.3 ± 1.0 (18) 4.0 ± 1.2 (18) 95% 33:40

late 6 3.7 ± 1.4 (6) 4.0 ± 0.0 (3) 4.0 ± 0.0 (3) 50% 7:5

both 25 4.6 ± 0.9 (25) 4.3 ± 0.9 (21) 4.0 ± 1.1 (21) 84% 40:45

2018 early 13 4.9 ± 0.3 (13) 4.2 ± 0.9 (11) 3.8 ± 1.3 (11) 85% 27:19

late 1 5 ± 0.0 (1) − − 0% −

both 14 4.9 ± 0.3 (14) 4.2 ± 0.9 (11) 3.8 ± 1.3 (11) 79% 27:19

Overall early 71 4.9 ± 0.7 (71) 4.2 ± 1.2 (64) 3.5 ± 1.3 (58) 88% 135:117

late 24 4.2 ± 1.1 (24) 3.8 ± 1.1 (20) 2.5 ± 1.2 (17) 81% 39:33

both 95 4.8 ± 0.8 (95) 4.1 ± 1.2 (84) 3.3 ± 1.3 (75) 86% 174:150


24

Table 1.3. Comparison of nest initiation dates between years of American kestrels nesting in nest boxes in Lubbock County, TX from

2013–2018.

Year

Earliest

Initiation

Latest

Initiation

Median

Initiation

Mean Initiation ±

SD (days)

Early Attempts Mean

Initiation ± SD (days)

Late Attempts Mean

Initiation ± SD (days)

2013 9-May 14-June 27-May 27-May ± 25.5 - 27-May ± 25.5

2014 24-March 12-June 6-April 20-April ± 32.8 31-March ± 7.0 6-June ± 6.0

2015 18-March 10-June 30-March 20-April ± 31.2 25-March ± 4.5 30-May ± 8.4

2016 9-March 8-June 25-March 6-April ± 31.2 22-March ± 9.5 31-May ± 6.3

2017 12-March 30-May 23-March 31-March ± 25.3 21-March ± 6.7 29-May ± 1.5

2018* 23-March 28-April 26-March 30-March ± 11.1 30-March ± 11.1 -

Overall 9-March 14-June 26-March 10-April ± 30.0 25-March ± 8.7 31-May ± 8.8

*Nest initiation dates for one nest in 2014, three in 2015, four in 2017, and three in 2018 are not known due to nest failure occurring

during incubation (egg) stage and are not included in initiation analyses.


25

Table 1.4. Examination of nest initiation phenology of marked adult kestrels that were captured in consecutive years in Lubbock,

County, TX from 2015 to 2017.

Bird #

∆ between 2015 and

2016 initiation date





1 -16 days -13 days +3 days

2 -7 days -5 days +2 days

3 ---- ---- -6 days

4 ---- ---- -4 days

Avg. ∆ -11.5 days -9 days -1.3 days

This table demonstrates the differences of nest initiation dates between and across years by four individuals.

It is unknown if this is due to random variation or an individual’s response to its environment and local conditions.


26

Table 1.5. S(clutch) was chosen as the best fit model from 15 originally created to assess American kestrel nest survival from nest data

in Lubbock County, TX from 2013−2018. S(clutch) had the lowest AICc value while the next best model S(constant) had an AICc

value of 114.592.

Model Name AICc ΔAICc AICc Weight Model Likelihood No. Parameters Deviance

S(clutch) 108.639 0.0 0.95 1.0 1 106.64


27

Table 1.6. Comparison of American kestrel productivity among published studies.

Location (reference)

T (years)

No. boxes

available

Occupancy

(%)

Obs. Success

(%)

Clutch Size

Brood

Size

Fledglings

per Brood

Texas

this study

5

29-32

47

87

4.8

4.1

3.3

NC Florida

Smallwood and Collopy 2009

4

86-388

31

67

4.3

2.8

2.4

NW Florida

Gault et al. 2004

2

68*

-

63

4.4

3.5

2.9

Pennsylvania

Rohrbaugh and Yahner 1997

5

130

40

49

–

–

–

SW Idaho

Steenhof and Peterson 2009

21

103

48

64

4.8

4.0

2.6

Saskatchewan

Bortolotti 1994

6

17-19

74

67

4.7

3.9

–

Missouri

Toland and Elder 1987

4

22-61

53

73

5.0

–

4.5

*Gault et al. (2004) studied natural cavities existing within woodpecker holes or snags in longleaf pine trees, not artificial nest boxes.


28

Figure 1.1. Location of the study area in relation to the Llano Estacado.


29

Figure 1.2. Location of three study sites in Lubbock County, TX used to assess American

kestrel breeding ecology. The green dots represent individual nest box locations. Site "A"

is the Lubbock Lake Landmark site, "B" is the Reese Technology Center site, and “C” is

the Texas Tech University Native Rangeland site. The average distance of the nearest

nest box at site “A” is 241.2 m, 608.3 m at site “B”, and 84 m at site “C”.

A B

C


30

Figure 1.3. Daily Survival Rates (DSR) modelled in Program MARK from American

kestrel nest data in Lubbock County, TX from 2013−2018. The model, S(clutch),

demonstrated that larger clutch sizes have higher daily survival rates. The error bars

represent the standard error associated with each estimate.


31

Figure 1.4. Nest survival modelled in Program MARK from American kestrel nest data in

Lubbock County, TX from 2013−2018. The model, S(clutch), demonstrated that larger

clutch sizes are more likely to survive the nesting period. The error bars represent the

95% confidence intervals associated with each nest survival estimate.


32

CHAPTER II

ADULT SURVIVAL AND FIDELITY ESTIMATES OF AMERICAN KESTRELS

NESTING IN ARTIFICIAL NEST BOXES IN LUBBOCK COUNTY TEXAS

INTRODUCTION

American kestrels (Falco sparverius) are a numerous and widespread falcon

species found across North America (Smallwood and Bird 2002). Populations of

American kestrels (hereafter, kestrels) have been declining over the last few decades. It is

estimated that kestrel populations in North America are declining by 47% based on

collective data from nest box monitoring programs, USGS Breeding Bird Surveys,

Christmas Bird Counts, and raptor migration counts from 1966 to 2010 (American

Kestrel Partnership, hereafter AKP, 2018; Farmer and Smith 2009; Smallwood et al.

2009). Research has yet to identify a definite cause for the declines, but it is most likely a

combination of biological and ecological factors such as landscape changes leading to

increased human-caused stress in disturbed areas (Strasser and Heath 2013), climate

change (Steenhof and Peterson 2009a), chemical exposure (Mitra et al. 2011), loss of

suitable nesting habitat, competition for nesting cavities, and disease (Smallwood and

Bird 2002). Based on abundance estimates from Christmas Bird Counts, kestrels

occupying the shortgrass prairie on the High Plains were once suspected of experiencing

population growth, but now population trends are uncertain (Bird Conservation region

18; AKP 2015; AKP 2018; North American Bird Conservation Initiative 2016). Despite

the nationwide downward trend and considerable need for information, few studies have

been conducted on the kestrel population in the Southern High Plains. A better

understanding of survival could inform us to localized population patterns.


33

Surprisingly, there are very few survival estimates that have been published for

American kestrels. Adult survival rates of kestrels in Florida were recently estimated to

be 75% for males and 74% for females, as derived from mark-recapture events

(Hinnebusch et al. 2010). Other studies have used band recoveries from dead birds or

nest-site occupancies to estimate mortality and survival rates for kestrels (Roest 1957;

Henny 1972). Roest (1957) reported an average annual survival rate of 43% based on

band recoveries over six years from 150 adult and juvenile birds. Henny (1972) used

nest-site occupancy to evaluate annual mortality rates and reported an annual adult

survival rate of 55%. Annual adult female survival, modelled from mark-recapture data

(n=113) in southeast Florida, was estimated at 39% (95% C.I. 29.0-51.0; Brown and

Collopy 2013). Low survival during the juvenile life stage may be causing kestrel

populations to decline in regions across North America (Stupik et al. 2015). By using

radiotelemetry, Stupik et al. (2015) estimated a daily survival rate of 0.27 for kestrel

fledglings in Connecticut, through the Kaplan-Meier procedure, for the period of post-

fledging up to the onset of migration. Estimates from Henny (1972) found that hatch-year

survival rate of kestrels is 31%. Brown and Collopy (2013) used mark-recapture data to

estimate annual post-fledging survival of juvenile kestrels at 8.2% (95% C.I.: 2.6-20.0).

Roest (1957) reported a mean life expectancy of 1 year and 2 months for juveniles and an

expected mean lifespan for adults of 2 years and 3 months. However, American kestrels

in captivity can live up to 14-17 years, with a mean life expectancy of 5 years and 2

months (Roest 1957; Bird and Palmer 1988).

Survival may be influenced by mate fidelity and/or nest fidelity. Differing levels

of both mate and site fidelity, have been reported in American kestrels. Steenhof and


34

Peterson (2009b) reported a female American kestrel used the same nest box for five

consecutive years and a male that used one for six consecutive years, clearly

demonstrating nest box fidelity among some individuals. However, low nest box fidelity

rates of 13−23% were reported in Idaho (Steenhof and Peterson 2009b). Of 143

individually marked males in Idaho, 28% stayed with their previous mate in successive

years. Of 113 individually marked females in Idaho, 37% stayed with their previous mate

in successive years (Steenhof and Peterson 2009b). Kestrels in other areas likely engage

in some level of fidelity to territories, nest boxes, and mates. High or low rates of fidelity

could play a substantive role in survival and population trends of kestrels in the Southern

High Plains.

I conducted research to better understand the ecology of kestrels in the Llano

Estacado region of the Southern High Plains of Texas. My objectives were to estimate

rates of American kestrel mate and site fidelity and to estimate annual adult survival of

kestrels that bred in nest boxes within my study area.

METHODS

Study Area

The Llano Estacado, also referred to as the Southern High Plains, is considered

the southern tip of the larger Great Plains (Figure 2.1). The Llano Estacado is in

Northeast New Mexico and the panhandle of Texas, which lies within the Bird

Conservation Region 18 (North American Bird Conservation Initiative 2016). The area is

characterized by high flat land that lies between the Caprock Canyon to the east in Texas

and Mescalero Ridge to the west in New Mexico (Leatherwood 2010). While the

southern portion lacks a distinct physical boundary, the Johnson Creek branch of the

Colorado River is considered the lower extent of the Llano Estacado (Leatherwood


35

2010). The southern ridge of the Canadian River valley delineates the north border

(Leatherwood 2010).

My study area is Lubbock County, TX which lies within the Llano Estacado; it is

an area of relatively level land with 0-3% of gentle slopes at an elevation varying around

975 m (Soil Survey Staff 2015). Lubbock County has a semiarid climate with an average

annual precipitation of 467.6 mm, the majority of which occurs between May and

October (Graves 2010). The maximum summer temperature in July averages 33.3° C and

the minimum winter temperatures average -3.9° C in January (Graves 2010). Lubbock

County, TX is part of the southern shortgrass prairie in the High Plains, characterized as a

short and mixed grass prairie composed primarily of blue grama (Bouteloua gracilis) and

buffalograss (Bouteloua dactyloides), and dotted with numerous playas and few shrubs

(Diamond 2010; Graves 2010).

Compared to its historic prairie state, the High Plains of Texas is now a very

different ecosystem (Leatherwood 2010). Anthropogenic development, both urbanization

and agricultural development, of the landscape altered the vegetative community from

historic continuous prairie to the fragmented landscape of primarily irrigated cropland

and urban housing (Diamond 2010). Today, the majority of land in the county is

cropland, dispersed farmhouses, and small towns and cities. In 2012, cropland accounted

for approximately 73% of landcover in Lubbock County (1715 km2 of 2333 km2 total)

(USDA 2012). Lubbock County, TX is known for producing cotton, despite occasional

droughts (Graves 2010). Of the total cropland in 2012, 48.5% were cotton fields (832

km2 of 1,715 total cropland km2; USDA 2012). Additionally, fire suppression allowed

woody species, such as oak (Quercus spp.), cedar (Juniperus spp.), elm (Ulmus spp.), and


36

mesquite (Prosopis spp.), to expand their distributions and invade parts of the High

Plains (Leatherwood 2010). The expansion of these woody species allowed secondary

cavity nesters, such as the American kestrel, to also expand their breeding range

(Smallwood and Bird 2002).

My study area consists of three sites, the Lubbock Lake Historic Landmark, the

Reese Technology Center, and the Texas Tech Native Rangeland (Figure 2.2). All of the

sites are classified as rangeland ecological sites of differing transitional stages (Soil

Survey Staff 2015). Transitional stages refer to the seral stages of succession which is

described as a gradient for the three sites. Within these three sites, nest boxes were placed

in remnant native prairie communities that are bordered by asphalt roads/highways,

runways, farmlands, golf courses, or urban neighborhoods. All three sites have different

amounts of prairie coneflower (Ratibida columnifera) and other annual forbs, cacti

(Opuntia spp.), honey mesquite (Prosopis glandulosa), elm, yucca (Yucca spp.), and

native grasses. The study sites support numerous wildlife species of interest and concern

such as the Texas horned lizard (Phrynosoma cornutum), black-tailed prairie dog

(Cynomys ludovicianus), burrowing owl (Athene cunicularia), prairie falcon (Falco

mexicanus), and ferruginous hawk (Buteo regalis) (Texas Parks and Wildlife Department

2016).

The Lubbock Lake Historic Landmark (Figure 2.2; hereafter, Lubbock Lake) is a

136 ha area in central Lubbock County. The site is primarily managed as an

archaeological and educational center. Of the three sites, Lubbock Lake most closely

resembles a pre-European native prairie ecosystem. Lubbock Lake has several hiking

trails scattered throughout the property with varying topography and is similar to the


37

Reese Technology Center site with a primarily shortgrass/blue grama dominant

vegetation community with some woody species present. Lubbock Lake is open to the

public so this site experiences low to moderate levels of human activity on the trails

throughout the year.

The largest study site is the Reese Technology Center (Figure 2.2; hereafter

RTC), a decommissioned United States Air Force base on the west side of Lubbock

County, TX. The RTC has >700 ha with the majority of the vegetation classified as a

shortgrass/blue grama dominant vegetation community (Soil Survey Staff 2015). The

RTC falls in between Lubbock Lake and the Texas Tech Native Rangeland in terms of

seral stages of succession. Concrete runways, one industrial sized turbine and three scaled

wind turbines, playas, and black-tailed prairie dog colonies are present within the

landscape. The RTC experiences daily amounts of manufacturing and machinery activity

mostly due to Zachary and Forterra, an electrical manufacturing company and a water-

related infrastructure manufacturer, respectively. Most of this activity occurs on the

perimeter of RTC.

The Texas Tech Native Rangeland (Figure 2.2; hereafter, Rangeland) is 55 ha in

central Lubbock County. The site encompasses a playa and is comprised of dense honey

mesquite shrubs, broom snakeweed (Gutierrezia sarothrae), threeawn (Aristida spp.),

Russian thistle (Salsola kali), ragweed (Ambrosia spp.), and gramas (Bouteloua spp.)

(Soil Survey Staff 2015; S. Mullican pers. obs.). The Rangeland study site can be

described as a remnant native prairie that has been suppressed of fire and intensely

invaded by mesquite. The density of mesquite and abundance of threeawn, can be used as

an indicator of poor quality rangelands (Stubbendieck 2011). The Rangeland is the most


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degraded ecosystem of the three sites. Graduate and undergraduate students at Texas

Tech University conduct research at the Rangeland site, but it is not open to the public.

Field Methods

Survival rate analyses require individuals to be identifiable at different

encounters. For American kestrels, this necessitates capturing and marking them with

unique bands. To trap adult kestrels during the breeding season, I set a bal-chatri (double

walled) trap with a mouse (Mus musculus) lure near the target bird associated with a nest

box (Bloom et al. 2007). Immediately after the bird was caught, I removed it from the

nooses and processed it. Occasionally, adult kestrels were captured inside their respective

nest box during periods of incubation or brooding of offspring. In all cases, unbanded

kestrels received a USGS numeric aluminum leg band and three coil-style plastic color

leg bands (black, blue, light green, medium green, red, white, and/or yellow). All color

bands were sealed using a soldering iron to prevent band loss (Nisbet 1991). Each

breeding pair of kestrels received a unique color band combination based on year-specific

color bands. Since kestrels are sexually dimorphic, the same color combination could be

used on a male and a female. To the extent possible, each banded breeding pair received

the same color band combination. This allowed for the maximum number of color

combinations to be used each year. Additionally, measurements of wing chord, tail chord,

tarsus width and depth, hallux length, and weight were collected from each adult kestrel.

Procedures followed the AICUC protocol #16034-04 and TTU AICUC SOP 043.

Analysis

Annual adult survival, X, was estimated by reformulating an existing daily

survival equation (Conroy and Carroll 2009)


39

𝑋 = (𝑌

𝑁)

1𝑍⁄

where X is the minimum annual adult survival rate, N is the total number of individuals

banded, Y is the minimum number of individuals that survived more than one year, and Z

is the average minimum age at last recapture.

I calculated a naïve estimation of mate fidelity for both male and female kestrels,

as the proportion of individuals nesting with the same mate divided by the total of banded

birds and birds who were banded but their mate was unknown. Site fidelity of males and

females were calculated based on methods described in Tschopp et al. (2018). I

bootstrapped my sample size 100 times to minimize error associated with small sample

sizes. I then averaged the larger sample and proceeded calculating site fidelity using the

index, IH4 (Tschopp et al. 2018).

𝐼𝐻4 = 2

1𝐼𝑇 +

1𝐼𝑡

Occurrence (IO) is the sum of recapture events per individual divided by the total number

of recapture occasions over the sampling period. Permanence (IT) is the proportion of

time between capture and last recapture (Fi), over the entire time spent sampling (F).

𝐼𝑇 =𝐹𝑖

𝐹

Periodicity (It) is the return of an individual and is stated as the ratio between IO and IT.

𝐼𝑡 =𝐼𝑂

𝐼𝑇


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RESULTS

Adult Survival

From 2013−2018, a Texas Tech University research team captured and banded 46

adult kestrels. Females were more likely to be recaptured than males in subsequent years

due to the greater inclination to be inside the nest box incubating eggs or brooding

nestlings. Twenty-five of the 46 adult kestrels were only caught in a single year (Table

2.1); however, 21 individuals were captured in multiple years. The average minimum age

at last recapture was 2.67 ± 0.97 (1 SD). Annual adult survival was estimated at a rate of

74.5%.

Mate Fidelity

The majority of the 46 adult individuals captured in this study were only detected

nesting in a single event (Table 2.2). However, 21 individuals were recaptured or

resighted in multiple years. Mate fidelity was higher among males (33.3%) than females

(21.4%; Table 2.2). Unfortunately, this calculation does not include pairs from nest boxes

that were never caught. As a result, the fidelity rates I report likely do not reflect the true

overall fidelity rates of the total breeding population.

Site Fidelity

I observed 21 individual kestrels using the same nest box (or territory) in

consecutive years (Table 2.3). One female in my study area used the same nest box for 5

consecutive years (Table 2.3). According to the IH4 index, site fidelity did not appear to

be significantly influenced by sex. Site fidelity for females was estimated at 39.68 ±

0.001 (1 SD) and site fidelity for males was estimated at 39.44 ± 0.002 (1SD). When

fidelity histories of both sexes are calculated together, the estimated site fidelity was

39.70 ± 0.001 (1 SD).


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DISCUSSION

There are few adult survival studies of American kestrels with which to compare

my results. I calculated annual adult survival to be approximately 74.5% without

differentiation between sexes. This is likely a naïve estimate of adult survival due to the

low total sample size and low numbers of recaptures. However, this is only five years of

data and it is likely that future estimates using the same method or, if sample sizes

eventually allow, a Cormack-Jolly-Seber approach, can be calculated with much stronger

support. Even though my estimated annual survival should be viewed within context of

sample size, it still closely resembles the results of Hinnebusch et al. (2010) who used 14

years of capture-recapture data from a southeast Florida population to estimate survival

(74% for female kestrels and 75% for male kestrels). In contrast, my adult kestrel

survival estimate is much higher than the 43% (Roest 1957) and 55% (Henny 1972)

reported from earlier studies. Higher adult survival within my study area may be a

combination of several factors: climate, individual age and experience, and wintering

strategy. My study area has a semiarid, temperate climate that rarely sees multiple days of

extreme, inclement weather. Roest (1957) concluded that kestrels who survived their first

winter had a higher probability of surviving the oncoming years. It is tempting to

conclude that the individuals who were only seen in one year probably dispersed from the

study area or died. However, overall capture success of kestrels nesting within my study

area was low (roughly 22-56% depending on the year). Based on this knowledge, I

cannot infer whether these individuals died, dispersed, or simply went undetected. In

order to make stronger inferences based on the capture-recapture data, it is imperative to

increase capture efforts and success in the future.


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I recorded 2 instances (out of 244 fledglings) in which locally produced male

kestrels were recruited into the local breeding population in the summer following their

year of hatching. Juvenile survival (or hatch-year survival) is a critical life stage in

raptors and this has gone largely unstudied in the literature (Roest 1957; Henny 1972;

Brown and Collopy 2013; Stupik et al. 2015). My data are too sparse to draw meaningful

conclusions other than local recruitment does occur. Banded birds from other regions,

such as Idaho or New Mexico, were not recaptured or resighted during the study period.

My estimate of mate fidelity (33.3% males and 21.4% females) is relatively

similar to rates reported by Steenhof and Peterson (28% males and 37% females; 2009b).

However, my estimates suggest that males demonstrate slightly higher mate retention

than those reported by Steenhof and Peterson (2009b). This could be a result of low

capture success and low sample size, bias toward female captures, or undetected sex-

specific differences in survival in my study or others. Low mate fidelity is likely

associated with migratory populations while high mate fidelity is associated with resident

populations (Steenhof and Peterson 2009b). Given my mate fidelity rate of 21-33%,

inferences can be made that the kestrel population in my study area is partially migratory.

I did not find any evidence of differences in site fidelity between the sexes in my

nest box program. This rate is slightly higher than site fidelity rates of 23% for males and

13% for females reported by Steenhof and Peterson (2009b). To my knowledge, there are

no other American kestrel studies that have examined site fidelity using banded

individuals in a long-term data set. Kestrels are short-lived raptors and demonstrate

varying levels of nest site fidelity. Yet, site fidelity is usually associated with longer-lived

birds (Hestbeck et al. 1991; Choudhury 1995). Site fidelity rates shown by kestrels in my


43

study area are likely a result of past reproductive history, age and experience of the

individual, winter strategy, a relatively stable climate, and habitat quality. Many

individuals that nested within my study area were also observed in the following winters.

My population is at the very least, partially migratory, given the number of residents seen

through the winter (S. Mullican pers. obs.). This winter strategy may allow higher

retention of both nest sites and mates due to familiarity with territories, environmental

conditions, and risk areas.


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LITERATURE CITED


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http://kestrel.peregrinefund.org/index.php?action=map. Accessed 9 Sept 2018.

Bloom, P. H., W. S. Clark, and J. W. Kidd. 2007. Capture Techniques. Pages 193−219 in

D. M Bird and K. L. Bildstein, editors. Raptor Research and Management

Techniques, Hancock House Publishers, Surrey, Canada.

Bird, D. M. and R. S. Palmer. 1988. American Kestrel. Pages 253-290 in R. S. Palmer,

editor. Handbook of North American birds. Vol. 5: diurnal raptors. Pt. 2. Yale

University Press, Connecticut.

Brown, J. L. and M. W. Collopy. 2013. Immigration stabilizes a population of threatened

cavity‐nesting raptors despite possibility of nest box imprinting. Journal of Avian

Biology 44:141–148.

Choudhury, S. 1995. Divorce in birds: a review of the hypotheses. Animal Behaviour

50:413–429.

Conroy, M. J., and J. P. Carroll. 2009. Estimation of survival from radiotelemetry,

nesting success studies, and age distributions. Pages 173−177 in Quantitative

Conservation of Vertebrates. Wiley-Blackwell, West Sussex, UK.

Diamond, D. D. 2010. Grasslands. Handbook of Texas Online. Texas State Historical

Association. https://www.tshaonline.org/handbook/online/articles/ryl02. Accessed

16 Aug 2016.

Farmer, C. J., and J. P. Smith. 2009. Migration monitoring indicates widespread declines

of American kestrels (Falco sparverius) in North America. Journal of Raptor

Research 43:263–273.

Graves, L. L. 2010. Lubbock County. Handbook of Texas Online. Texas State Historical

Association. https://www.tshaonline.org/handbook/online/articles/ryl02. Accessed

11 Aug 2016.

Henny, C. J. 1972. An analysis of the population dynamics of selected avian species-with

special references to changes during the current modern pesticide era. U.S. Fish

and Wildlife Service, Washington, D.C.

Hestbeck, J. B., J. D. Nichols, and R. A. Malecki. 1991. Estimates of movement and site

fidelity using mark‐resight data of wintering Canada geese. Ecology 72:523−533.


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Hinnebusch, D. M., J.-F. Therrien, M-A. Valiquette, B. Robertson, S. Robertson, and K.

L. Bildstein. 2010. Survival, site fidelity, and population trends of American

kestrels wintering in southwestern Florida. Wilson Journal of Ornithology

122:475–483.

Leatherwood, A. 2010. Llano Estacado. Handbook of Texas Online. Texas State

Historical Association. https://www.tshaonline.org/handbook/online/articles/ryl02


Mitra, A., C. Chatterjee, and F. B. Mandal. 2011. Synthetic chemical pesticides and their

effects on birds. Research Journal of Environmental Toxicology 5:81–96.

Nisbet, I. C. T. 1991. Problems with darvic color-bands on Common Terns: band losses

and foot injuries. North American Bird Bander 16:61–63.

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Map.” 2016. http://nabci-us.org/resources/bird-conservation-regions-map/#bcr18.

Accessed 13 May 2017.

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Smallwood, J. A., and D. M. Bird. 2002. American kestrel (Falco sparverius). Account

602 in A. Poole, editor. The birds of North America. Cornell Lab of Ornithology,

Ithaca, New York, USA.

Smallwood, J. A., M. F. Causey, D. H. Mossop, J. R. Klucsarits, B. Robertson, S.

Robertson, J. Mason, M. J. Maurer, R. J. Melvin, R. D. and Dawson. 2009. Why

are American kestrel (Falco sparverius) populations declining in North America?

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Agriculture. 2015. Web Soil Survey. http://websoilsurvey.nrcs.usda.gov/.


Steenhof, K., and B. E. Peterson. 2009a. American kestrel reproduction in southwestern

Idaho: annual variation and long-term trends. Journal of Raptor Research 43:283–

290.

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dispersal in American kestrels. Wilson Journal of Ornithology 121:12–21.

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Stubbendieck, J., S. L. Hatch, and N. M. Bryan. 2011. North American wildland plants: a

field guide. University of Nebraska Press.

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boxes. Northeastern Naturalist 22:20–31.

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Accessed 16 Aug 2016. http://tpwd.texas.gov/gis/rtest/

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Table 2.1. Adult kestrel age classes based on capture-recapture/resight data in Lubbock County, TX from 2013−2018. The majority of

banded birds were only caught once. The oldest birds breeding within the nest box program are at least 5 years old.

Age of Bird Count

5 2

4 1

3 6

2 12

1 25


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Table 2.2. Status of American Kestrels nesting in successive years with marked mates in nest boxes in Lubbock County, TX from

2013–2018.

Males Females n % n %

Same box, same mate 6 33.3 6 21.4

Same box, different mate

Mate still alive* 0 − 0 −

Status of former mate unknown 2 11.1 7 25

Individuals who only nested 1 year 10 55.5 15 53.6

Totals 18

28

*We did not detect any evidence of mate divorce occurring in our study area.


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Table 2.3. Number of years that male and female American kestrels demonstrated site fidelity by using nest boxes within 20 active

territories in consecutive years in Lubbock County, TX from 2013−2018.

No. Years Male Female Total

1 10 15 25

2 4 8 12

3 4 2 6

4 0 2 1

5 0 1 2

Total 18 28 46

*Fidelity is only demonstrated when a kestrel uses the same nest box(es) in more than a single year. Several kestrels are assumed to

have an alternate nest box within its territory.


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Figure 2.1. Location of the study area in relation to the Llano Estacado.


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Figure 2.2. Location of three study sites in Lubbock County, Texas used to assess

American kestrel adult survival, mate fidelity, and site fidelity. The green dots represent

individual nest box locations. Site "A" is the Lubbock Lake Landmark site, "B" is the

Reese Technology Center site, and “C” is the Texas Tech University Native Rangeland

site. The average distance of the nearest nest box at site “A” is 241.2 m, 608.3 m at site

“B”, and 84 m at site “C”.

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