EDITED Karmacharya Draft Five_SDT FINAL

Population Dynamics of Northern Cardinal and Carolina Wren in an Urban Forest Fragment:

Safe Refuge or Ecological Trap?

A Thesis

Presented to the

Graduate Faculty of the

University of Louisiana at Lafayette

In Partial Fulfillment of the

Requirements for the Degree

Master of Science

Binab Karmacharya

Fall 2015

© Binab Karmacharya

2015

All Rights Reserved

Population Dynamics of Northern Cardinal and Carolina Wren in an Urban Forest Fragment:

Safe Refuge or Ecological Trap?

Binab Karmacharya

APPROVED:

Scott M. Duke-Sylvester, Chair Joseph E. Neigel

Assistant Professor of Biology Professor of Biology

Paul L. Klerks Mary Farmer-Kaiser

Professor of Biology Dean of the Graduate School

ACKNOWLEDGMENTS

I thank my adviser, Dr. Scott M. Duke-Sylvester, for his guidance throughout my

graduate study and for his patience, critical suggestions, and encouragement during the

preparation of this manuscript. I also thank my committee members, Drs. Joseph E. Neigel

and Paul L. Klerks, for their help in improving this manuscript.

I am very grateful to three individuals: Jeff Hostetler, Erik Johnson, and Jared Wolfe.

Jeff helped me with data analysis and Erik and Jared taught me important aspects of field

ornithology, avian ecology, and bird banding. I cannot imagine completing this work without

their help.

I am grateful to many people and agencies for supporting my graduate degree and this

project. Thanks to all the volunteers at the Louisiana Bird Observatory, who worked hard

under tough conditions to collect the data. The Department of Biology, University of

Louisiana at Lafayette, and the Louisiana Board of Reagent supported my graduate program,

and the Graduate Student Organization at the University of Louisiana provided funding for

field equipment.

I am grateful to my wife, Janabi, daughter, Ojaswi, and my parents for support and

encouragement throughout the graduate program.

TABLE OF CONTENTS

ACKNOWLEDGMENTS ................................................................................................. iv

LIST OF TABLES ............................................................................................................. vi

LIST OF FIGURES .......................................................................................................... vii

INTRODUCTION ...............................................................................................................1

METHODS ..........................................................................................................................5

RESULTS ..........................................................................................................................10

DISCUSSION ....................................................................................................................13

TABLES ............................................................................................................................18

FIGURES ...........................................................................................................................21

LITERATURE CITED ......................................................................................................25

ABSTRACT .......................................................................................................................34

BIOGRAPHICAL SKETCH .............................................................................................36

LIST OF TABLES

Table 1: Model comparison table for Cormack–Jolly–Seber capture–mark–

recapture analysis to investigate the best base model for capture probability (p)

and survival (φ) for Northern Cardinals at the Bluebonnet Swamp Nature Center,

Baton Rouge, Louisiana, from 2010 to 2014. The table includes the number of

parameters (K), difference in AICc (∆AICc), and model weights (relative

likelihood of models in the set). Only the ten best-supported models are presented. ..............18

Table 2: Model comparison table for Cormack–Jolly–Seber capture–mark–

recapture analysis to investigate the best base model for capture probability (p)

and survival (φ) for Carolina Wrens at the Bluebonnet Swamp Nature Center,

Baton Rouge, Louisiana, from 2010 to 2014. The table includes the number of


likelihood of models in the set). For this analysis capture probability (p), was

modeled as p ((season) + effort; Table S2). Only the ten best-supported models

are presented ............................................................................................................................19

Table 3: Model comparison table for reverse-time capture-recapture Pradel model

to investigate the best model for realized population growth rate (λ) for Northern

Cardinals and Carolina Wrens at the Bluebonnet Swamp Nature Center, Baton

Rouge, Louisiana, from 2010 to 2014. The table includes the number of


likelihood of models in the set). For this analysis capture probability (p) was

modeled as p (season + effort + sex) and survival rate (φ) was modeled as φ (year

+ season + sex) for Northern Cardinals, and p (season + effort) and φ (year) for

Carolina Wrens. .......................................................................................................................20

LIST OF FIGURES

Figure 1: Annual, seasonal and sex-specific variation in monthly apparent

survival estimates (± SE) of resident Northern Cardinals in the Bluebonnet

Swamp Nature Center, Baton Rouge, Louisiana, from 2010 to 2014. Open circles

represent females and solid triangles represent males. ............................................................21

Figure 2: Effect of size apparent survival estimates (± SE) of Northern Cardinals

and Carolina Wrens in the Bluebonnet Swamp Nature Center, Baton Rouge,

Louisiana, from 2010 to 2014. .................................................................................................22

Figure 3: Seasonal variation in monthly realized population growth rate (± SE) of

Northern Cardinals and Carolina Wrens in the Bluebonnet Swamp Nature Center,

Baton Rouge, Louisiana, from 2010 to 2014. Open circles represent Carolina

Wrens and solid triangles represent Northern Cardinals .........................................................23

Figure 4: Annual variation in monthly apparent survival estimates (± SE) of

Carolina Wrens in the Bluebonnet Swamp Nature Center, Baton Rouge,

Louisiana, from 2010 to 2014. ................................................................................................24

INTRODUCTION

Habitat loss and fragmentation has been a primary cause of the global biodiversity

crisis (Wilson 1992). The fragmentation of native landscape often results in networks of

small and isolated habitat patches that offer varying degrees of ecological value to resident

and migrant bird communities (Stephens et al. 2004). Although the creation of small nature

preserves embedded within urban matrixes is often perceived as a possible mitigation

strategy to anthropogenic development, these isolated habitat patches may actually serve as

ecological traps where bird species prefer to settle, but experience reduced survival and

reproduction (Gates and Gysel 1978, Robertson and Hutto 2006). Ecological traps can

manifest in small nature preserves for several reasons: edge effects may support populations

of native and exotic predators, increase contact with emerging diseases and environmental

contaminates, and resulting structural changes in vegetation can reduce or alter food and

shelter resources (Batten 1973, Dunn and Tessaglia 1994, Franklin et al. 2000, Rolstad

1991). In general, birds adapted to habitat heterogeneity, such as edge-specialists, tend to fare

better in small nature preserves as opposed to bird species which are more reliant on pristine

habitats (Temple and Cary 1988, Villard 1988, Whitcomb et al. 1981). As urban centers

continue to expand into wild and rural areas, researchers have begun to focus on ways to

improve the quality of urban areas for birds sensitive to human development (Rosenzweig

2003). Clearly, to prevent causing more harm than good, land managers need to understand

how small nature preserves within urban matrices influence bird populations (Donnelly and

Marzluff 2004, Wolfe et al. 2013).

To precisely determine the ecological value of nature preserves to birds, we must

move beyond measures of abundance and focus on species-specific vital rates, such as

2

survival, recruitment, and population growth, to better understand processes that determine

population-level persistence (Van Horne 1983). Additionally, by examining demographic

processes that influence avian persistence within small nature preserves, we can begin to

identify those behavioral and physiological attributes that make certain bird species more

successful than others within a fragmented landscape.

As such, capture-mark-recapture (CMR) methodologies have been used extensively

to generate survival, recruitment and population growth estimates for a diversity of bird

species (Pradel 1996). CMR frameworks are particularly useful for disentangling

mechanisms responsible for variation in vital rates by modeling survival, recruitment, and

population growth as a function of habitat and physiological attributes of individual birds

(Johnson and Omland 2004). For example, a study carried out in the boreal forest of Canada

revealed that Ovenbirds (Seiurus aurocapilla) suffered lower survival in habitat patches

surrounded by a ‘harder’ agricultural-edge than birds in fragments with a ‘softer’ forestry-

edge (Bayne and Hobson 2002). The asymmetrical response to edge effects suggest that birds

residing in nature preserves within urban matrices must be well-adapted for ecotones, or,

conversely, birds in small nature preserves suffer from high mortality rates and populations

are subsequently replenished by immigrants from more salubrious habitats.

Birds are adapted to predictable and seasonal changes in climate, length of day, and

availability of food resources resulting in periods of opportunity and stress. In this context,

non-migratory birds have evolutionarily streamlined three energetically intensive annual

cycle events to maximize individual and population level success throughout the year: spring

and summer reproduction, molting in the late summer and fall, and increased

thermoregulatory needs during the winter season. Non-migratory birds presumably structure

3

the two most energetically taxing phases of the annual cycle, breeding and molt, around the

availability of seasonal food resources (Jacobs and Wingfield 2000). Additional energetic

expenditure during periods of breeding and molt may negatively affect survival despite

increased food availability. Conversely, reduced or unpredictable food resources coupled

with cold weather during the winter periods can have a strong negative influence on survival

(Desrochers et al. 1988, Jansson et al. 1981). Measuring differences in survival across the

annual cycle represents a critical step towards identifying the phase of the life cycle where

non-migratory bird populations are most vulnerable to perturbation and would benefit most

from protection.

In addition to variation throughout the annual life cycle, variation in individual traits

like age, sex, and body size may also influence vital rates (Lomnicki 1988, DeAngelis and

Gross 1992, Lebreton 1992). Overall, body size is an important indicator of individual fitness

and wing length is one of the best proxy-measurements of body size in birds (James 1970,

Hamilton 1961). Clearly, the physiological condition of individual birds influences survival

probability as exemplified by studies of blackbird demography, where a positive correlation

between body size (based on wing length) and survival rate was detected (Searcy and

Yasukawa 1981). Both age and sex can also play a role in determining individual

survivorship. Birds in their first year of life are often less likely to survive than adults

(Woolfenden 1984); differences in survival between age classes may be attributed to superior

competitive ability and a wider range of learned skills amongst adults (Siriwardena et al.

1998, Armstrong et al. 2002). In addition to differences in survival between age classes,

differences in survival between sexes could arise from the cost of sexually selected traits and

asymmetric costs of reproduction (Promislow 1992, Promislow et al. 1992). For example,

4

some blackbird species exhibit sexual skew in survival where dichromatism may result in

higher mortality amongst males with brightly colored plumage (Searcy and Yasukawa 1981).

Mortality amongst males was higher than females during an outbreak of Mycoplasma

infection amongst House Finches (Carpodacus mexicanus) (Nolan et al. 1998).

In this study, we sampled Northern Cardinal (Cardinalis cardinalis) and Carolina

Wren (Thryothorus ludovicianus) populations in a 41.7-ha nature preserve embedded in an

urban matrix to ascertain the influence of sex, age, body size, and seasonal and annual

variation in survivorship, recruitment, and population growth. Specifically we explored if or

how these two species persist in a small habitat fragment, and which vital rate characteristics

are most limiting to their persistence. Our results represent the first published full annual

cycle estimates of survival and population growth relative to age, sex, and body size for non-

migratory passerines.

METHODS

Study Sites –- Our study was carried out at Bluebonnet Swamp Nature Center located

within the city of Baton Rouge, Louisiana (lat: 30.369 and long: -91.107). The swamp is a

41.7-ha reserve containing lowland hardwood forest and forest residential edges dominated

by bald cypress (Taxodium distichum L) and water tupelo (Nyssa aquatica L). The

understory is composed of emergent wetland species in areas that are perennially flooded,

and of dense stands of understory shrubs dominated by Chinese privet (Ligustrum sinense).

Once part of a larger bottomland hardwood system, much of the watershed was converted to

agriculture by the 1940s, followed by a rapid conversion of agricultural to residential use in

the 1960s continuing through today (BREC 2013). To protect the remnant swamp from

continuing urban expansion, the study site was designated as conservation area in 1997 after

being purchased by The Nature Conservancy and donated to the Parks and Recreation

Commission of East Baton Rouge Parish. The park remains an isolated patch of protected

forest and swamp embedded in the dense urban habitat (Wolfe et al. 2013).

Study species – Northern Cardinals are a non-migratory passerine distributed

throughout eastern and central North America from southern Canada into Mexico (Halkin

and Linville 1999). They are found in a variety of habitats with shrubs and/or small trees,

including forest edges and interior, shrubby areas in logged and second-growth forests, marsh

edges, grasslands with shrubs, successional fields, hedgerows in agricultural fields, and

plantings around buildings (Dow 1969, Halkin and Linville 1999). Carolina Wrens are also a

non-migratory passerine distributed throughout the eastern United States and into Mexico

and they inhabit a wide range of habitats including brushy clearcuts, lowland cypress

(Taxodium sp.) swamps, forests, and ravines densely populated by hemlock (Tsuga sp.) and

6

rhododendron (Rhododendron sp.) (Dickson et al. 1984, Horn 1984). In spite of their similar

geographic distribution and habitat preference, Northern Cardinals and Carolina Wrens have

distinctly different life history strategies. Northern Cardinals are sexually dimorphic with

males being bright red and larger relative to the browner-colored females. Northern Cardinals

form pair bonds and defend their territory only during the breeding season, whereas during

the non-breeding season they live in non-cohesive nomadic flocks (Filliater and Breitwisch

1997). Male and female Carolina Wrens are indistinguishable in appearance, with males

being only slightly larger than females (Pyle 1997, Twedt 2004). Also in contrast to Northern

Cardinals, Carolina Wrens maintain a year-round pair bond and home territory, maintained

by calls and songs rather than plumage ornaments (Haggerty et al. 2014).

Field Methods – We sampled bird populations twice a month year-round from April

2010 to March 2014. Each sampling period started at sunrise and lasted for five hours. Birds

were captured using 12 m x 2.6 m mist-nets with a 36-mm mesh size. Nets were deployed at

a set of permanent net runs located within the nature center and positioned in areas with

varying plant densities. From April 2010 to February 2013, we used an initial set of 15 net

locations, which was sampled twice a month. In March of 2013, we added a second set of 15

net location and alternated between the original set of 15 and the new set of 15 net, with each

set being sampled once per month. On a few occasions we ran as few as 10 nets (due to

damaged nets) or as many as 30 nets during a sampling period, and adjusted for variation in

sampling effort in our data analysis as necessary (see below). Nets were checked

approximately every 30 minutes, but during cold weather they were checked as often as

every 15 minutes. All birds extracted from nets were placed individually into clean cotton

7

bags with drawstrings with the time and net number of each capture noted on the bag. Birds

were returned to a central station for processing.

Newly caught individuals were marked with uniquely numbered USFWS metal bands

and released near the capture site. For all captured individuals, we recorded date, net ID,

band number (if previously captured), sex, age, molt limit, molt cycle, reproductive

condition, mass, and wing length. Birds were aged using molt limit criteria described by Pyle

(1997). For this study, we categorized individuals into one of two age groups, immature or

adult. Individuals in their pre-formative molt or earlier in life were categorized as "immature"

(approximately < 5 months old), whereas individuals in their formative and definitive basic

plumages were categorized as "adult" (approximately > 5 months old).

We conducted 104 capture occasions over the four-year period. To analyze the data,

we divided each year into three seasons: breeding, molting, and winter. The study period was

divided into annual cycles spanning between April of one year and March of the subsequent

year. This meant that each study year started with the breeding season and ended with the

winter season. Observed breeding and molting records of birds captured at our study site

were used to determine the onset and termination of those seasons. Based upon these

observations, we categorized April 1 to July 15 as the breeding season, July 16 to October 31

as the molting season, and November 1 to March 31 as the non-breeding season. This was

consistent with the breeding and molting cycle reported in the previous studies (Halkin et al.

1999, Haggerty et. al. 2014). Standardized right wing length (standardized to have a mean of

0 and a standard deviation of 1) was used as an index of body size.

Statistical Methods – We analyzed our data using the program MARK (White and

Burnham 1999) with the R package RMark (R Development Core Team 2013, Laake and

8

Rexstad 2008). We performed a goodness-of-fit test using the median ĉ approach (White and

Burnham 1999) and found no evidence of over-dispersion or lack of fit (ĉ = 1.00). Model

selection was accomplished with Akaike’s Information Criterion corrected for small sample

size (AICc) (Burnham and Anderson, 2002). The model with the lowest AICc value is

considered the best supported model, although models with a difference in AICc (∆AICc) of

< 2 are considered to have similar support with no evidence for difference among models

being compared; 2 ≤ ∆AICc ≤ 4 suggests evidence for considerable difference, 4 ≤ ∆AICc ≤

7 suggests substantial evidence for difference, and ∆AICc > 7 is generally indicative of

overwhelming evidence for difference in support received by the models being compared

(Burnham and Anderson 2002). If no single model received overwhelming support,

suggesting model selection uncertainty, we averaged parameter estimates weighed by model

certainty (Burnham and Anderson 2002). We investigated various aspects of demography

using three classes of CMR models: Cormack Jolly Seber (CJS), multi-state, and reverse-

time Pradel models.

Apparent survival rate (φ) and recapture probability (p) were modeled and estimated

with CJS models (Lebreton et al. 1992). Model selection was carried out in a sequential

approach. First we investigated the influence of sex, season, year, capture effort (the number

of nets used during each capture occasion), and capture occasion on p. The best model of p

was used to test for effects of sex, transient individuals, season, size, year, and their additive

and two-way interactive effects on φ. Time-since-marking (TSM) models that accounted for

transients were used to account for survival deflation due to presence of transient individuals

moving thorough the study area (Pradel 1996, Pradel et al. 1997). Finally, multi-state models

9

were used to test for variation in age-specific (immature and adult) apparent survival rate

(Brownie et al. 1993).

A reverse-time Pradel model was used to model and estimate realized population

growth rate (λ) (Hines and Nichols 2002, Pradel 1996). We tested for the effect of season,

year and their additive effect on λ. Even though reverse time models cannot account for the

negative bias in survival rate introduced by transient individuals, the estimates of λ will be

unbiased, as underestimation of survival rates is balanced by overestimation of recruitment

rate (Saracco and DeSante 2008). Recruitment rates (f) and seniority parameters (γ) were

derived from λ and φ (estimated with CJS model accounting for transients) using the formula

f = λ - φ and γ = φ/ λ (Saracco et al. 2008). Proportional contribution of φ and f to λ was

explored by using seniority parameter (γ), the probability that an individual in the population

was also present in the population previous period (Nichols et al. 2000).

The time intervals between capture occasions were adjusted in the program MARK

such that monthly (30-day) estimates of population parameters were provided. We report

estimates of parameters and confidence intervals as monthly rates and report annual estimates

as monthly rates raised to the 12th power. Unless indicated otherwise, all means were

presented as ± 1 standard error (SE).

RESULTS

Northern Cardinal – During our four-year study period, we captured 362 (189 males and

173 females) individual cardinals a total of 843 times. Our initial model analysis using CJS

models revealed that the capture probability (p) was best described by an additive effect of

sex, season, and capture effort (Table 1). The best-supported model for monthly apparent

survival, φ, included an interactive effect of sex and body size, and an additive effect of year,

season, and transients (Table 1). Because the top ten models differed by ΔAICc < 7,

indicating model selection uncertainty, we averaged the top ten models to obtain model-

averaged parameter estimates. The latter estimates were similar to the estimates from the

best-supported model.

Overall average monthly survival (φ) was 0.947 ± 0.006 during the study period.

Estimates from the best-supported CJS model suggested that average monthly φ was highest

for females during the winter season of year 2011-12 (0.977 ± SE 0.009) and lowest for

males during the breeding season of year 2010-11 (0.811 ± 0.057) (Figure 1). Average

monthly φ was 0.954 ± 0.007 for females and 0.938 ± 0.009 for males. Average monthly φ

was lowest in the year 2010-11 compared to rest of the study period (Figure 1). Average

monthly φ was 0.913 ± 0.020 for the breeding season, 0.952 ± 0.014 for the molting season,

and 0.963 ± 0.012 for the winter season. The top CJS models included body size as an

explanatory variable, with bigger birds exhibiting higher survival (Figure 2). The overall

average monthly survival rate of adults (0.940 ± 0.006) was higher than that of immatures

(0.870 ± 0.060).

The best-supported Pradel model indicated that seasonal variation was an important

factor for the population growth rate (λ) (Table 3). Average monthly population growth rate

11

(λ) was highest during the breeding season (1.116 ± 0.035) and lowest during the winter

season (0.957 ± 0.019) (Figure 3). Overall average monthly λ was 1.007 ± 0.004 and average

annual λ was 1.092 ± 0.051. Average monthly recruitment rate (f) and seniority rate (γ)

during the breeding season were 0.203 and 0.818.

Carolina Wren – During our four-year study period, we caught 149 individual wrens

a total of 432 times. Due to the absence of obvious sexual dimorphism, we did not categorize

individuals as male or female. Our initial model analysis with the CJS method revealed that

capture probability (p) was best described by an additive effect of season and capture effort

(Table 2). The best supported model for monthly apparent survival (φ) included an additive

effect of year, body size, and transients (Table 2); however, the top ten models differed by

ΔAICc < 7, indicating model selection uncertainty. We employed model averaging to obtain

model-averaged parameter estimates. Model averaged parameter estimates were similar to

the best-supported model and there was no indication of seasonal variation. Survival

estimates are provided only for the resident birds with transients excluded.

Overall monthly survival rate φ was 0.919 ± 0.011. The best-supported model showed

that average monthly apparent survival rate (φ) was highest in year 2010-11 (0.955 ± 0.015)

and lowest in the year 2012-13 (0.897 ± 0.020) (Figure 4). The top CJS models included

body size as an explanatory variable, with bigger birds exhibiting higher survival (Figure 2).

Monthly survival rate for immatures (0.656 ± 0.053) were significantly lower than that of

adults (0.938 ± 0.010).

The most parsimonious model for population growth rate (λ) included a model with

seasonal variation (Table 3). Average monthly λ was highest during the breeding season

(1.246 ± 0.058) and lowest during the molting season (0.883 ± 0.038) (Figure3). Overall

12

average monthly λ was 1.010 ± 0.002 and average annual λ was 1.133 ± 0.075. Average

monthly recruitment rate (f) and seniority rate (γ) during the breeding season were 0.304 and

0.756, respectively.

DISCUSSION

Annual life cycle estimates of survival, recruitment and population growth rates for non-

migratory birds are surprisingly rare as studies often focus on breeding or winter ecology

exclusively. Despite a growing interest in measuring vital rates for migratory birds during

breeding, migratory, and winter periods (Hostetler et al. 2015, Taylor and Norris 2010), we

are unaware of studies that report annual life cycle estimates of multiple vital rates for non-

migratory passerines. In this vein, we used 4 years of year-round mark-recapture data to

examine variation in survival and population growth of Northern Cardinals and Carolina

Wrens within a nature preserve throughout their annual cycles. The overall annual survival

rate (0.520 ± 0.050) of the Northern Cardinal was similar to the regional baseline estimate of

0.537 ± 0.008 (DeSante and Kaschube 2009) and overall annual survival rate (0.349 ± 0.050)

of Carolina Wren was similar to the regional baseline estimate of 0.358 ± 0.022 (DeSante

and Kaschube 2009). In spite of their contrasting life history strategies, both species had

stable or even slightly growing populations (annual growth rate >1) suggesting that the 41.7-

ha refuge served as a source, not an ecological trap, for these two resident bird species.

Our study did not find support for a proposed facet of life history tradeoff theory,

which predicts lower survival during cold winter months when food resources become scarce

(Desrochers et al. 1988, Jansson et al. 1981). The paucity of winter food resources has been

elicited as a selective pressure that led to the long-distance movements of Nearctic-Neotropic

migrant birds (Cox 1968, Levey and Stiles 1992). Five non-mutually exclusive processes

may explain the lack of evidence for a demographic tradeoff between the breeding and winter

seasons at our study site. First, the subtropical environment of central Louisiana provides the

winter resources necessary to maintain relatively high survival for both species. Second, the

14

availability of supplemental food at bird feeders may mitigate the paucity of natural food

resources during winter months for cardinals (wrens do not feed at feeders). Third, some

dominant nonnative plants, such as Chinese privet (Ligustrum sinense), may provide the food

and structural resources necessary to sustain relatively high survival through the cold winter

months. More specifically, Chinese privet is not deciduous thereby potentially providing the

structural resources for insectivorous understory birds, like Carolina Wren, to acquire the

food necessary to persist through the winter. Furthermore, Chinese privet provides an

abundant winter crop of fruit that Northern Cardinals commonly consume (J. D. Wolfe,

personal observations). Fourth, an absence of some migrant birds may provide an

intraspecific competitive release for both species. Fifth, these species have evolved to adjust

metabolic needs in winter and adjust their diet in concordance with seasonally shifting food

resources. Expanding this study across latitudes and habitats would provide valuable tests of

these hypotheses.

Seasonal variation in survival was detected only for the Northern Cardinal, where

survival was lowest during the breeding season in both sexes, suggesting a cost associated

with breeding in the study area. The cost of reproduction on survival is an important tradeoff

around which life histories are thought to evolve (Williams 1966, Stearns 1992); tradeoffs

affecting survival during the breeding season include metabolic drain or increased

susceptibility to pathogens, such as West Nile virus and avian malaria, and increased

vulnerability to predators (Bennett and Cameron 1974, Harshman and Zera 2007,

Magnhagen 1991, Post and Gotmark 2006, Reisen et al. 2006). Northern Cardinal and

Carolina Wren have different life history strategies, which may help explain interspecific

differences in seasonal survival. For example, Northern Cardinals live in large non-cohesive

15

nomadic flocks during non-breeding seasons, and pair bond and defend their territory during

breeding season (Filliater and Breitwisch 1997). Thus, during the breeding season, Northern

Cardinals lose anti-predator benefits of living in large flocks, which may partly explain their

lower survival during this period (Lima 2009). In contrast, the Carolina Wren maintains a

year-round pair bond and home territory such that they don't make drastic changes in

territorial behavior during breeding and non-breeding seasons (Haggerty et al. 2014).

Maintaining a single year-round territory and liberty from demands of finding a mate for pair

bonding may make breeding a less costly endeavor for Carolina Wrens relative to Cardinals.

We suggest that lower survival in male Northern Cardinals may be due to costs

associated with bright-red plumage, larger size and generally elevated exposure to predators.

Sexually selected traits in males, like carotenoid pigmentation, are not only costly to produce,

but also may increase predation risk (McGraw et al. 2005). Unfortunately, we could not

ascertain sex for many captured Carolina Wrens to examine sexually mediated differences in

survival where males do not invest in sexually selected traits, but are generally larger than

females.

Immature birds of both species had significantly lower survival than adults; however,

this difference was more pronounced in Carolina Wrens than Northern Cardinals. Due to

year-round territory defense, young Carolina Wrens may be subjected to intraspecific

competition with their adult counterparts once they are fledged. Conversely, Northern

Cardinals live in non-territorial flocks, thereby potentially releasing young cardinals from the

negative effects of intraspecific competition.

Body size (wing length) in both Northern Cardinal and Carolina Wren was found to

positively vary with survival. This effect was present even after accounting for potential

16

biases due to sex (female Northern Cardinals are smaller than males) and age specific

differences in body size (immatures are generally smaller than adults). We also tested

quadratic models for size effect to see if an intermediate size is optimal for either species, but

found no support for these alternatives. Although larger birds tended to exhibit higher

survival, presumably a threshold exists where larger birds incur a cost for their size or,

conversely, food provisioned to nestlings, has cascading effects on adult body size, thereby

allowing large birds to acquire and defend better territories later in life. Disentangling the

processes responsible for differences in survival relative to body size represents an

interesting avenue for further research.

To our knowledge, our population growth rate estimates are the first for these two

species in an urban nature reserve. Population growth rate is a critical parameter of interest in

the study of wildlife population because it provides information regarding the long-term

viability of focal populations (Nichols et al. 2000). Average annual growth rate estimates

were greater than 1.0 for both species, indicating stable and perhaps slightly increasing

populations. Despite lower survival relative to Northern Cardinals, Carolina Wrens were able

to maintain a stable population through higher recruitment. During the breeding season,

monthly population growth was estimated to be about 24% and 11% for Carolina Wrens and

Northern Cardinals, respectively. The proportional contribution of survival and recruitment

towards population growth as measured with seniority rate, indicated that recruitment yielded

a higher contribution for Carolina Wrens than it did for Northern Cardinals. The lower

survival rate of Carolina Wrens relative to Northern Cardinal appears to be offset by its

higher reproductive rate, as Carolina Wrens tend to have larger average clutch size and

17

higher reproductive success (Haggerty et al. 2014, Halkin et al. 1999, Crowell and Rothstein

1981).

This study provided detailed vital rate estimates for two birds population in an urban

nature preserve and provided a realistic demographic target for conservation efforts of more-

sensitive species in human-modified landscapes. Unfortunately, the statistical techniques

necessary to provide robust estimates of survival and recruitment are data intensive and this

fact precludes doing such analysis for many species of conservation concern because they are

inherently rare. Thus, estimating the demographic response of model bird species to habitat

degradation may provide insight into those factors that limit the recovery of more-sensitive

species.

TABLES

TABLE 1 —Model comparison table for Cormack–Jolly–Seber capture–mark–

recapture analysis to investigate the best base model for capture probability (p) and survival

(φ) for Northern Cardinals at the Bluebonnet Swamp Nature Center, Baton Rouge, Louisiana,

from 2010 to 2014. The table includes the number of parameters (K), difference in AICc

(∆AICc), and model weights (relative likelihood of models in the set). Only the ten best-

supported models are presented.

Model K ∆AICc a Model weight

Capture probability

p (sex + season + effort) 13 0.000 0.415

p ((sex * season) + effort) 15 0.420 0.336

p (season + effort) 12 1.600 0.186

p (year + season + effort) 15 4.142 0.052

p ((year * season) + effort) 21 7.337 0.011

p (effort) 10 16.916 0.000

p (sex + year + effort) 14 17.749 0.000

p ((sex * year) + effort) 17 18.574 0.000

p (year + effort) 13 19.039 0.000

Survival rate

φ ((sex * size) + year + season + transients) 20 0.000 0.208

φ (sex + year + season + size + transients) 19 0.454 0.166

φ ((sex * size) + transients) 15 0.674 0.149

φ (year + season + transients + size) 18 1.336 0.107

φ (season + size + transients) 15 1.440 0.101

φ (sex + transients + size) 14 1.640 0.092

φ (size + transients) 13 1.780 0.085

φ (year + size + transients) 16 3.162 0.043

φ (season + transients) 14 5.301 0.015

φ (year + transients + season) 17 6.403 0.000

a Values of AIC for the top-ranked models for capture probability and survival rate were

3727.516 and 3694.173, respectively.

19

TABLE 2 —Model comparison table for Cormack–Jolly–Seber capture–mark–

recapture analysis to investigate the best base model for capture probability (p) and survival

(φ) for Carolina Wrens at the Bluebonnet Swamp Nature Center, Baton Rouge, Louisiana,

from 2010 to 2014. The table includes the number of parameters (K), difference in AICc

(∆AICc), and model weights (relative likelihood of models in the set). For this analysis

capture probability (p), was modeled as p ((season) + effort; Table S2). Only the ten best-

supported models are presented.


Capture Probability

p (season + effort) 12 0.000 0.727

p (year + season + effort) 15 2.084 0.257

p (season) 6 8.546 0.010

p ((year + season) + effort) 21 10.521 0.004

p (year + season) 9 12.285 0.002

p (year * season) 15 13.451 0.001

p (effort) 10 30.808 0.000

p (year + effort) 13 34.581 0.000

p (constant) 4 51.037 0.000

p (year) 7 53.238 0.000

Survival rates

φ (year + transients + size) 15 0.000 0.332

φ (size + transients) 12 0.603 0.246

φ (year + transients) 14 2.045 0.119

φ (transients) 11 2.060 0.119

φ (year + season + size + transients) 17 3.718 0.052

φ (season + size + transients) 14 3.849 0.048

φ (season + transients) 13 5.584 0.020

φ (year + season + transients) 16 5.888 0.017

φ (size) 11 6.237 0.015

φ (year + size) 14 6.556 0.013

a Values of AICc for the top-ranked models for capture probability and survival rate were


20

TABLE 3 —Model comparison table for reverse-time capture-recapture Pradel model

to investigate the best model for realized population growth rate (λ) for Northern Cardinals

and Carolina Wrens at the Bluebonnet Swamp Nature Center, Baton Rouge, Louisiana, from

2010 to 2014. The table includes the number of parameters (K), difference in AICc (∆AICc),

and model weights (relative likelihood of models in the set). For this analysis capture

probability (p) was modeled as p (season + effort + sex) and survival rate (φ) was modeled as

φ (year + season + sex) for Northern Cardinal, and p (season + effort) and φ (year) for

Carolina Wren.


Northern Cardinal

λ (season) 19 0.000 0.827

λ (year + season) 22 3.254 0.163

λ (constant) 17 9.491 0.007

λ (year) 20 11.433 0.003

Carolina Wren

λ (season) 18 0.000 0.854

λ (year + season) 21 6.124 0.106

λ (constant) 16 18.517 0.040

λ (year) 19 24.020 0.000

a Values of AICc for the top-ranked models for Northern Cardinals and Carolina Wrens were


FIGURES

FIGURE 1 —Annual, seasonal and sex-specific variation in monthly apparent survival

estimates (± SE) of resident Northern Cardinals in the Bluebonnet Swamp Nature Center,

Baton Rouge, Louisiana, from 2010 to 2014. Open circles represent females and solid

triangles represent males

22

FIGURE 2 —Effect of size on apparent survival estimates (± SE) of Northern Cardinals and

Carolina Wrens in the Bluebonnet Swamp Nature Center, Baton Rouge, Louisiana, from

2010 to 2014.

23

FIGURE 3 —Seasonal variation in monthly realized population growth rate (± SE) of

Northern Cardinals and Carolina Wrens in the Bluebonnet Swamp Nature Center, Baton

Rouge, Louisiana, from 2010 to 2014. Open circles represent Carolina Wrens and solid

triangles represent Northern Cardinals.

24

FIGURE 4 — Annual variation in monthly apparent survival estimates (± SE) of Carolina

Wrens in the Bluebonnet Swamp Nature Center, Baton Rouge, Louisiana, from 2010 to

2014.

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Karmacharya, B. Bachelor of Science, Tribhuvan University, 2003; Master of Science,

University of Florida, Fall 201, Master of Science

Major: Biology

Title of Thesis: Population Dynamics of Northern Cardinal and Carolina Wren in an Urban

Forest Fragment: Safe Refuge or Ecological Trap?

Thesis Director: Dr. Scott M. Duke-Sylvester

Pages in Thesis: 36, Words in Abstract: 298

ABSTRACT

Conserving bird populations in urban landscapes often depends on interactions

between extinction, recolonization, and survival in remnant habitat patches such as small

nature preserves. Thus, determining the ecological value of small nature preserves to birds is

a necessary step towards an informed conservation strategy. As such, I conducted a year

round capture-mark-recapture study from April 2010 to March 2014 to examine population

dynamics of Northern Cardinals (Cardinalis cardinalis) and Carolina Wrens (Thryothorus

ludovicianus) in a 41.7-ha nature preserve embedded in an urban matrix. More specifically,

we examined variation in survival, recruitment, and realized population growth rates relative

to year, season, sex, age, and wing length (as a proxy for body size) to investigate attributes

that affect individual survival and to assess whether the reserve served as a population source

or sink. The overall annual apparent survival rate of Northern Cardinals (0.520 ± SE 0.050)

was higher than that of the Carolina Wrens (0.349 ± 0.050), and estimates in both species

were similar to regional baseline estimates. The survival rates for adults were significantly

higher than for immatures in both species, with body size having a positive influence on

survival. Seasonal variation in survivorship was evident only in Northern Cardinals, being

highest in the winter and lowest during the breeding season. Average annual population

growth rate was slightly greater than 1.0 for both species, indicating stable or perhaps

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modestly increasing populations. These results represent the first published full annual cycle

estimates of survival and population growth relative to age, sex, and body size for non-

migratory passerines. Our results suggest that urban forests can provide the necessary

resources to sustain growing populations of locally common birds. Furthermore, our

demographic estimates derived from two healthy bird populations can serve as target values

for other species of conservation concern within human-modified landscapes.

BIOGRAPHICAL SKETCH

Binab Karmacharya is a native of Nepal. Binab completed his masters in Wildlife

Ecology and Conservation from the University of Florida, Gainesville, Florida. He studied

the effect of longleaf pine management practices on the population dynamics of small

mammals in southeastern United States. After his masters, he worked as a wildlife biologist

for environmental consultancy Normandeau Associates in Gainesville, Florida, studying

potential impacts of windmills on different avian species. This got him interested in avian

ecology and he joined the graduate school at the University of Louisiana at Lafayette to study

population dynamics of passerine birds in an urban forest. He completed his masters in Fall

2015.

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