Patterns of Brown-Headed Cowbird Parasitism in a Recently Invaded Area and Potential Mechanisms Limiting Cowbird Reproduction

PATTERNS OF BROWN-HEADED COWBIRD PARASITISM IN A RECENTLY INVADED AREA AND POTENTIAL MECHANISMS LIMITING COWBIRD REPRODUCTION

MATTHEW J. REETZ

A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY

UNIVERSITY OF FLORIDA

2008

To LeAnn, my favorite wife

ACKNOWLEDGMENTS

Invaluable field help was provided by Andrew Cook, Steve Daniels, Dan Dawson,

Elizabeth Bauman, Nia Haynes and Rebecca Hamel. I thank the Florida Department of

Environmental Protection, Danny Driver, David Armstrong, Joel McQuagge, Jesse Smalls, Steve

Coates, Mats Troedsson, and Bruce Christensen of the University of Florida (UF), and Geoff

Parks of the Gainesville Nature Operations Division for access to field sites. I am indebted to

Michael Avery, Eric Tillman, John Humphrey, Kandy Keacher and Michael Milleson of the

USDA National Wildlife Research Center for support with trapping and maintenance of captive cowbirds. Brian Branciforte and Kate Haley of the Florida Fish and Wildlife Commission and

Monica Lindberg, Caprice McCrae, Laura Hayes and Sam Jones of UF provided valuable logistical support. Research design of this project was improved through comments of various avian discussion groups. My deepest thanks go to my committee members Michael Avery, Doug

Levey, David Steadman, and especially co-chairs Kathryn Sieving and Scott Robinson.

Research protocols D539 and D954 were approved by the UF Institutional Animal Care and Use

Committee. This work was made possible by the Florida Fish and Wildlife Conservation

Commission’s program, Florida’s Wildlife Legacy Initiative, and the U.S. Fish and Wildlife

Service’s State Wildlife Grants program (SWG04-031). Finally, my thanks go to C. LeAnn

White for her steadfast support, especially when the light at the end of the tunnel grew dim.

TABLE OF CONTENTS

page

ACKNOWLEDGMENTS ...... 4

LIST OF TABLES...... 7

LIST OF FIGURES ...... 8

ABSTRACT...... 10

CHAPTER

1 PATTERNS OF PARASITISM IN NORTH-CENTRAL FLORIDA, A REGION OF RECENT COWBIRD EXPANSION ...... 12

Introduction...... 12 Patterns of Cowbird Expansion in the Southeast ...... 15 Objectives...... 16 Methods ...... 17 Results...... 18 Discussion...... 20 Cowbird Parasitism in North-Central Florida ...... 20 Potential Limits on Parasitism Rates or Their Accurate Detection...... 22 Conclusions and Management Implications...... 26

2 RESPONSES OF FOUR SONGBIRD SPECIES TO EXPERIMENTAL COWBIRD PARASITISM IN A RECENTLY INVADED AREA ...... 32

Introduction...... 32 Methods ...... 35 Study Design ...... 35 Selection of Study Species ...... 36 Results...... 38 Discussion...... 39 Rejection Responses of Florida Hosts...... 39 Acceptance of Cowbird Eggs ...... 41 Conclusions ...... 43

3 DOES VARIATION IN FEMALE COWBIRD FECUNDITY EXPLAIN OBSERVED LOW LEVELS OF PARASITIZATION?...... 46

Introduction...... 46 Limitations to Cowbird Reproduction...... 47 Low Nest Parasitism in Florida: Low Cowbird Fecundity?...... 48 Methods ...... 49 Cowbird Collection ...... 49

Reproductive Assays ...... 51 Clutch Size Estimates...... 53 Fecundity Estimates...... 55 Results...... 56 Cowbird Collection ...... 56 Reproductive Assays ...... 57 Clutch Size Estimates...... 61 Fecundity Estimates...... 63 Additional Trapping Observations ...... 65 Discussion...... 66 Clutch Size Estimates...... 68 Fecundity Estimates...... 69 Cowbird Collection ...... 71 Conclusions ...... 73

4 ALTERNATIVE HYPOTHESES FOR THE “SLOW” COWBIRD INVASION OF FLORIDA...... 84

Introduction...... 84 Host Community Composition...... 86 Nest Predation...... 92

APPENDIX

A STUDY SITE DESCRIPTION AND HOST SPECIES SURVEYED...... 102

LITERATURE CITED ...... 110

BIOGRAPHICAL SKETCH ...... 129

LIST OF TABLES

Table page

1-1 Summary of nests and family groups for 32 species monitored in the study areas, 2004−2006...... 30

2-1 Results of experimental parasitism of four common Florida species...... 45

3-1 Summary of means of morphometric data taken from breeding female Brown-headed cowbirds collected in Florida and Texas, 2005−2007 and results of statistical comparisons...... 75

3-2 Estimate of clutch size of 2005 Texas cowbirds...... 75

3-3 Estimate of clutch size of 2006−2007 Florida cowbirds...... 76

4-1 Host species documented as breeders in Florida also known to have fledged Brown- headed Cowbirds...... 100

LIST OF FIGURES

Figure page

1-1 Breeding Bird Survey data of Brown-headed Cowbirds detected per route in Florida from 1966−2006...... 29

1-2 Estimated number of Brown-headed Cowbirds per Breeding Bird Survey route for northern and southern Florida, 1966−2006...... 29

3-1 Location of Brown-headed Cowbird collection sites from April-August, 2005-2007...... 77

3-2 Number of female cowbirds collected per month at sites in north-central Florida in 2006 and 2007...... 77

3-3 Mean number of female cowbirds collected per trap day at sites in north-central Florida from April to July, 2006−2007...... 78

3-4 Number of female cowbirds collected per month in Florida by trap and by pellet gun from April−July 2007...... 78

3-5 Mean weekly mass of ovaries of all Florida and Texas female cowbirds collected between 15 April and 15 July, 2005−2007...... 79

3-6 Mean weekly mass of oviducts of all Florida and Texas female cowbirds collected between 15 April and 15 July, 2005−2007...... 79

3-7 Mean weekly mass of ovaries of breeding Brown-headed Cowbirds collected in Florida, 2006−2007...... 80

3-8 Mean weekly mass of oviducts of breeding Brown-headed Cowbirds collected in Florida, 2006−2007...... 80

3-9 Mean weekly mass of ovaries of breeding Brown-headed Cowbirds collected in Texas, 2005...... 81

3-10 Mean weekly mass of oviducts of breeding Brown-headed Cowbirds collected in Texas, 2005...... 81

3-11 Mass of female Brown-headed Cowbirds collected in Florida between 23 April and 12 July, 2006−2007...... 82

3-12 Mass of female Brown-headed Cowbirds collected in Texas between 19 April and 6 July, 2005...... 82

3-13 Mean area of post-ovulatory follicles (POFs) of Brown-headed Cowbirds collected in Texas and Florida from April to June, 2005−2007...... 83

3-14 Estimated laying rate (eggs produced per day) of breeding cowbirds collected in Florida and Texas during four periods of the cowbird breeding season...... 83

4-1 Distribution of Brown-headed Cowbirds in Florida. Redrawn from Breeding Bird Survey abundance map (1994−2003)...... 97

4-2 Proportion of total abundance composed of species of varying quality as cowbird hosts ...... 97

4-3 Total number of host species of three types at locations in three regions...... 98

4-4 Mean abundance of three types of Brown-headed Cowbird hosts in three regions...... 98

4-5 Mean abundance of forest species that represent good quality cowbird hosts in Illinois and Florida...... 99

Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy

PATTERNS OF BROWN-HEADED COWBIRD PARASITISM IN A RECENTLY INVADED AREA AND POTENTIAL MECHANISMS LIMITING COWBIRD REPRODUCTION

Matthew J. Reetz

May 2008

Chair: Kathryn E. Sieving Cochair: Scott K. Robinson Major: Wildlife Ecology and Conservation

Despite conservation implications surrounding recent Brown-headed Cowbird (Molothrus ater) expansion into the southeastern United States, there are few ecological data on cowbirds in

Florida. Objectives for this project were to (1) quantify parasitism rates to determine the extent of brood parasitism and identity of host species and (2) test hypotheses explaining patterns of parasitism and cowbird distribution in Florida.

I surveyed parasitism of 35 host species breeding in Florida through nest monitoring and observations of family groups with fledglings. Although cowbirds are moderately abundant in the study area, only 1.6% of 1117 nests and 6.1% of 278 family groups with fledglings were parasitized. While rates of parasitism are extremely low across the community and for individual hosts, my results suggest that cowbirds may preferentially parasitize a few host species nesting in the midstory and canopy.

To help explain observed low cowbird parasitism rates, I tested egg rejection behavior of four common Florida songbird species known to reject parasitism elsewhere. I experimentally parasitized 78 nests using eggs obtained from a captive cowbird colony. Three of four species

tested typically accepted experimental cowbird eggs, rejecting parasitism at significantly lower

rates than those published in the literature for each species. Rejection behaviors do not appear to limit Brown-headed Cowbird reproduction in north-central Florida.

I tested the hypothesis that reduced physiological fecundity of female Brown-headed

Cowbirds contributed to low observed parasitism rates. I compared ovarian reproductive condition of breeding female cowbirds in Florida with those from Texas, where high parasitism rates have been recorded. Based on reproductive assays, I estimated that average female cowbirds in Florida and Texas have similarly high fecundity at 34 eggs per female per breeding season.

Low observed cowbird parasitism frequencies combined with moderate abundance of

cowbirds in north-central Florida presents a paradox that my data do not explain. Cowbird

reproduction may be limited by high nest predation rates or limited diversity and abundance of

host species. I present a summary of census and reproductive data that suggest that these

mechanisms may contribute to observed patterns of cowbird parasitism in Florida.

CHAPTER 1 PATTERNS OF PARASITISM IN NORTH-CENTRAL FLORIDA, A REGION OF RECENT COWBIRD EXPANSION

Introduction

The Brown-headed Cowbird (Molothrus ater) is one of three brood-parasitic cowbird

species in North America. It was prehistorically mainly found in the grasslands of the Great

Plains region of central North America where it foraged with bison herds and parasitized locally

breeding host populations. With the arrival of European settlers that caused habitat

fragmentation, destruction of bison populations, and establishment of permanent feeding areas,

cowbirds began expanding their range in response to habitat fragmentation and the replacement

of nomadic bison with largely sedentary cattle. Until the 1800s, Brown-headed Cowbirds were

uncommon in the eastern United States, probably because of a lack of suitable open feeding

habitats among large tracts of forest (Mayfield 1965). With the increase in human population,

however, cowbirds began to appear in states successively farther east (Lowther 1993) and local records of cowbirds changed from rare or absent to common or abundant (Mayfield 1965).

Presently, the breeding range of the Brown-headed Cowbird includes the entire contiguous

United States and much of North America (Sauer et al. 2006).

Because of its recent, widespread expansion, the Brown-headed Cowbird has been of

great conservation concern. It is a generalist obligate brood parasite that continually parasitizes

new host species, and has a prodigious reproductive output with females laying 40 eggs per

season (Scott and Ankney 1980) in the nests of over 220 host species (Friedmann and Kiff

1985). Prior to human alteration of the landscape, cowbirds were naturally limited to host

species with which they were sympatric during the breeding season. The spread of the Brown-

headed Cowbird brought it into contact with a new suite of host populations and species, most of

which lacked recent historical exposure to brood parasitism. Significant songbird population

declines during the 20th century were thought to be due to the steady increase of cowbird

populations continent-wide (Mayfield 1977, Grzybowski et al. 1986, Robbins et al. 1989, Trail

and Baptista 1993).

Where Brown-headed Cowbird populations are well established, brood parasitism can

significantly reduce reproductive success of host species and communities (Friedmann et al.

1977, Robinson et al. 1995b). For example, Brown-headed Cowbird parasitism contributes to

negative net reproduction of some songbird populations, which are likely maintained only by

immigration from other areas (Robinson et al. 1995b). Cowbirds limit reproduction of hosts in a

variety of ways, including reduced clutch sizes (Mayfield 1961, Burhans et al. 2000, Hoover

2003) through removal and predation of host eggs or young (Friedmann 1963, Sealy 1992,

Arcese et al. 1996, Granfors et al. 2001, Hoover and Robinson 2007), or reduced hatching

(McMaster and Sealy 1998, Burhans et al. 2000) and host fledging success (McMaster and Sealy

1998, Burhans et al. 2000). Costs can also be incurred beyond the nest through increased

juvenile and adult mortality (Payne 1965, Hoover and Reetz 2006).

Overall costs of cowbird parasitism are particularly evident at local scales in contact zones where cowbirds begin parasitizing novel hosts that are highly susceptible to parasitism.

Cowbirds spread rapidly throughout California beginning in the early 1900s, for example, and

brood parasitism has contributed to the extirpation of local populations of Least Bell’s Vireo

(Vireo bellii) in much of its California range (USFWS 1998). Furthermore, parasitism by

Brown-headed Cowbirds has threatened the extinction of species with limited ranges such as

Kirtland’s Warbler (Dendroica kirtlandii) in northern Michigan (Mayfield 1992). As a result,

Brown-headed Cowbirds became a regional management problem for particular songbird species

and were often aggressively controlled to improve reproductive success of hosts (Kelly and

DeCapita 1982, Beezley and Rieger 1987, Eckrich et al. 1999). Identifying situations where

management of cowbird populations is necessary to prevent such declines is of importance in

conservation of some populations of host species (Rothstein and Cook 2000), particularly since

the cowbird has spread over most of North America, coming into contact with new host species

and populations.

There are many reasons to monitor the reproductive success (and thereby, cowbird

parasitism rates) of songbirds for conservation and management of avian habitats, populations, and species. Potential impacts of cowbird parasitism can be identified using rates of parasitism as an initial cue, with further detailed work leading to conservation of affected host species.

Research to characterize Shiny Cowbird (Molothrus bonariensis) expansion to Puerto Rico

(since the 1940s), for example, identified extremely high rates of parasitism (75−100%) on

multiple native species (Post and Wiley 1977, Wiley 1985). This work ultimately led to a

conservation program for the endangered, endemic Yellow-shouldered Blackbird (Agelaius

xanthomus), which had suffered population declines of >80% as a result of intense parasitism

(Wiley et al. 1991). A large-scale cowbird trapping program at Fort Hood, Texas reduced

parasitism of the federally endangered Black-capped Vireo (Vireo atricapilla) from 91% in 1987

to 8.6% ten years later (Eckrich et al. 1999). Cowbird management is also an ongoing and

critical component in the conservation of the threatened and endangered populations of

Southwestern Willow Flycatcher (Empidonax traillii extimus), Least Bell’s Vireo, Black-capped

Vireo, California Gnatcatcher (Polioptila californica), Kirtland’s Warbler, and Golden-cheeked

Warbler (Dendroica chrysoparia).

Where cowbirds have clear negative effects on host populations and communities, rates of

parasitism (percentage of nests/broods with cowbird eggs/chicks) can be as high as 80−100 %

(Norris 1947, Hatch 1983, Grzybowski et al. 1986, Robinson 1992) and often above 50% (Elliott

1978, Burgham and Picman 1989, Hahn and Hatfield 1995, Trine et al. 1998). Therefore, monitoring songbird reproductive success and rates of cowbird parasitism can help determine the

potential for serious (or unimportant) consequences of cowbird activity in conservation planning.

This is particularly important in areas where cowbirds have most recently expanded their

breeding range, such as the southeastern United States.

Patterns of Cowbird Expansion in the Southeast

The first records of Brown-headed Cowbirds breeding in the southeastern United States

were in far western North Carolina in 1933 (Pearson et al. 1959) and far western South Carolina

in 1934 (Sprunt and Chamberlain 1970). Breeding cowbirds reached northern Georgia in the late

1940s (Parks 1950). By the mid-1960s, cowbirds were parasitizing nests in many parts of the

Carolinas but were still scarce in the eastern Piedmont and inner coastal plain regions, although

they were first reported during the breeding season near the coastal cities of Wilmington, NC and

Charleston, SC in 1967 and 1965, respectively (Potter and Whitehurst 1981).

Brown-headed Cowbirds were first documented breeding in Florida during the mid-1950s, when a fledgling was sighted in Escambia County in the far western panhandle (Monroe 1957).

Cowbirds were subsequently confirmed breeding in Jacksonville in 1965 (Ogden 1965) and south to Alachua County in 1980 (Edscorn 1980). By the end of the 1980s, the breeding range of the Brown-headed Cowbird had expanded to incorporate all of Florida (Hoffman and

Woolfenden 1986, Paul 1989) and thus the entire southeastern portion of the United States.

Brown-headed Cowbirds are presently confirmed breeding in at least 50 of Florida’s 67 counties

(FFWCC 2007). Since the mid-1960s, the population of Brown-headed Cowbirds in Florida grew slowly but steadily (Figure 1-1) and also increased in Breeding Bird Survey stratum three,

which includes northern Florida (Figure 1-2; Sauer et al. 2006). Cowbirds are roughly as

abundant in the northern portion of Florida as they are in much of the rest of the continent (Sauer

et al. 2006), so there is good reason to expect that they may represent a threat to breeding

songbird populations.

Objectives

Most of the approximately 50 songbird species breeding in Florida are known to be

parasitized by cowbirds in other parts of their ranges, and some are known as suitable hosts (i.e.,

have reared cowbird young) in other areas (Ortega 1998). Some of these species are locally rare

in north-central Florida (e.g., Hooded Warbler Wilsonia citrina, Yellow-breasted Chat Ictera virens; FFWCC 2007) or are experiencing significant declines in the southeast (e.g., Red-eyed

Vireo Vireo olivaceus, Northern Parula Parula americana; Sauer et al. 2006) and would, therefore, be particularly susceptible to any negative effects of brood parasitism they may incur.

Isolated records of parasitism of breeding songbirds in Florida suggest that many species may be at risk to Brown-headed Cowbird parasitism (see review in Cruz et al. 1998). Despite widespread concern over potential impacts of cowbird parasitism for native breeding songbirds, only scattered data on parasitism were collected in Florida during the 60 years since cowbirds began breeding in the state.

To determine potential impacts of Brown-headed Cowbird expansion into Florida, I

conducted a comprehensive survey of cowbird parasitism in songbird communities in north-

central Florida, including (1) monitoring understory bird nests for rates of occurrence of cowbird

eggs and nestlings in host nests and (2) surveying post-fledgling family groups (adults with fledglings) of both understory and canopy songbird species to determine the rates of successful

brood parasitism by cowbirds among the species sampled. More than 30 species were included

in my study; each is known to have been parasitized elsewhere in their breeding range by Brown-

headed Cowbirds. This represents the first community-wide survey documenting cowbird

parasitism rates in both nests and post-fledging families of host species breeding in Florida.

Methods

I surveyed nests from a diversity of potential hosts to determine natural parasitism rates in

north-central Florida. Nest searches were conducted daily between 0700 and 1900 EST at sites

within 25 km of Gainesville, FL (ca. 29º39’ N, 82º19’W) that included many different types of

habitats (Table A-1). Sites ranged from urban (downtown Gainesville) to rural (no or low nearby

housing density), from semi-natural (e.g., city parks) to natural (e.g., managed state preserves),

and from disturbed (e.g., regenerating rangeland) to quasi-natural (large natural communities in

state preserves). Nests were monitored every 1 to 3 days. Data were supplemented from nest

records from an avian monitoring project (Sieving & Contreras, unpubl. data) conducted in

Myakka River State Park near the southwest Florida gulf coast in Sarasota and Manatee Counties

(ca. 27°14’ N, 82°14’ W) from April to July, 2001−2005. Cowbird abundance in this area is

approximately one-quarter of that in northern Florida (Sauer et al. 2006) but large mixed-sex

roosting flocks with 1500+ cowbirds have been seen during the breeding season in nearby St.

Petersburg, FL (D. Margeson, pers. comm.).

Data on nest success for canopy species such as Red-eyed Vireo, Northern Parula, and Pine

Warbler (Dendroica pinus) are rare or non-existent because their nests are generally high in the

canopy (>10m), and therefore difficult to monitor reliably using current technologies. Nests of

cryptic species (e.g., Carolina Wren Thryothorus ludovicianus) are also under-represented in typical nest searching/monitoring samples due to low detection rates. Therefore, to account for these biases in the data set, I conducted intensive surveys for family groups (adults and fledglings) for all species that nest throughout the understory and canopy (Verner and Ritter

1983). Monitoring of adults feeding young outside of the nest is the main component of

reproductive indices used to estimate reproductive success (Vickery et al. 1992) and are useful

when nest monitoring is logistically infeasible (Bonifait et al. 2006).

Family groups were found by watching foraging adults and following adults carrying food

or by tracking fledgling begging calls. Fledglings of some species give more audible begging

calls, but I listened for any call that might represent fledgling feeding or presence. Family groups of the study species that I encountered were followed for at least ten minutes to determine if a cowbird fledgling was present. Multiple species have rarely been observed feeding a single cowbird fledgling (Klein and Rosenberg 1986), so I assumed cowbird fledglings were being fed

by the species that raised them. I typically observed repeated feedings of cowbirds by the same

species and never witnessed a feeding of a given cowbird by more than one host species. Lower

nesting (understory) species tend to be better represented in standardized samples of forest bird

communities. To overcome any potential bias this may represent, I spent approximately 75% of

time conducting visual scans for adult birds and families in the midstory and canopy and 25% in

lower strata at forested sites. Many times, family groups for lower nesting species were located

opportunistically when searching for and monitoring nests.

Results

From March−August, 2004−2006, I monitored the parasitism status of 35 potential host

species breeding in Florida (Tables 1-1, A-2). Nest surveys in north-central Florida included

monitoring of 994 nests of 29 potential cowbird host species. Nesting data from Myakka River

State Park added 123 nest records of nine host species and included 26 records for three species

not monitored in this study (32 species total). Only 18 nests (1.6%) of eight potential host

species contained a cowbird egg or nestling. The most records of parasitism (N=6) were from

Northern Cardinal (Cardinalis cardinalis), but constituted only 2.5% of cardinal nests monitored.

Other species parasitized with at least five nest records were Blue-gray Gnatcatcher (Polioptila caerulea; 20%), Hooded Warbler (20%), Blue Grosbeak (Guiraca caerulea; 14.3%), White-eyed

Vireo (Vireo griseus; 10.5%), Bachman’s Sparrow (Aimophila aestivalis; 9.1%), Eastern Towhee

(Pipilo erythrophthalmus alleni; 6.1%), and Northern Mockingbird (Mimus polyglottos; 0.2%).

One of three (33%) Common Yellowthroat nests was parasitized. The only Yellow-throated

Vireo (Vireo flavifrons) nest contained a lone cowbird nestling. Of nests of the 10 species parasitized, 2.4% (18 of 750) contained cowbird eggs or nestlings. Nest records of Northern

Mockingbird constituted the largest proportion of the sample of parasitized birds (N=397) and may have biased average parasitism rate over multiple species. However, even after excluding these records, the parasitism frequency of nests was still low at 5.7% (17 of 299) of nests.

Cowbirds fledged successfully from five nests (one each of Yellow-throated Vireo, Blue-gray

Gnatcatcher, Common Yellowthroat, Bachman’s Sparrow, Northern Cardinal). Bachman’s

Sparrow nests are known to be parasitized (Ortega 1998), but this represents the first report of this species raising a cowbird to fledging and the first record of cowbird parasitism on the

Bachman’s Sparrow in Florida (Reetz et al. 2008). Among the few nests of midstory and canopy species that I was able to monitor, 1.7% (2 of 116) were parasitized compared to 1.6% (16 of

1001) of nests of the remaining species. Excluding Northern Mockingbird nests from the sample of lower nesting species, 2.5% (15 of 592) of nests were parasitized.

I monitored 278 family groups of 20 species, including three species (Carolina Chickadee

Poecile carolinensis, Red-eyed Vireo, and Northern Parula) for which no nest data were available. Seventeen (6.1%) of these family groups contained a cowbird fledgling (Table 1-1).

Of the six species with at least one parasitized family group, 10.6% (17 of 160) of families

contained a cowbird fledgling. Family groups of midstory or canopy nesting species were more

likely to be found feeding a cowbird young than lower nesting species (Log Odds Ratio=3.04,

CI=4.65−93.61). Indeed, of the 17 parasitized family groups monitored, 15 (88.2%) were of midstory or canopy nesting species, and 17.9% (15 of 84) of midstory or canopy nesting family groups observed were parasitized. For example, 3 of 5 (60%) Summer Tanager (Piranga rubra),

7 of 18 (38.9%) Northern Parula, 3 of 9 (33%) Blue-gray Gnatcatcher, 1 of 4 (25%) Red-eyed

Vireo, and 1 of 14 (7.1%) Pine Warbler family groups monitored were observed feeding a

cowbird fledgling. Northern Cardinals were the only lower nesting species that was observed

feeding a fledgling cowbird (2 of 93 family groups or 2.2%).

Discussion

Cowbird Parasitism in North-Central Florida

Despite the apparent abundance of cowbirds and their feeding habitat in northern Florida, I detected very low rates of Brown-headed Cowbird parasitism across the community of potential host species breeding in north-central Florida. Less than 2% of nests were found with cowbird eggs and approximately 6% of family groups contained a cowbird fledgling. While this represents the first community-wide survey of Brown-headed Cowbird parasitism in Florida, some more narrowly focused reproductive studies present similarly low rates of cowbird parasitism. For example, Prather and Cruz (1995) found no parasitism cases among 42 Prairie

Warbler (Dendroica discolor paludicola) and 20 Yellow Warbler (Dendroica petechia

gundlachi) nests in south Florida, and only two cases in 108 nests of ten species on the

southwestern gulf coast (2002). During the six years (1986−1991) of the Florida Breeding Bird

Atlas, a collaborative project designed to record breeding distributions of all avian species in

Florida, volunteers and researchers monitored more than 2400 nests of the same species surveyed

in this study. Less than 0.5% of nests were found with cowbird eggs or nestlings (FFWCC

2007). Species seen feeding fledgling Brown-headed Cowbirds during the Atlas project were similar to those in this survey and included Blue-gray Gnatcatcher, Red-eyed Vireo, Northern

Parula, Pine Warbler, and Summer Tanager, among others. Taken together, available monitoring data of Florida host species show very limited Brown-headed Cowbird nest parasitism.

Results from parasitism surveys in this project are nonetheless surprising given the relative abundance of Brown-headed Cowbirds in north-central Florida, ranging from 3−10 birds per

Breeding Bird Survey route (1994−2003; Sauer et al. 2006). Such abundances are comparable to those in most of the eastern and far western United States, where reproductive studies have recorded much higher community-wide parasitism rates than reported here. For example, nest parasitism rates of 36 host species in northeastern New Mexico are 20.8% of 850 nests (Goguen and Mathews 1998) and of 32 host species in southeastern New York are 16.53% of 301 nests

(Hahn and Hatfield 1995). Parasitism frequency of common host species in recently-invaded

South Carolina and Georgia is lower at 8.2% of nests (Kilgo and Moorman 2003), but still significantly higher than the frequency reported here.

Even species with high rates of parasitism elsewhere in their ranges seem to elude it in

Florida. Northern Cardinals had parasitism rates of 2% of nests and family groups in this study,

but suffer parasitism rates of 25−100% of nests in other parts of their breeding range (reviewed

in Ortega 1998). Rates of parasitism are also very high (40−83%) for Eastern Towhee and

White-eyed Vireo in other regions (Norris 1947, Young 1963, Goertz 1977, Hopp et al. 1995),

yet were only 5.3% and 10.5%, respectively, in north-central Florida. Finally, fewer than 2% of

nests of understory nesting species contained cowbird eggs, yet 13% of nests in a similar suite of

understory species were parasitized elsewhere in the southeastern U.S. (Whitehead et al. 2002).

Various procedural and ecological-evolutionary mechanisms may underlie the low rates of parasitism detected in this study.

Potential Limits on Parasitism Rates or Their Accurate Detection

First, rates may have been affected by inclusion of certain species that are likely incidental or inappropriate hosts. For example, rates of nest parasitism in southeastern Ontario, where cowbirds are abundant, are only 6.7% across all 87 species for which parasitism has been recorded at least once (Peck and James 1987). Many of these species, however, are accidental or unsuitable cowbird hosts (e.g., Virginia Rail Rallus limicola) not known to fledge cowbirds

(Ortega 1998). Nine of the ten most parasitized species in Ontario, however, are appropriate hosts and some averaged as high as 48% of nests parasitized (Peck and James 1997, 1998a, b).

Therefore, parasitism rates of at least marginally suitable hosts are likely significantly higher than the 7% reported for the entire community of breeding birds. I surveyed a total of 35 host species, all of which have been recorded as hosts of Brown-headed Cowbirds (Ortega 1998). but some hosts included in the survey, (e.g., Mourning Dove Zenaida macroura, Blue Jay

Cyanocitta cristata, Northern Mockingbird, Brown Thrasher Toxostoma rufum and House Finch

Carpodacus mexicanus) are inappropriate or poor hosts due to life-history traits (e.g., nestling diet, defense behaviors) incompatible with cowbird parasitism. When all nests of the 14 unsuitable host species surveyed are removed from the sample, the parasitism rate of the remaining 21 species is still only 3.6% of nests (18 of 486) and 7.5% of family groups (17 of

238; Table 1-1). Therefore, surveys of parasitism frequency do not appear to be biased by selective inclusion of host species.

Second, many hosts have nests that are difficult to find (e.g., Common Yellowthroat) or extremely difficult to monitor (e.g., Northern Parula and other canopy nesting species), and thus parasitism may go undetected. For example, very low rates of nest parasitism (1%) were

detected across a host community in the Sierra Nevada of California (Verner and Ritter 1983), a

survey that included species heavily parasitized elsewhere in the region. Observations of family

groups still showed low parasitism rates (4%), but the authors’ analyses suggested that parasitism

was biased toward species with nests that are difficult to monitor. Results from my surveys also

suggest that nest parasitism may go undetected due to preference for particular species infeasible

to survey. I was able to monitor few midstory and canopy nest, and parasitism rate (1.9%) in this

limited sample was similar to that for lower nesting species (1.5%). However, I note that the vast majority (88%) of parasitized family groups I observed was of species nesting in the higher strata, presenting an overall parasitism rate of approximately 20% for the nesting guild with the

lowest nest detectability in nest monitoring surveys. As a result, it appears that low observed frequencies across the community of hosts is at least partially due to difficulties in obtaining adequate samples for all potential species.

Third, estimates of cowbird parasitism that are based on the percent of parasitized nests

can underestimate actual parasitism levels if nests are abandoned, nest contents are readily

depredated (Robinson et al. 1995a), or cowbird eggs are ejected. Many species nesting in north-

central Florida practice egg rejection behaviors (i.e., egg ejection, egg burial, and abandonment

of nests) in some parts of their ranges. Many of the most abundant songbird species that breed in

Florida are known to readily eject cowbird eggs or abandon parasitized nests. For example,

Northern Mockingbirds and Brown Thrashers eject up to 69% (Peer et al. 2002) and 100%

(Rothstein 1975b, Haas and Haas 1998) of experimental cowbird eggs placed in their nests. If

anti-parasitism behaviors are prevalent in many of the host species breeding in a region, a large

portion of actual nest parasitism will go undetected leading to lower observed parasitism rates

overall. Egg rejection behaviors do not appear to be prevalent in common hosts in Florida

(Chapter 2).

Fourth, though Brown-headed Cowbird fecundity is usually very high with an average

female producing 40 eggs in a breeding season (Scott and Ankney 1980), there is significant

variation in fecundity over the species’ range (Payne 1965, 1973a, 1976, Scott and Ankney

1980). It is possible, therefore, that while cowbirds are abundant in north-central Florida, host

communities may show reduced parasitism of nests simply because breeding female cowbirds

lay fewer eggs. Cowbird fecundity has been measured using a variety of methods, and limited

reproduction would provide a parsimonious explanation for low rates of parasitism in Florida.

Females breeding in north-central Florida show high fecundity (Chapter 3), with a rate of egg

production similar to the highest rate estimated for the species.

Finally, low parasitism rates may be related to the limitation of suitable hosts. Cowbirds

have been identified as “generalist” parasites because they are known to parasitize over 220

species (Ortega 1998). Because parasitism often affects many or sometimes most potential host species in a community, parasitism may not be host-specific because eggs are spread among

hosts according to their abundance (“shotgun” hypothesis; Rothstein 1975b). In this case, ‘good’

hosts (those able to successfully fledge cowbird young) are parasitized as frequently as ‘poor’

hosts. Host-specific characteristics can affect the survival probability of cowbird young so that

selection may favor parasitism of hosts successful at raising cowbirds. As a result, parasitism of

nests may be biased toward a few select species (“host selection” hypothesis; Rothstein and

Robinson 2002). Host selection is supported by data showing nonrandom parasitism frequencies

of a suite of potential hosts (Woolfenden et al. 2003, Ellison et al. 2006). In areas where

cowbirds are very abundant, individuals are likely to employ both host selection and shotgun

tactics because competition for resources (nests of “good” hosts) is more intense.

For successful expansion and population growth of cowbirds in new regions such as

Florida, the availability of at least some suitable hosts that accept cowbird eggs and appropriately

nourish juveniles is critical. Resultant parasitism frequencies are often higher than those that

reject eggs (Sealy and Bazin 1995) or feed young a diet inappropriate to cowbirds (Middleton

1991), but patterns of parasitism across the host community have not typically been addressed in

areas where cowbirds have recently expanded. In a newly colonized area such as Florida, low

initial cowbird densities should favor host selection because of low competition for nests. In this case, the availability (abundance/diversity) of quality hosts may represent a limiting factor to reproduction.

Data presented in this chapter are consistent with host selection because certain species,

particularly those that nest in the midstory or canopy, appear to have been preferentially

parasitized. This result probably does not reflect a selection bias by cowbirds toward higher

nests, per se, but rather toward suitable host species. Lower nesting species such as Northern

Mockingbird, Northern Cardinal, and Carolina Wren are probably poor cowbird host species

because of anti-parasitism behaviors, nest site choice, or other life-history traits (Rothstein

1975b, Haggerty and Morton 1995, Halkin and Linville 1999). In north-Florida, these and

similar species constitute a relatively higher proportion of lower nesting species abundance than

in other regions with a more diverse and abundant community of quality understory nesters (e.g.,

emberizids, parulids, etc.). Species such as Indigo Bunting (Passerina cyanea), Yellow-breasted

Chat and Orchard Oriole are quality understory hosts and are moderately parasitized in other

southeastern states where they are common (Whitehead et al. 2002, Kilgo and Moorman 2003).

However, north Florida represents the southern extent of their breeding range where they are

found in very low densities (Sauer et al. 2006). As a result, a subset of midstory and canopy

nesting species that is preferentially parasitized may be bearing the brunt of cowbird parasitism

in north-central Florida because it represents the highest abundance of quality hosts but has

relatively low species diversity.

Conclusions and Management Implications

Expansion of breeding Brown-headed Cowbirds into Florida was greeted with widespread

concern, particularly with the additional arrival of the Shiny Cowbird in south Florida. Atherton

and Atherton (1988:63) stated “it is conceivable that the breeding ranges of the two cowbird

species will meet within the next few years and that both Prairie Warbler and Black-whiskered

Vireo will be in grave danger of being extirpated from the state.” The observation of a flock of

1000+ Brown-headed Cowbirds with six Shiny Cowbirds in south Florida in July 1988 (Paul

1988:1281) prompted the dramatic prediction that it represented “... the end of birding as we

know it in south Florida.” To date, there has been only one published breeding observation (Paul

1988), and one breeding specimen collected (Chapter 3) that confirm reproduction of Shiny

Cowbirds in Florida. Furthermore, through monitoring of nests and family groups with

fledglings, I detected very low rates of Brown-headed Cowbird parasitism on the community of

potential host species breeding in north-central Florida. Therefore, brood parasitism by cowbirds

does not appear to currently represent an urgent threat to most of Florida’s breeding songbirds.

My nest parasitism surveys suggest, however, that a subset of host species, particularly

midstory and canopy nesting species, accounts for a disproportionate amount of recruitment of

juvenile cowbirds in north-central Florida; these species could be suffering negative consequences of parasitism. Despite the logistical difficulties in locating and monitoring nests of midstory and canopy nesting species, I stress a critical need nest data to determine parasitism

rates. In tandem, monitoring of populations of these species is needed to determine if declines are occurring from negative effects of brood parasitism. In addition, the relationship between cowbirds and their hosts should continue to be monitored across different host communities and in areas with different abundances of cowbirds. Brown-headed Cowbird abundance is highest in

northwestern Florida, where diversity and abundance of good quality hosts also are the highest in

the state (Sauer et al. 2006). Finally, nest parasitism frequencies of species that are locally

abundant but have restricted distributions should be determined. In particular, the breeding

distribution of the Painted Bunting (Passerina ciris) in Florida is concentrated in the northeastern

corner of the state with smaller populations scattered along the Atlantic coast (FFWCC 2007).

At a site in South Carolina, Painted Buntings had 35.3% of nests parasitized by Brown-headed

Cowbirds (Whitehead et al. 2002). Such a rate could threaten small, isolated populations breeding in Florida.

Perhaps the most useful conclusion of this study to management is that monitoring of

family groups may provide a more comprehensive technique for assessment of the true incidence

of successful cowbird parasitism in any area affected by increasing cowbird populations. Of

individual species with reasonable sample sizes, the highest parasitism rates recorded were 39%

(Northern Parula) and 33% (Blue-gray Gnatcatcher) of family groups. While these rates are

considered moderate among those nest parasitism rates published in the literature (Ortega 1998),

they do not consider cowbird eggs that do not hatch or are lost from nests, or increased nest

predation of parasitized nests (Burhans et al. 2002). On the other hand, these rates may also

overestimate relative parasitism due to sampling biases inherent in the methodology. For

example, cowbird fledglings are notoriously and relentlessly boisterous when begging for food

from host adults, which may make fledged families more conspicuous. I attempted to avoid

over-sampling of families with noisy cowbird fledges by finding and following family groups by scanning for adults with food, or listening for begging of fledglings of all species. Fledglings of some species have relatively soft calls and cryptic behaviors, making them more difficult to detect from a distance than cowbird young, especially high in the canopy. Therefore, this method is at least representative of parasitism in species’ nests that successfully fledge young. In any case, these data suggest that parasitism rates of selected species may be much higher than community-wide nest surveys alone would detect. Therefore, in addition to cowbird population monitoring and nest monitoring, family group surveys may be a particularly valuable means of assessing the overall intensity of cowbird parasitism affecting songbird communities. It is also, perhaps, the most efficient technique for identifying the host species most likely to recruit young cowbirds into the breeding population.

Figure 1-1. Breeding Bird Survey data of Brown-headed Cowbirds detected per route in Florida from 1966−2006.

All Routes 20 Northern Florida

. Southern Florida 15

10 Birds/Route Birds/Route

0 1966 1971 1976 1981 1986 1991 1996 2001 2006

Figure 1-2. Estimated number of Brown-headed Cowbirds per Breeding Bird Survey route for northern and southern Florida, 1966−2006. BBS Routes considered Southern Florida include all routes south of Marion County (ca. 28°57’35” N), the county immediately south of Alachua, where this study was conducted. Data presented represent raw counts and only routes recording Brown-headed Cowbird at least once were included.

Table 1-1. Summary of nests and family groups for 32 species monitored in the north-central Florida from March to August, 2004−2006. U: species nesting in the midstory to canopy, L: species nesting low or on ground. Nesting category based on field observations and Ehrlich et al. (1988). Species totals for each nesting category are given for nests/family groups monitored. Number of supplemental nests (number parasitized) from Contreras & Sieving (unpubl. data, 2000−2004) are: MODO = 19, GCFL = 12, BLJA = 8, EABL = 7, NOMO = 18, COYE = 2 (1), EATO = 43 (3), BACS = 11 (1), EAME = 3. Species with an asterisk are considered incidental or poor hosts. Unparasitized Parasitized Unparasitized Parasitized Common Name Latin Name Category Nests Nests Families Families Eurasian Collared Dove* Streptopelia decaocto U 3 − − − Mourning Dove* Zenaida macroura U 52 − − − Great-crested Flycatcher* Myiarchus crinitus U 12 − − − Acadian Flycatcher Empidonax virescens U 14 − 9 − Loggerhead Shrike* Lanius ludovicianus U 5 − 1 − White-eyed Vireo Vireo griseus L 17 2 25 − 30 Yellow-throated Vireo Vireo flavifrons U − 1 5 − Red-eyed Vireo Vireo olivaceus U − − 4 1 Blue Jay* Cyanocitta cristata U 9 − − − Carolina Chickadee* Poecile carolinensis U − − 1 − Eastern Tufted Titmouse* Baeolophus bicolor U 2 − − − Carolina Wren Thryothorus ludovicianus L 10 − 12 − Blue-gray Gnatcatcher Polioptila caerulea U 4 1 9 3 Eastern Bluebird* Sialia sialis L 51 − 3 − Wood Thrush Hylocichla mustelina U 3 − − − Northern Mockingbird* Mimus polyglottos L 397 1 28 − Brown Thrasher* Toxostoma rufum L 80 − 7 − Northern Parula Parula americana U − − 18 7 Pine Warbler Dendroica pinus U 3 − 14 1

Table 1-1. Continued. Unparasitized Parasitized Unparasitized Parasitized Common Name Latin Name Category Nests Nests Families Families Prothonotary Warbler Protonotaria citrea L 1 − − − Common Yellowthroat Geothylpis trichas L 2 1 2 − Hooded Warbler Wilsonia citrina L 4 1 7 − Yellow-breasted Chat Icteria virens L 1 − − − Summer Tanager Piranga rubra U 1 − 5 3 Bachman’s Sparrow Aimophila aestivalis L 10 1 − − Eastern Towhee Pipilo erythrophthalmus alleni L 54 3 12 − Northern Cardinal Cardinalis cardinalis L 237 6 93 2 Blue Grosbeak Guiraca caerulea L 6 1 − − Indigo Bunting Passerina cyanea L 2 − 3 − 31 Eastern Meadowlark* Sturnella magna L 3 − − − Red-winged Blackbird Agelaius phoeniceus L 81 − − − Common Grackle* Quiscalus quiscala U 2 − − − Boat-tailed Grackle* Quiscalus major U 2 − − − Orchard Oriole Icterus spurius U 2 − 3 − House Finch* Carpodacus mexicanus L 29 − − − TOTALS 35 1099 18 261 17 Midstory/Canopy Species 17/10 114 2 69 15 Lower Nesting Species 15/10 985 16 192 2 Quality Hosts 469 17 221 17

CHAPTER 2 RESPONSES OF FOUR SONGBIRD SPECIES TO EXPERIMENTAL COWBIRD PARASITISM IN A RECENTLY INVADED AREA

Introduction

Multiple direct and indirect costs are associated with brood parasitism by Brown-headed

Cowbirds that reduce survival and fecundity of host species (see Chapter 1). As a result, nest parasitism can impose strong selection on hosts for behaviors to reduce the number of cowbird eggs that host females incubate (Rothstein 1990). Two predominant behaviors commonly expressed by hosts that reject parasitism are removal of cowbird eggs from the nest and desertion of parasitized clutches. Indeed, quantification of the frequency of egg ejection and clutch desertion behaviors, via experimental emulation of cowbird nest parasitism (egg additions), is used to determine the prevalence of rejection responses within a species or host community

(reviewed in Peer and Sealy 2004a). In this way, host species can be categorized as either ejecters (species that physically remove parasitic eggs from their nest) or accepters. Species that initially accept eggs into their nests may still reject parasitism by abandoning the parasitized clutch; either by deserting the nest (Rothstein 1975b, reviewed in Ortega 1998:192) or by burying the eggs under a new nest lining (Sealy 1995). Despite its apparent benefit, only a relatively small subset of species (ca. 29 of >220) that are parasitized exhibit at least intermediate egg ejection frequencies (Table 2.1 in Ortega 1998). Furthermore, many species that do not eject cowbird eggs from their nests do not subsequently abandon nests or bury cowbird eggs.

Two major explanations are proposed for why potential host species accept nest parasitism.

Under the evolutionary equilibrium hypothesis, nest parasitism is tolerated as a result of conflicting selection pressures (Zahavi 1979, Rohwer and Spaw 1988). Costs of ejection errors or abandonment of nests, representing losses of host eggs and energy expended in replacement nesting, may outweigh the costs of raising parasite young; thus acceptance could be

evolutionarily favored (Lotem and Nakamura 1998). Under conditions of coexistence between cowbirds and host species that accept parasitism under this scenario, selection simultaneously favors cowbird traits that maximize efficiency of host use and host traits that minimize costs of parasitism (Robertson and Norman 1976, Dawkins and Krebs 1979). Under the evolutionary lag hypothesis (Rothstein 1975b, Dawkins and Krebs 1979, Davies and Brooke 1989), acceptance of parasitic eggs occurs because adaptations that eliminate or reduce the frequency of brood parasitism have not yet developed, spread, or become fixed in the host population (Rothstein

1975a). As a result, host populations are not able to recognize parasitic eggs (Rothstein 1982) or

females (Smith et al. 1984, Bazin and Sealy 1993), or lack appropriate responses to foreign eggs.

In host populations in recent contact with expanding populations of Brown-headed

Cowbirds, evolutionary lag is the most reasonable explanation for a general lack of adaptive

response to novel brood parasitism (Rothstein 1975b, Burgham and Picman 1989). In such

situations, naive host populations may be at increased risk of decline due to high rates of nest

failure (Cruz et al. 1989) if they are unable to recognize or respond to nest parasitism (Rothstein

et al. 1980). Despite having only come into contact with Shiny Cowbirds in the 1990s, however,

Gray Kingbirds (Tyrannus dominicensis) in the Bahamas are known to eject artificial cowbird eggs (Baltz and Burhans 1998), a behavior that is also expressed where the two species have

been sympatric for longer periods (Cruz et al. 1985, Post et al. 1990). In populations no longer

sympatric with cowbirds, multiple species have shown similar retention of cowbird egg rejection

behaviors displayed in populations still exposed to parasitism (Rothstein 2001, Peer et al. 2007).

Therefore, retention of behavioral defenses against brood parasites in some species and

populations may buffer them from immediate negative consequences of recent cowbird

expansion.

Colonization of Florida by Brown-headed Cowbirds has occurred over the last 50−60 years

(see Cruz et al. 2000) and was first thought to represent a significant threat to many breeding songbirds (Cruz et al. 1998). However, observed parasitism frequencies for individual hosts and across an entire host community are unusually low despite moderate Brown-headed Cowbird abundance (Chapter 1). Elsewhere in the southeastern United States, many host species are known to have displayed anti-parasitism behaviors during the recent southward expansion of cowbirds (Whitehead et al. 2002). These behaviors are likely maintained by gene flow from regions where cowbirds have consistently been breeding over the last two centuries (Rothstein

1990). In addition, fossil evidence suggests that Brown-headed Cowbirds (Hamon 1964, Ligon

1966, Emslie 1998) and another extinct cowbird species (Pandanerus floridana; Brodkorb 1957) were common in Florida in the late Pleistocene. During this time rejection behaviors may have evolved in some species in response to heavy brood parasitism and may be retained in current populations in the absence of cowbirds. Indeed, at least one species endemic to Florida, the

Florida Scrub Jay (Aphelocoma coerulescens), is known to readily eject artificial cowbird eggs

(Fleischer and Woolfenden 2004). Species known as ejecters (e.g., Northern Mockingbird,

Brown Thrasher) and deserters (e.g., Northern Cardinal) elsewhere within their breeding distribution are among the most abundant songbird species currently breeding in Florida.

I tested the hypothesis that the prevalence of egg rejection behaviors in Florida hosts contributes to limiting the reproduction of Brown-headed Cowbirds. I experimentally parasitized four common host species that are known to vary in the degree of their rejection response, but that commonly respond adaptively to parasitism in some part of their range. I predicted that parasitism rejection behaviors in populations of Florida songbird species would be comparable in frequency to populations exhibiting moderate to high rejection frequencies in other parts of their

range. If rejection behaviors are generally less prevalent or absent in Florida study species,

reflecting a typical evolutionary lag in adaptive responses to cowbird expansion, factors other

than effective host rejection of parasitism are more likely to be contributing to limited Brown-

headed Cowbird reproduction in Florida. While most studies of egg ejection have used artificial

cowbird eggs to examine host responses (e.g., Rothstein 1975b, Baltz and Burhans 1998, Robert

and Sorci 1999), nests in this experiment were parasitized with actual cowbird eggs obtained

from a captive colony raised for this purpose. The goal was to simulate parasitism as accurately

as possible and avoid potential complications inherent in the use of artificial eggs (Rothstein

1976, 1977, Martin-Vivaldi et al. 2002).

Methods

Study Design

In September 2004, I captured 40 hatch-year cowbirds (20 male, 20 female) in Alachua

County, FL using baited drop-down decoy traps. From March−August 2005 and 2006, 10 pairs

were separated for breeding in individual outdoor pens (4 x 2 x 2m) at the USDA National

Wildlife Research Center, Florida Field Station in Gainesville, FL under an approved animal use

protocol (IACUC D539). Birds were fed a specialty diet that met caloric and dietary needs for

reproduction (Holford and Roby 1993). Each pen contained two or more open-cup or domed

artificial nests and an occasional inactive Northern Cardinal nest. Some artificial nests contained a single wooden artificial egg that was similar in size, appearance, and weight to the egg of a

Red-eyed Vireo. To improve the probability that a captive female cowbird would lay eggs, cowbird pairs were housed with Zebra Finches (Taeniopygia guttata) and Society Finches

(Lonchura striata domestica). These finches bred readily in captivity and provided stimuli

associated with active bird nests (e.g., nest-building, incubation behavior, warm eggs). Pens

were monitored daily for the presence of cowbird eggs.

From 28 April to 14 July, 2004 and 2005, eggs obtained from captive birds were used to experimentally parasitize four primary species that are abundant during the breeding season in

Florida. Nests were searched for daily in a variety of habitats at sites within 25 km of

Gainesville, FL (Chapter 1). Sites included nature preserves (Ordway-Swisher, Bolen Bluff,

Split Rock), wetlands (Newnan’s Lake) and numerous Gainesville city parks and neighborhoods.

A single experimental cowbird egg was placed in each host nest between the egg-laying stage and 2−3 days following initiation of incubation as determined by candling of eggs (Lokemoen and Koford 1996). In one case, a Northern Cardinal nest received an experimental egg before the female laid her first egg. Eggs were warmed slightly in the hand prior to being placed in the nest. To mimic the common removal of a host egg by a female cowbird (Nolan 1978), a single host egg was often removed (75% of cases) from nests containing at least two host eggs. Nests were visited daily and host responses were categorized as “rejected” or “accepted” based on

Rothstein (1975b). Two types of rejection behavior were distinguished. Eggs were considered

“ejected” if the cowbird egg or fragments were found outside the nest, or if the egg was missing with no apparent decrease in numbers of host eggs. Nests were considered “abandoned” if eggs were cold and nests undisturbed with no sign of nest activity for three consecutive days. If no change occurred on subsequent days and the nest remained active, the egg was considered

“accepted”. To assure that cowbird eggs were missing due to ejection and not partial nest predation, I excluded experimental nests that showed partial or complete host egg loss within two days following apparent cowbird egg ejection.

Selection of Study Species

Study species chosen are known to respond to cowbird parasitism through ejection or nest abandonment behaviors in other regions: Brown Thrasher, Northern Mockingbird, Northern

Cardinal, and Red-winged Blackbird. Study species were categorized as either ejecters or accepters based on criteria in Peer et al. (2002) using ejection rates of experimentally parasitized nests in populations with at least recent exposure to cowbird parasitism. Brown Thrashers were considered ejecters (75−100% ejection rate) whereas Northern Cardinal and Red-winged

Blackbirds were categorized as accepters (0−20% ejection rate). Northern Mockingbirds were categorized as intermediate ejecters (21−74%) because ejection rates vary significantly among populations. Nests of these species were the focus of search efforts in this experiment and represented an array of generally high to low parasite rejection rates allowing determination of relative rejection rates in the study. Observed frequencies of rejection and acceptance behaviors for each study species were compared to expected frequencies based on artificial parasitism studies in other regions. Expected responses from multiple studies were compared to observed responses using two-tailed Fisher’s Exact Probability Test. Expected responses (number of cases of rejection/acceptance) were 62/28 for Brown Thrasher (Rothstein 1975b, Haas and Haas 1998)

29/26 for Northern Mockingbird (Rothstein 1975b, Friedmann and Kiff 1985, Peer et al. 2002).

For Red-winged Blackbird and Northern Cardinal, I derived two expected frequencies, a conservative estimate and one incorporating the highest frequencies reported for each species

(Graham 1988, Clotfelter and Yasukawa 1999). Conservative expected frequencies were 16/143 for Red-winged Blackbird (Rothstein 1975b, Ortega 1991, Prather and Cruz 2006), and 4/60 for

Northern Cardinal (Rothstein 1975b, Eckerle and Breitwisch 1997, Whitehead et al. 2002). High expected values were 97/284 for Red-winged Blackbird and 33/85 for Northern Cardinal. If rejection behaviors contribute to limiting cowbird reproduction in Florida, I predicted that observed rejection frequencies for each species would be similar or higher in Florida populations than in representative populations from other regions. If observed frequencies are significantly

lower than expected, the hypothesis will be rejected indicating that rejection of parasitism does

not explain low observed parasitism of hosts in Florida.

Results

Ten pairs of captive cowbirds produced 34 eggs in 2005 and 122 eggs in 2006 (mean

mass=2.88g, range 1.16−3.54g). At least some experimental eggs were viable because three eggs that were accepted into nests subsequently hatched, but 47% (27 of 58) of accepted eggs did

not hatch. All cowbirds from hatched eggs were depredated in the nest by natural predators. All

other experimental eggs were ejected by hosts, abandoned in nests, or depredated. For a limited

number of Brown Thrasher nests (N=9), I made observations of behaviors following artificial

parasitism. Egg ejection always occurred within twenty minutes after the return of the female,

and some females that eventually ejected the egg initially sat on the parasitized clutch.

Eggs obtained from the cowbird colony were used to artificially parasitize 75 nests of the

four selected cowbird hosts (Table 2-1). Brown Thrasher was the only species tested that

consistently rejected cowbird parasitism of their nests while all other species typically accepted

cowbird eggs. Northern Mockingbirds, Northern Cardinals and Red-winged Blackbirds accepted

cowbird eggs in 85%, 95% and 100% of cases, respectively. Observed rejection frequencies

were higher than expected based on the literature for Brown Thrasher, and lower for the other

three study species. Northern Mockingbird showed significantly different rejection frequencies

than expected (P<0.01; Table 2-1). Rejection frequencies for Northern Cardinals and Red-

winged Blackbirds were not significantly different than conservative expected values, but were

significantly lower than high expected values (P<0.05).

Discussion

Rejection Responses of Florida Hosts

The hypothesis that egg rejection behaviors of Florida hosts explain low observed parasitism frequencies was not supported by the data. Results show a low prevalence of egg

rejection in three of four common cowbird hosts that were experimentally parasitized. Northern

Mockingbird, Northern Cardinal and Red-winged Blackbird accepted cowbird eggs placed in

their nests in the overwhelming majority (93%) of cases. Results are consistent with similarly

high acceptance rates of artificial cowbird eggs found for Red-winged Blackbirds in southern

Florida (Prather and Cruz 2006). Northern Cardinals breeding elsewhere in the southeast have

been shown to abandon 50% of naturally parasitized nests (N=4; Whitehead et al. 2002). This

pattern was not observed in north-central Florida where cardinals accepted eggs in 95% of nests

experimentally parasitized, many of which were parasitized before completion of the clutch. In

the one case in which a lone experimental egg was placed in a cardinal nest before host eggs

were laid, the host female still laid an entire clutch in the nest. Surprisingly, Northern

Mockingbirds ejected very few experimental eggs, accepting parasitic eggs in most cases.

Mockingbirds breeding elsewhere show intermediate responses with frequencies of cowbird egg

ejection ranging from 25−100% (Rothstein 1975b, Post et al. 1990, Peer et al. 2002). Frequency

of egg ejection by Northern Mockingbirds in this experiment is the lowest recorded for this

species. Therefore, while all four study species display rejection and abandonment behaviors in

other parts of their distributions, three of four study species do not appear to have recently

developed or retained these behaviors in Florida populations.

Brown Thrashers have only recent exposure to cowbird parasitism in Florida but ejected

nearly all experimental cowbird eggs and abandoned one parasitized nest. Other studies have

shown a high prevalence of ejection behaviors in populations no longer exposed to brood

parasitism (Rothstein 2001, Peer and Sealy 2004b, Peer et al. 2007). For example, Gray Catbirds

(Dumetella carolinensis) introduced to Bermuda retain ejection behaviors despite an absence of

brood parasites there (Rothstein 2001). There are three possible explanations for observed high

ejection frequency of experimental cowbird eggs by thrashers in this experiment. First,

Rothstein (1975a) estimated that it would take 20−100 years for some parasitized populations to

switch from primarily accepting to ejecting cowbird eggs. As the overall intensity of parasitism on the host community in north-central Florida is probably not significant (Chapter 1), rapid

evolution of rejection in Brown Thrashers is unlikely. Second, Florida populations may be receiving genes for egg rejection from areas with longer exposure to cowbirds (Rothstein 1990).

All four study species, however, have sedentary populations and are year-long residents in

Florida. Sedentary populations have been shown to have significantly lower gene flow than migratory populations (Arguedas and Parker 2000) suggesting gene flow of “rejection alleles” is unlikely for Florida populations. Finally, some species may retain rejection behaviors for extremely long periods of time because the rejection response is selectively neutral in the absence of parasitism (Rothstein 1975a). For example, Boat-tailed Grackles (Quiscalus major) may have retained ejection behaviors for as long as 800,000 years without exposure to brood parasitism (Peer and Sealy 2004b). Fossil evidence suggests that all four study species were sympatric with cowbird species in central Florida at least 10,000 years ago (Hamon 1964, Emslie

1998). During this time rejection behaviors may have evolved in all of the study species but were retained only by Brown Thrashers when cowbird species went extinct or changed their geographic distribution.

Acceptance of Cowbird Eggs

Northern Mockingbirds, Northern Cardinals, and Red-winged Blackbirds typically accepted experimental parasitism of their nests in this experiment. These species may have accepted cowbird eggs because they were unable to discriminate cowbird eggs from their own

(Rothstein 1982). Northern Cardinals eggs are similar in size, color, and patterning to Brown- headed Cowbird eggs whereas the other study species have eggs that differ significantly in appearance. However, the degree of dissimilarity between host and parasite eggs would not be important in populations lacking the ability to discriminate eggs. In these populations, any egg recognition errors may result in hosts damaging or ejecting their own egg when attempting to remove cowbird eggs (Rothstein 1977). Therefore, costs associated with host egg loss or abandonment of unparasitized nests would outweigh the costs associated with retaining rejection behaviors, and would select for acceptance of eggs in populations with absent (Røskaft et al.

2006) or limited parasitism pressure, as in Florida.

Many species may accept parasitism because they have ratios of bill length to egg-width too small to eject cowbird eggs by grasping them (Rothstein 1975b). Some smaller species are known to eject cowbird eggs by puncturing (Rothstein 1976, 1977, Sealy 1996) but this behavior is not widespread. Perhaps the unusual strength of cowbird eggs (Picman 1989) makes them difficult to puncture, resulting in incidental damage to host eggs (Spaw and Rohwer 1987). The species experimentally parasitized in this experiment are all similar in size or larger than Brown- headed Cowbirds with bills large enough to grasp cowbird eggs. Therefore, study species did not accept eggs simply because of morphological constraints. Indeed, Rothstein (1975b) artificially parasitized four accepter species with miniature cowbird eggs capable of being grasp- ejected, but eggs were still accepted in 87% of nests. However, many of the species that constitute the host community in north-central Florida are smaller species (e.g., parulids,

vireonids) that may accept parasitism due to inability to remove larger cowbird eggs, and may

instead rely on other behavioral defenses to cowbird parasitism. For example, early nest attentiveness (Neudorf and Sealy 1994) and active nest defense (Gill and Sealy 1996) by Yellow

Warblers may deter female cowbird intruders.

Acceptance of cowbird eggs may have occurred in the study species because cues

associated with a female cowbird intruder were not simulated. For example, Meadow Pipits

(Anthus pratensis) eject experimental European Cuckoo (Cuculus canorus) eggs more frequently

when a model cuckoo is placed at the nest (Moksnes et al. 1991). However, other studies have

found no correlation between parasite presence and rejection behaviors (Moksnes and Roskaft

1988, Sealy 1995, Soler et al. 2000). In this experiment, lack of response to cowbird eggs in the nest was likely not due to the host missing the cue of female cowbird presence at the nest.

Responses occurring before cowbird eggs are placed in the nest, however, may be important to

reducing overall cowbird parasitism. Red-winged Blackbirds, for example, often mob cowbirds

and chase them from nesting colonies (Robertson and Norman 1976). Density of nesting Red-

winged Blackbirds has been negatively correlated with parasitism rate suggesting that group

defense may be an important cowbird deterrent (Freeman et al. 1990). Northern Cardinals and

Northern Mockingbirds are also known to practice aggressive behaviors toward many types of

nest intruders (Breitwisch 1988, Nealen and Breitwisch 1997), which may limit the number of

eggs that female cowbirds are able to place in their nests.

The two species known to commonly abandon parasitized nests, Northern Cardinal and

Red-winged Blackbird, did not do so in this experiment. Nest abandonment was not observed

perhaps because it is not strictly a response to cowbird parasitism in some populations. For

example, adults often only abandon nests if too many host eggs are removed (Rothstein 1982,

Rothstein 1986). Furthermore, Least Bell’s Vireos only abandoned nests when removal of a host

egg took place one day before it was replaced with a cowbird egg (Kosciuch et al. 2006). These

studies suggest that adults may choose to desert a nest in response to the selection pressure of nest predation, not necessarily brood parasitism. During experimental parasitism of nests in this study, a single host egg was removed and replaced by a cowbird egg only in cases when two or more host eggs were present. Therefore, it is not known if Florida birds would abandon nests as a result of partial clutch loss instead of addition of cowbird eggs. Individuals in other

populations of Northern Cardinal and Red-winged Blackbird abandon naturally parasitized nests

at significantly higher rates than unparasitized nests (Graham 1988, Ortega 1991, Clotfelter and

Yasukawa 1999, Whitehead et al. 2002). Florida populations in this study appear to lack the

ability to abandon nests strictly in response to brood parasitism, but may employ abandonment

behavior in response to other pressures.

Conclusions

In several species, loss of rejection behavior has been demonstrated in populations that are no longer exposed to parasitism. For example, American Robins (Turdus migratorius) eject cowbird eggs only in areas where Brown-headed Cowbirds have historically bred (Briskie et al.

1992). Cruz and Wiley (1989) showed that the prevalence of egg ejection in Village Weavers

(Ploceus cucullatus) declined significantly in a population introduced to Hispaniola where the selective pressure of brood parasitism was absent. Interestingly, ejection frequencies in the same population increased from 14% to 83% in 16 years, perhaps due to novel parasitism pressures exerted by colonizing Shiny Cowbirds (Robert and Sorci 1999). In Florida, I observed low rejection frequencies for three of the four study species and yet also observed relatively low natural parasitism frequencies of each species and the entire host community, overall. Results

reported here suggest that selective pressure currently imposed by Brown-headed Cowbirds on

ejection behaviors of certain hosts in Florida is likely weak.

I assumed that the prevalence of rejection behaviors in hosts commonly known to practice

them would be representative of the community of hosts in north-central Florida. However,

cowbirds are known to show preferences for specific hosts (Woolfenden et al. 2003, Ellison et al.

2006) on which they may exert more intense selective pressure for development of anti- parasitism behaviors. Indeed, observations of family groups feeding fledglings indicate that species such as Northern Parula and Blue-gray Gnatcatcher may be preferred hosts in north- central Florida (Chapter 1). Small species such as these that are incapable of ejecting eggs may instead rely on abandonment behaviors or active defense. Indeed, some species show a positive relationship between nest defense and degree of parasitism (Robertson and Norman 1976,

Strausberger and Burhans 2001). While results of this study suggest that rejection responses of some species do not explain parasitism frequencies in north-central Florida, behavioral responses of preferred host species may be more important to limiting Brown-headed Cowbird reproduction in the region.

Table 2-1. Results of experimental parasitism of four common Florida species. All species are known to eject cowbird eggs or abandon parasitized nests. Brown Thrashers consistently ejected cowbird eggs whereas the other species tested generally accepted cowbird eggs experimentally placed in their nests. P-values indicate results of Fishers Exact Tests comparing observed frequency of rejection behavior with expected values for each species derived from the literature. For Red- winged Blackbird and Northern Cardinal, p-values represent comparisons with both conservative and high expected values. Asterisks (*) indicate comparisons in which observed rejection frequencies were significantly lower than expected for each species. Species N Accepted Ejected Abandoned % Rejected P-value Brown Thrasher 17 2 14 1 88.2 0.1433 Northern Mockingbird 20 17 3 0 15.0 0.0038* Red-winged Blackbird 19 19 0 0 0.0 1.00, 0.0058* Northern Cardinal 19 18 1 0 5.3 0.2433, 0.0427*

CHAPTER 3 DOES VARIATION IN FEMALE COWBIRD FECUNDITY EXPLAIN OBSERVED LOW LEVELS OF PARASITIZATION?

Introduction

Many ultimate morphological, physiological, and ecological factors are thought to

determine reproductive life-history traits such as clutch size (Moreau 1944, Skutch 1949, Lack

1954, Ashmole 1963, Cody 1966). Intraspecific clutch size increases with latitude (Koenig

1984, Jarvinen 1989, Dunn et al. 2000) and longitude (Johnston 1954, Bell 1996), and decreases

with elevation (Badyaev and Ghalambor 2001, Johnson et al. 2006). These gross geographic

patterns are altered by regionally specific proximate factors. For example, fecundity of Spotted

Owls (Strix occidentalis) was negatively affected by weather patterns prior to and during the

breeding season (LaHaye et al. 2004), perhaps because decreased precipitation and temperature

altered the abundance of food or water resources. Under experimental conditions, breeding birds

given supplemental water (Coe and Rotenberry 2003) or food (Arcese and Smith 1988, Boutin

1990, Nager et al. 1997) will lay larger clutches than birds in control groups. Food availability

and other nutritional factors, therefore, may underlie regional variation in reproductive

parameters such as clutch size or fecundity.

Because both additional energy and specific nutrients are required for egg production, diet quality and availability of suitable foods may be equally or more important than food abundance in determining fecundity, clutch size and overall reproductive output. Reproduction of captive

Zebra Finches given food resources ad libitum, for example, was constrained more by nutrient reserves, energy expenditure (Deerenberg et al. 1996), and diet quality (Semel and Sherman

1991) than by food abundance. Calcium, in particular, is not available in quantities sufficient for

egg production in many environments (Barclay 1994, Graveland and van Gijzen 1994) and birds

often supplement calcium intake during the breeding season (Simkiss 1975, St. Louis and

Breebaart 1991, Graveland 1996). Indeed, Great Tits (Parus major) supplemented with calcium

increased clutch sizes (Tilgar et al. 1999). Songbirds breeding in areas with limited calcium

have reduced reproductive output (Graveland 1990, Graveland et al. 1994, Graveland and Drent

1997). Because the abundance of exchangeable calcium in the soil shows extensive geographic

variation, reproductive indices such as clutch size or fecundity may be correlated with calcium

availability over large spatial scales (Patten 2007).

Brown-headed Cowbirds have the highest known fecundity of any passerine in North

America (Smith and Rothstein 2000). Females can produce as many as 77 eggs in 89 days of

breeding (Holford and Roby 1993). This high rate of egg production is made possible because

the Brown-headed Cowbird is the only known songbird species that lacks ovarian regression

following clutch completion (Lewis 1975) and can start new clutches in as little as one day (Scott

and Ankney 1980). Brown-headed Cowbird fecundity (number of eggs laid per breeding

season), however, shows dramatic geographic variation. Female cowbirds, for example, are

estimated to lay 2−8 eggs per breeding season in New York, 10−12 eggs in Michigan (Payne

1965, 1976), 24−30 eggs in Oklahoma and California (Payne 1965, 1973a), and 40 eggs in southeastern Ontario (Scott and Ankney 1980). This extreme intraspecific variability in fecundity suggests that the impacts of cowbird parasitism on hosts may also be highly variable.

In this chapter, I explore the possibility that limited reproduction of cowbirds may underlie low observed parasitism rates.

Limitations to Cowbird Reproduction

Various nutritional factors have been implicated in regional differences in reproductive values of Brown-headed Cowbirds over their breeding distribution. Cowbirds exhibit high

fecundity in areas where grain-based agriculture is widespread (e.g., California) probably

because food sources are accessible and abundant. Where food resources are not abundant or are

sparsely distributed, food acquisition may increase reproductive costs. For example, egg-laying rate of Brown-headed Cowbirds was significantly lower in birds that commuted long rather than short distances between feeding and breeding areas (Curson and Mathews 2003). Differences between West Indian islands in abundance of Shiny Cowbirds are attributed to availability of food rather than hosts (Post et al. 1990). Finally, as the most fecund North American songbird

species, Brown-headed Cowbirds require large amounts of calcium for egg synthesis. As a

result, cowbirds are known to supplement calcium through consumption of removed host eggs

(Scott et al. 1992) and a variety of other calcareous sources (Ankney and Scott 1980). Any

variation in supplemental calcium availability can thus explain regional differences in fecundity,

as well as observed parasitism rates (Holford and Roby 1993). Furthermore, as nest parasites

cowbirds may be uniquely limited by the abundance of nests of suitable hosts. Cowbirds in

northern Michigan do not breed until approximately a month later than those in southern

Michigan, because suitable hosts have not yet begun breeding (Payne 1965).

Low Nest Parasitism in Florida: Low Cowbird Fecundity?

Despite relatively high breeding season abundance of Brown-headed Cowbirds in north-

central Florida (3−10 birds per Breeding Bird Survey Route; Sauer et al. 2006), observed

parasitism frequency is unusually low; only 1.6% of nests of 29 potential host species were

parasitized (Chapter 1). Rates of parasitism on specific host species are also lower than in other

regions, even those that cowbirds have recently invaded. Parasitism rates on Northern Cardinals

(Cardinalis cardinalis), for example, are 20.1% in Ontario (Peck and James 1998b), 7.5% in

Texas (Barber and Martin 1997), 5.4% in South Carolina (Whitehead et al. 2000), and only 2.3%

in north-central Florida.

To help explain low observed parasitism frequencies, I tested the hypothesis that

reproductive output of Brown-headed Cowbirds was limited in north-central Florida by low

fecundity. To determine differences in reproductive parameters that may influence patterns of

parasitism, I analyzed female reproductive organs to estimate fecundity and clutch size using a

methodology that has been performed for a variety of species (Hannon 1981, Kennedy et al.

1989, Arnold et al. 1997, Pearson and Rohwer 1998, Lindstrom et al. 2006), including Brown- headed Cowbirds (Payne 1965, 1973a, 1976, Scott and Ankney 1980, 1983). Reproductive condition of female cowbirds from Florida was compared with cowbirds from Fort Hood, Texas, an area with high rates of cowbird parasitism (e.g., Barber and Martin 1997). I predicted that

cowbird fecundity, in particular, would be lower in Florida than in Texas but did not specifically

test any mechanism that would explain observed differences.

Methods

Cowbird Collection

Brown-headed Cowbirds were collected at sites in both north-central Florida and east-

central Texas (Figure 3-1). Sites were located at roughly similar latitude to control for potential

biases. Cowbirds breeding in Florida are the subspecies M. ater ater which tend to be larger

(Lowther 1993) than those breeding in Texas, which are typically the dwarf cowbird subspecies

M. a. obscurus (Summers et al. 2006). Texas specimens were trapped or shot at Fort Hood in

Bell and Coryell Counties in central Texas (ca. 31°11’ N, 97°40’ W) between April and July,

2005 by the Nature Conservancy according to protocol used by Summers et al. (2006). Cowbird control has been conducted at Fort Hood since 1988 (Eckrich et al. 1999) due to negative impacts of parasitism on native breeding birds. Cowbirds were either captured using a series of baited decoy traps typically placed near free-ranging cattle or shot from within 30 m using a 20- gauge shotgun, full choke, number 9 lead shot and 2-3/4 inch shotshells. Birds were shot

between 0800−1100 CST when foraging in groups on the ground. Potential local breeders were

distinguished from migrant females based on morphometric, phenotypic and physiological characters (Summers et al. 2006).

Females were captured in Florida from April−July of 2006 and 2007 in baited drop-down

decoy traps. Traps were baited daily with F-R-M Game Bird Starter (Flint River Mills,

Bainbridge, GA) placed on a mesh tray below trap openings. In 2006, two trap sizes were used.

Large traps (2.4 x 2.4 x 1.8 m) owned by the USDA National Wildlife Research Center, Florida

Field Station were constructed of wood and wire mesh. Medium-sized traps (1.7 x 1.4 x 1.4 m)

were based on the “pick-up trap” design of the Texas Parks and Wildlife Department (2008) and

were primarily constructed of one-inch hardware cloth. Small traps (1.45 x 1.2 x 1.2 m) of

similar design were used in 2007. Live cowbird decoys placed in traps typically included one

female and one or more male cowbirds collected in September 2004 and housed at the USDA

Wildlife Research Center in Gainesville, FL. Traps were checked daily from 0900−1100 or

1400−1600 EST and females were euthanized humanely on site via cervical dislocation

according to IACUC-approved protocol. Trapping effort ranged from one to eight open traps per

day at up to three sites. In 2006, trapping was conducted at the University of Florida (UF) Dairy

Research Unit (DRU), Beef Research Unit (BRU), Horse Teaching Unit, and Santa Fe Beef

Research Unit in Alachua County, FL. In 2007, trapping was conducted at the same sites in

addition to the UF Large Animal Clinic and Beef Teaching Unit (BTU).

With the onset of the breeding season in May, the number of female cowbirds that can be

trapped often drops significantly (Beezley and Rieger 1987, Summers et al. 2006). Shooting of

females in feeding areas allows a steady collection of specimens throughout the breeding season

(Summers et al. 2006). Trapping of cowbirds may also be biased toward individuals of poorer

condition (Dufour and Weatherhead 1991). Therefore, trapping was supplemented in Florida in

2007 with collection of individual birds using pellets and a Crosman 664 Powermaster pneumatic air rifle. Birds were shot opportunistically between 1400 and 1800 EST at DRU,

BRU, and BTU sites in or near fields where they foraged with cattle. I used playback of female chatter calls in some cases to improve shooting efficiency as recommended by Rothstein et al.

(1987). All collected birds were labeled and frozen until analyzed.

Reproductive Assays

All females collected were analyzed at the Florida Museum of Natural History in

Gainesville, FL. Breeding condition of females was assessed using pre-ovulatory and post- ovulatory follicles (POFs; Scott and Ankney 1983). Pre-ovulatory follicles represent developing ova that are subsequently released into the abdominal cavity and taken into the oviduct for egg production. After ovulation, POFs remain attached to the ovary and are reabsorbed over time.

The condition of the oviduct was examined for presence of an oviducal egg or signs that an egg had passed (straight vs. convoluted). Ovaries were examined with a dissection probe, #5 tweezers, and Bausch & Lomb Stereo Zoom 4 (7−30x magnification) dissecting microscope because macroscopic counts of follicles may be unreliable (Arnold et al. 1997).

I recorded the number and size of pre−ovulatory follicles (ones that could potentially produce the yolk of an egg) in each specimen. The widths of pre-ovulatory follicles were measured to the nearest 0.01mm using a Mitutoyo Absolute Digimatic caliper (model CD6:CS).

Round, orange-yellow pre-ovulatory follicles were distinguished from degenerating or atretic follicles which are irregular in shape and cream-colored. Pre-ovulatory follicles generally grow quickly and sequences of developing follicles may represent eggs of the same clutch. K-means hierarchical cluster analysis (maximum 10 iterations) using SPSS (2003) was used to group

follicles into categories A, B, or C based on size (sensu Payne 1965, Scott 1978) to identify distinguishable sequence breaks representing separate clutches. A follicles are those that would

likely have ovulated on the next day following collection, B follicles two days from collection

and so on. Although smaller pre-ovulatory follicles (e.g., D follicles) were sometimes present in

birds in the sample, some follicles are known to regress before ovulation (Payne 1976).

However, Scott (1978) found only 3 cases of atresia in 69 follicles >5mm indicating that larger

follicles (i.e., A and B) are unlikely to regress before releasing the yolk. Therefore, analysis was

limited to three clusters to improve the probability of including only follicles that would reach

ovulation.

I also recorded the number and size of all recognizable POFs which are flat and oval-

shaped and have an open slit (stigma) through which ova are released into the abdominal cavity

during ovulation. Only those follicles with ruptured stigma were considered POFs. POFs were

distinguished from burst atretic follicles (Pearson and Rohwer 1998) by examining color and

other differentiating characteristics (Scott and Ankney 1983). Burst atretic follicles are ruptured

but typically contain a cream-colored residue that is lacking in POFs. The area of each flattened

POF was estimated as the product of width and maximum lengths measured to the nearest

0.01mm.

Ovaries, oviducts, and oviducal eggs were removed and weighed to the nearest 0.0001g using a Mettler Toledo Analytical Balance (model AB54-S). Average start and end dates of the cowbird breeding season were determined separately for Texas and Florida based on presence of enlarged oviducts, large pre-ovulatory follicles (i.e., A follicles) and oviducal eggs. Collected birds found in reproductive condition with enlarged oviducts (>0.7g), oviducal eggs, or the presence of one or more large pre-ovulatory (A follicles) or post-ovulatory follicles are

“breeding”. All other birds are considered “non-breeding” birds. Breeding birds that were

collected with a developing egg anywhere in the oviduct are called “laying” birds while all other

breeding birds are called “non-laying” (sensu Scott and Ankney 1980).

Shapiro-Wilk tests of normality indicated that body mass, ovary mass, and oviduct mass

variables from each collection location were normally distributed (P>0.15 for all). Therefore,

measures of reproductive condition of breeding birds were compared between Florida and Texas,

and laying and non-laying cowbirds using two-tailed t-tests assuming unequal variances in SPSS

(2003). Change in body mass over the course of the breeding season at each collection location

was analyzed using simple linear regression. Finally, as a result of the change in size of

reproductive organs with the beginning and end of the breeding season, I used regression

analysis to test if mass of oviducts and ovaries of breeding birds differed between Florida and

Texas. PROC GLM (SAS Institute Inc. 2003) with Type IV sum of squares was used to fit a

general linear model to the data because it allows for both continuous and categorical predictor

variables. The response variables in each of the two models were oviduct mass and ovary mass

with collection week and location (Florida and Texas) as predictor variables. A quadratic

regression function was applied to the week variable in both models due to the observed growth

and regression curve of reproductive organs. Residuals plots were used to assure that there was a

good fit between the model and the data.

Clutch Size Estimates

Average clutch size for Brown-headed Cowbirds has been estimated based on sequences of

developing pre-ovulatory follicles and regressing post-ovulatory follicles (Payne 1965, Scott

1978). There are, however, some limitations to using this method. Some developing follicles may regress prior to ovulation (Payne 1976), released ova may get reabsorbed before entering the oviduct (Meyer et al. 1947) and post-ovulatory follicles may be difficult to distinguish from

burst atretic follicles (Payne 1965, Lindstrom et al. 2006). As a result, clutch size can be

overestimated when relying on assumed follicle sequences (Davis 1942, Payne 1965, Scott 1978,

Arnold et al. 1997). Clutch size can also be underestimated using post-ovulatory follicles if they

are reabsorbed quickly (Arnold et al. 1997). For these reasons, I estimated clutch size by

determining numbers of post-ovulatory follicles observed in breeding birds (Scott and Ankney

1983).

The method used relies on two clutch size estimates based on numbers of post-ovulatory follicles in (1) laying birds and (2) non-laying birds. First, I estimated the total number of eggs laying birds would have laid in their current clutch. This number was derived by finding the

total number of clearly-identifiable POFs in the sample population of laying birds, which should

represent the average midpoint of laying sequences. That is, a sample of 10 laying cowbirds

with an average clutch size of 5 would lay 50 eggs and develop 50 POFs over the laying period.

At the midpoint of their sequences they should have 30 POFs because the third POF represents

the midpoint of a 5-egg clutch. Thus, the midpoint number of POFs can be used to determine the

total number of eggs the sample would have laid by multiplying it by 2, and subtracting the

sample size (i.e., (30 x 2)–10=50). Because size breaks in regressing follicle sequences indicate

distinct clutches (Payne 1976, Hannon 1981), I only included POFs that were clearly in the same

laying sequence.

Second, clutch size of non-laying birds was estimated. Non-laying birds do not have oviducal eggs by definition, so they are likely in between clutches. Therefore, recent clutch sizes were estimated based on total number of POFs divided by the sample size. In some cases, non- laying birds did not have clearly identifiable POFs so were assumed to have laid a clutch earlier.

This assumption is supported by the observation that all non-laying birds showed evidence of

egg production (e.g., enlarged and convoluted oviducts). Because it is assumed that these birds

had the same frequency distribution of clutch sizes, the contribution of these birds was estimated

as the same proportion of POFs recorded in all other non-laying birds. For example, if a sample

of 50 non-laying birds included 40 birds with 60 POFs (1.5 POFs per bird) the remaining 10

individuals without identifiable POFs would be estimated to have 15 POFs (10 birds x 1.5). To

estimate average clutch size, the sum of (1) total number of eggs that laying birds would have

produced and (2) the estimated number of POFs of non-laying birds, were divided by the total

number of birds in the sample. Estimates were performed separately for Florida and Texas

samples.

Inter-clutch interval (number of days between distinct clutches) was estimated using the

proportion of non-laying birds in a particular portion of the non-laying period (Scott and Ankney

1979). For example, some birds without an oviducal egg may have finished laying their clutch

on the morning before collection. These birds would have large POFs that are less than a day old indicating ovulation occurred on the day prior to collection. This would represent the first day of the non-laying period for these females. Similarly, large pre-ovulatory follicles (A follicles) in a cowbird without an egg in the oviduct would represent the end of the non-laying interval since ovulation was likely to occur on the following day. The reciprocal of the proportion of non- laying birds in each of these stages should represent the inter-clutch interval. For example, in a sample of non-laying birds with an inter-clutch interval of two days, roughly one-half of the birds should be in one of the two non-laying days. These two non-laying states were used to estimate the non-laying interval in Florida and Texas female cowbirds.

Fecundity Estimates

Fecundity in this study is defined as the number of eggs that an average female cowbird lays over a breeding season of average length. Average annual cowbird fecundity in each study

area was determined as the product of daily fecundity (laying rate) and duration of the cowbird

breeding season (Scott and Ankney 1980). Daily fecundity was calculated using the proportions

of breeding birds collected that had an egg in the oviduct (Payne 1973b). Average dates of the

breeding season in Texas and Florida were estimated as the first and last days when

approximately half of birds had enlarged oviducts, large pre-ovulatory follicles or oviducal eggs.

Only breeding birds that were collected between respective breeding season dates in Texas and

Florida were included in fecundity estimates. I assumed that all birds had an equal probability of

being collected regardless of whether or not they were carrying a developing egg. Scott &

Ankney (1979) found that the proportion of collected birds carrying an egg was not affected by variables such as collection location, method, or time of day, among others.

Results

Cowbird Collection

A total 142 female cowbirds were collected from Texas between 22 March and 6 July,

2005. The majority (N=94) of these specimens were collected by shooting. A total of 71 and 63

female cowbirds were collected in Florida between 23 April and 12 July, 2006 and 2007,

respectively (Figure 3-2). Total effort (traps open x days open) was 146 trap days in 2006 and

463 in 2007. In general, more females were caught per trap day in large traps (0.415) than in small and medium traps combined (0.061; Figure 3-3). All 2006 specimens were collected in traps, the majority (59%) of which was trapped in July when a higher proportion of individuals was no longer in breeding condition. Trapping success was low from April to May in both years but remained low in June only in 2007 (Figure 3-3). Trapping success increased from May to

June in 2006 but the majority of these birds (74%) was collected after June 21 when some birds

were in declining or non-reproductive condition (see below). Shooting of individual female

cowbirds in 2007 greatly increased sample sizes during May and June (Figure 3-4).

Body mass of breeding Florida cowbirds was significantly lower in trapped than shot birds

(35.2 vs. 38.5g, t=-4.581, P<0.001) but there was no difference when mass of oviducal eggs was removed (34.3 vs. 31.6g, t=1.115, P=0.272). Ovaries (0.3250 vs. 0.5217g; t=-0.3838, P<0.001) and oviducts (1.0253 vs. 1.4126g, t=-4.495, P<0.001) were also significantly lighter in trapped birds. However, wing chord (t=0.013, P0.990) and tail length (t=-0.767, P=0.448) were similar in trapped and shot birds in Florida. Among 20 Florida cowbirds collected by shooting during the breeding season, 17 had an oviducal egg. Furthermore, laying birds that were shot were more likely to have an oviducal egg than trapped birds (Log Odds Ratio=2.76, CI=3.63−68.41).

Reproductive Assays

Reproductive assays were conducted on all female cowbird specimens collected. Of 142

Texas females collected, 79 had enlarged oviducts, POFs, large pre-ovulatory follicles, or

oviducal eggs and were categorized as breeding birds. Of breeding birds, 40 were found with an

egg in the oviduct and were called laying birds, and the remaining 39 were categorized as non-

laying birds. 134 specimens were collected in Florida and 56 were called breeding birds. Of

Florida breeding birds, 29 were laying birds and 27 were non-laying birds.

It was not possible to test for any effect of year on reproductive measures due to small

sample sizes for part of the breeding season in Florida in 2006. However, there were no

differences (independent samples t-test, P>0.20 for all) in the number of pre- and post-ovulatory

follicles, body mass, or mass of oviducts and ovaries of samples collected during 1 June to 21

June in 2006 (N=8) or 2007 (N=16). These variables were also not significantly different when

samples from 2006 were paired with randomly-chosen samples from similar collection dates (± 2

days) in 2007 (paired samples t-test, P>0.30 for all, N=8). Therefore, all data presented for

Florida birds represent pooled data over both collection years.

Ovary measurements (± SE) of Texas cowbirds collected between 22 March and 6 April showed no signs of breeding season development (26.89 ± 1.21mm2, 0.027 ± 0.002g, N=40).

Oviducts were similarly small and undeveloped during this time (0.043 ± 0.009g, N=7). For

both Florida and Texas females, growth and maturation of ovarian follicles led to a dramatic (ca.

14-fold in Texas) increase in mean ovary mass in mid- to late-April (Figure 3-5). Similarly,

oviducts of collected specimens increased significantly (ca. 28-fold in Texas) in mid- to late

April (Figure 3-6). Mean ovary and oviduct mass remained fairly consistent from May to early

June, although there was much individual variation due to the dynamic nature of ovarian follicle

growth and yolk release during ovulation (Figures 3-7 to 3-10).

On average, laying birds in Florida had significantly heavier ovaries (t=2.480, P=0.017)

and oviducts (t=3.429, P=0.001) than non-laying birds (Figures 3-7, 3-8). However, breeding

birds without oviducal eggs still had large oviducts (i.e., mean mass >1.0g), probably because

they recently completed clutches or were preparing to ovulate. Laying birds in Texas had

significantly larger ovaries (t=2.091, P=0.041), but not oviducts (t=1.263, P=0.212) than non-

laying birds (Figures 3-9, 3-10). Ovaries and oviducts began to regress in the second week of

June as resource allocation to reproduction was likely waning with the end of the cowbird

breeding season. Breeding females from Texas and Florida had similar ovary mass (t=0.805,

P=0.422) over the course of the breeding season (Table 3-1) but oviducts were significantly

heavier in Texas than Florida females (t=-2.304, P=0.023). Because growth and regression of

reproductive organs roughly followed a bell-shaped curve, I applied a general linear model to the

data to determine if oviduct and ovary mass differed between Florida and Texas cowbirds as a

function of week of the breeding season. The regression model indicated that collection location

was not a significant predictor of ovary mass (parameter estimate=0.037, F=0.88, P=0.350;

Figure 3-7). However, the model indicated that collection site was a significant predictor of

oviduct mass (parameter estimate=-0.129, F=5.17, P=0.025) with Texas females having heavier

oviducts than Florida females.

Body mass of Florida females declined significantly from 23 April until 12 July (R2=0.161,

P<0.001, N=110; Figure 3-11). Mass of breeding birds in Florida tended to decrease over time but not significantly (R2=0.024, P=0.263, N=53) especially when the mass of the oviducal egg

was removed from body mass (R2=0.006, P=0.585). Mean mass of breeding females (R2<0.001,

P=0.990, N=58) and all cowbirds collected in Texas between 19 April and 6 July (R2=0.001,

P=0.826, N=79) did not change significantly with time (Figure 3-12). Similarly, no body mass change was observed over time when oviducal egg mass was removed from body mass of laying birds (R2=0.012, P=0.429). Overall body mass decline from April to July in Florida birds was

likely due to mass differences between pre-breeding season birds prepared for reproduction and

post-breeding birds with regressing reproductive organs. That mass declined over the entire

collection period in Florida but not Texas is due to a significant difference in sample size of birds

collected after 25 June (Florida=81, Texas=6) when most birds are no longer in breeding

condition. Although the eastern subspecies of cowbird is generally larger than the dwarf

cowbird of Texas (Lowther 1993, Pyle 1997), mass breeding Florida and Texas females was not

different (t=-1.113, P=0.268). Similarly, wing chords of Florida breeders also were not

significantly different from those of Texas cowbirds (Table 3-1) collected at Fort Hood during

previous breeding seasons (Summers et al. 2006).

Increase in ovarian mass was associated with the development of pre-ovulatory follicles in

late April. Oocytes (immature ova) were generally ca. 1.0mm in diameter in March and early

April whereas breeding birds had one or more large orange-yellow yolky follicles representing

potential eggs. A follicles (those likely to ovulate the next day) were as large as 10.50mm in diameter in Texas and 9.89mm in Florida. Sequences of developing follicles were determined using cluster analysis. For Florida birds, A, B, and C follicles were those that were larger than

8.57, 5.85, and 3.39mm diameter, respectively. Similarly, Texas birds cluster cutoff values for

A-C follicles were 8.72, 6.38 and 3.37mm diameter, respectively. These values are similar to those reported by Scott (1978) who found that the three largest pre-ovulatory follicles clustered at >8mm, and ca. 6 and 4mm. Earliest and latest A follicles were found in cowbirds collected on

26 April and 28 June in Texas and 26 April and 27 June in Florida.

All but four breeding individuals had at least one post-ovulatory follicle (POFs), although

birds without POFs were still clearly in reproductive condition. All laying birds had one large

POF that was associated with the egg in their oviduct and which released the yolk one day prior

to collection (Day -1). Laying birds had as many as four POFs that showed a graded series in

size. Mean areas (± SE) of the sequence of POFs from largest (Day -1) to smallest (Day -4) in

laying birds in Florida were 22.89 ± 0.72, 13.01 ± 0.47, 7.80 ± 0.44, and 6.57 ± 1.50mm2, respectively. In Texas cowbirds, mean areas of POFs in this sequence were 25.89 ± 0.85, 25.42

± 0.73, 12.01 ± 0.75, and 10.43 ± 1.38mm2. Day -2 POFs that were identified as likely being

part of the same sequence showed ca. 40% decrease in size from the most recent follicle. This

indicated that POFs regressed rapidly and that follicles older than four days would likely be

difficult to identify. Furthermore, the three most recent POFs (Days -1, -2, and -3) of laying

birds were significantly larger in Texas than in Florida (t-tests, P<0.01 for all; Figure 3-13).

POFs from Days -1 and -2 were ca. 13% and 18.5% smaller in laying Florida specimens, a result

which may be caused by differences in the timing of specimen collection between the two study

areas. The sample of Florida cowbirds included individuals collected throughout daylight hours

whereas Texas cowbirds were all captured before 1100 hours. Ovulation in Brown-headed

Cowbirds occurs around 0700 hours (Scott and Ankney 1983) after which the follicle begins

regressing. I was not able to specifically test effect of time of collection on size of POFs because

appropriate data were not collected. However, all birds that were shot were collected in the

afternoon. Among laying birds collected in Florida, the largest POF was significantly smaller

(t=2.653, P=0.017) in birds collected by shooting (N=18) versus trapping (N=10). This may

explain observed differences between Florida and Texas in POF size. However, the total number

of POFs counted in laying birds in Florida was the same regardless of collection method (t=-

3.760, P=0752) suggesting that all POFs that were present following ovulation in the morning were recognizable in birds collected later in the day.

Clutch Size Estimates

Clutch size and inter-clutch interval estimates were determined using the sample of

breeding birds from each collection location. In Texas, 40 laying females had between one and

four readily identifiable POFs and most commonly three (Table 3-2). The mid-point of the laying sequence for these birds was estimated as 108 total POFs. Two non-laying birds were excluded from counts due to the poor condition of their ovaries. A total of 29 non-laying females had 66 recognizable POFs (2.3 POFs/bird). The remaining eight non-laying birds without POFs were therefore estimated to contribute 18 POFs (8 birds x 2.3). Based on these figures, average clutch size of breeding birds in Texas is estimated as 3.38 eggs per female

(Table 3-2).

In Florida, one laying bird was excluded from clutch size estimates because the ovary was

damaged when collected. A total of 28 laying females also had between one and four POFs,

with a mode of three (Table 3-3). These females produced 83 POFs. Twenty-three non-laying

birds had 46 distinct POFs (2.0 POFs/bird) so that the remaining four birds without POFs were

estimated to have eight POFs. Average clutch size of breeding birds in Florida is estimated as

3.49 eggs per female (Table 3-3).

Inter-clutch intervals were estimated using two methods that use the proportion of non-

laying birds with (1) size A pre-ovulatory follicles and (2) POFs large enough to have ovulated

an egg on the day before collection. In Texas, 13 of 37 non-laying birds had size A pre-ovulatory

follicles that would likely have ovulated on the next day. Of 27 non-laying birds in Florida, 12

had size A ovulatory follicles. The reciprocals of these proportions results in estimates of inter-

clutch interval of 2.85 and 2.25 for Texas and Florida, respectively.

For method 2, I determined the size of the smallest POF in all laying birds that could have

released the egg found in the oviduct. I assumed any non-laying bird with a POF larger than this

size ovulated on the previous day. Since POFs regress approximately 50% in 12 hours following

ovulation (Scott and Ankney 1983), I used the smallest POF of a laying bird collected in the

afternoon to establish a cutoff size for POFs in non-laying birds. These values are 18.82mm2 and

15.01mm2 for Texas and Florida, respectively. Twenty of the 37 non-laying birds in Texas had

POFs that indicated that the last day of ovulation was one day prior to collection. Of 27 non-

laying Florida birds, 14 had large POFs. The reciprocals of these two proportions result in inter-

clutch interval estimates of 1.85 and 1.92 days for Texas and Florida, respectively. Based on

both estimation methods, the average non-laying period of female Brown-headed Cowbirds is

approximately 2−3 days in Texas and 2 days in Florida.

In five captive Florida Brown-headed Cowbird females used to generate eggs for

experimental parasitism research (Chapter 2), sequences of actual egg-laying in artificial nests resulted in a mean estimate (± SE) of clutch size (3.68 ± 0.57 eggs) and inter-clutch interval

(1.80 ± 0.23 days) similar to those estimated above for wild-breeding populations.

Fecundity Estimates

Average fecundity of cowbirds was calculated based on the method of Scott & Ankney

(1980) using length of the breeding season and daily laying rate. Average start and end dates to

the laying season were estimated using the presence of enlarged oviducts, large pre-ovulatory

follicles and oviducal eggs. Of the eight Florida birds collected from April 23 to 29, three had

oviducal eggs and three of the five non-laying birds had large POFs and mid-sized (B) pre-

ovulatory follicles and were therefore in breeding condition. The earliest bird that had an

oviducal egg was collected on 26 April but had three distinct POFs indicating the ovulation

probably began as early as 23 April. I was not able to collect an adequate sample of birds prior

to this date. The earliest specimen was collected 23 April and had no POFs but some developing

follicles. Therefore, I assume that the average female begins breeding during the final week of

April. Therefore I estimate an average start date to the cowbird breeding season in Florida to be

26 April.

Females were collected in Florida as late as 17 July but the last female with an oviducal egg was collected on 6 July. This female is likely an outlier because only one of the remaining

17 females collected in the first week of July had post-ovulatory follicles that indicated laying of an egg could have occurred in early July. One of 46 birds collected after this date had a single small post-ovulatory follicle, indicating that laying had likely ceased after the first week of July.

To assign an average end date to the cowbird breeding season in Florida, I estimated the

last date when ca. 50% of the birds were in breeding condition. From 25 June to 1 July, only one

of 18 birds collected had an oviducal egg. Of the remaining 17, six were likely still in breeding

condition due to the presence of pre-ovulatory follicles larger than 5mm in ovaries with little or

no atresia. Therefore, 38% of birds were still in laying condition. From 18−24 June, four of

seven birds (57%) were in breeding condition (one with an oviducal egg). Central dates for each of these weeks are 21 June and 28 June respectively. If it is assumed that the percentage of birds in breeding condition decreases steadily between these dates, the first date when less than 50% of birds were in laying condition is estimated to be 24 June (48.9% of birds).

In Texas, birds were collected beginning in March and the first bird with an oviducal egg was collected on 26 April. The presence of two large POFs indicated the ovulation could have begun as early as 24 April. However, of the other six birds collected during 23−29 April none had ovaries with developing follicles so were not in breeding condition. Because the sample from Texas does not include sufficient sample sizes of identified breeders from mid-late April to early May to deduce an average start date, I estimate a similar start date for Texas birds as those from Florida. Reproductive behaviors of Brown-headed Cowbirds begin as early as late March but few cowbirds are collected with an oviducal egg until mid-April (S. Summers, pers. comm.).

Since the start date estimates when an average bird is laying (i.e., 50% of the population), 26

April represents at least a conservative estimate of the beginning of the breeding season in Texas.

The latest oviducal egg-bearing female captured at Fort Hood was collected 12 July, but for most years the latest is typically around 9 July (S. Summers pers. comm.). Consistent with these observations, the last Texas specimen analyzed that had an oviducal egg was collected on 6

July. From 25 June to 1 July, four of nine birds collected had an oviducal egg. Of the remaining five, none were likely still in breeding condition because they lacked developing follicles.

Therefore 44% of birds were still in laying condition. From 18−24 June, 10 of 16 birds were in breeding condition, seven of which had an oviducal egg. Using central dates and the assumption of a steady decrease in breeding condition, the first date when less than 50% of birds were in laying condition is estimated to be 26 June (49.6% of birds).

In sum, I estimate the average breeding season length as 60 days from 26 April to 24 June in Florida and 62 days from 26 April to 26 June in Texas. Both starting and ending dates are earlier than those reported for Brown-headed Cowbirds breeding in southern Ontario (Scott and

Ankney 1980) but estimated breeding seasons for Florida and Texas are 4−6 days longer.

Nine birds from Florida and seven birds from Texas that were in reproductive condition

were excluded from estimates of daily laying rate because they were collected after the average

breeding season end dates. Of excluded birds, two Florida and five Texas females had

developing eggs in their oviducts. Daily laying rate was estimated as the proportion of breeding

birds with eggs in the oviduct (Payne 1973b). In Texas, 40 of 71 breeding birds collected

between 26 April and 26 June had oviducal eggs resulting in an average daily laying rate of 0.56 eggs per female per day. In Florida, 27 of 47 breeding birds collected between 26 April and 24

June resulting in an average daily laying rate of 0.57 eggs. Small sample sizes precluded analysis, but daily laying rate showed some minor variation during the breeding season (Figure

3-14). Using average laying rates and the respective lengths of the breeding season, it is estimated that each female Brown-headed Cowbird in both Texas and Florida lays an average of

34-35 eggs in a breeding season (TX: 62 days x 0.56 eggs/day = 34.93, FL: 60 days x 0.57 eggs/day = 34.47). Because these estimates only include breeding individuals, they only represent average fecundity for birds in reproductive condition and not fecundity of the entire population of birds present in the breeding area.

Additional Trapping Observations

During collection of Brown-headed Cowbirds in Florida, we collected four specimens of

Shiny Cowbirds, three female and one male. Because there are few diagnostic features that

distinguish females of Shiny and Brown-headed Cowbird (see Pyle 1997), tissues from each

female specimen were sent to the Bell Museum in Minneapolis for genetic analyses to confirm species identification. DNA was extracted from the samples and PCR was used to amplify two different loci. The first locus was a 486-base-pair fragment of the cytochrome b gene from the mitochondrial genome. The second locus was a 565-base-pair fragment from the third intron of the MUSK gene on the Z chromosome. Sequences were compared to known M. ater and M.

bonariensis sequences deposited in GenBank and vouchered tissue museum samples to

determine species identity. All three specimens were confirmed as being Shiny Cowbirds.

These contribute to only a handful of reported cases of Shiny Cowbirds in north-central Florida, all occurring since 2000. Furthermore, one of the females collected (UF 46077) also had a developed ovary with three large pre-ovulatory follicles and a single potential post-ovulatory follicle. The oviduct was large (~1g) and convoluted indicating that it recently passed an egg.

These observations represent the best evidence to date of Shiny Cowbirds breeding in Florida.

Discussion

Based on assays of ovarian and oviducal condition, an average female Brown-headed

Cowbird breeding in north-central Florida and Texas lays approximately 35 eggs over the course

of an 8−9 week breeding season. This estimate of fecundity is slightly lower than the 40 eggs

determined for birds breeding in Manitoba (Scott and Ankney 1980) and higher than the 30 eggs

derived for cowbirds in California (Payne 1965, Fleischer et al. 1987). Estimated fecundity and

clutch size of Florida cowbirds were nearly identical to values derived for females breeding in east-central Texas, where nest parasitism has led to critical declines of some species of native songbirds (Eckrich et al. 1999). Similarly, clutch sizes and inter-laying intervals were similar between female cowbirds breeding in Florida and Texas. Therefore, the hypothesis that low fecundity of cowbirds in Florida explains their observed low rates of parasitism is not supported.

Factors that have been shown to affect reproductive indices in other studies do not appear

to significantly affect Brown-headed Cowbirds in Florida. For example, calcium availability can

limit reproduction in some populations. While some of the soils in north-central Florida are

underlain with sand or clay and are depauperate in calcium (Abrahamson and Hartnett 1990),

availability of calcium for egg production is probably not limited. The physiographic regions of

north and central Florida are characterized by many areas with limestone at or near the surface

(Brown et al. 1990), providing a rich source of calcium carbonate. Furthermore, the Floridan

Aquifer is one of the most productive carbonate aquifers in the world (USGS 1990) and

dissolved limestone delivers large concentrations of calcium into a water system that feeds the

entire state (USGS 1990). Therefore, calcium is at least secondarily available through calcareous

stone, or through bone and mollusc shells often eaten by cowbirds (Ankney and Scott 1980). In

addition, Schoech and Bowman (2003) found no difference in plasma calcium levels between

suburban and natural populations of Florida Scrub Jays (Aphelocoma coerulescens) breeding in

central Florida. The authors assumed that natural calcium sources would be highly limited in

natural scrub habitats due to low pH levels, yet jays were able to find ample calcium for

production of eggs.

Individual female Brown-headed Cowbirds breeding in north-central Florida and Texas

also appear to be able to get ample food resources for production of eggs. In 2005, Florida

ranked seventeenth in the United States in the total number of cattle (USDA 2002b) and Alachua

County currently has nearly 49,000 head of beef and dairy cows (USDA 2007) on approximately

86,000 acres of agricultural land (USDA 2002a). Therefore, there are numerous feeding

locations in the study area where female cowbirds can acquire resources for egg production.

Cowbirds were observed foraging at all trapping locations as well as on road sides, at city parks,

and private bird feeders. I did not specifically measure abundance of cowbirds at these locations.

However, abundance at feeding areas where collection was performed varied by site and time of the breeding season. At collection sites with the largest numbers of dairy and beef cows

(200−800), relatively few females were captured or seen foraging with cattle. Two sites located near the largest urban area in the county (Gainesville) had large mixed-sex flocks with as many as 100−150 individuals during peak foraging hours. Therefore, major nutritional factors thought to determine cowbird fecundity or clutch size in other regions or in other species, do not appear to be limiting fecundity in Florida cowbirds.

Clutch Size Estimates

Clutch sizes in this study were estimated using the number of readily recognizable post- ovulatory follicles that were in the same laying sequence. Clutch size estimates for Florida (3.68 eggs) and Texas (3.38 eggs) cowbirds were smaller than those (3.91−4.60 eggs) derived using a similar methodology for cowbirds in various locations in the United States (Payne 1976, Scott and Ankney 1983). Clutch sizes are known to increase with latitude (Koenig 1984, Jarvinen

1989, Dunn et al. 2000) which may partially explain lower observed clutch sizes than those from

Michigan and Ontario (Payne 1976, Scott and Ankney 1983).

Lower clutch size estimates for the cowbirds in this study may also have been due to subtle differences in methodology or study populations. Post-ovulatory follicles in Brown-headed

Cowbirds regress rapidly with 50% decrease in size every 12 hours in the two to three days following ovulation (Scott and Ankney 1983). Furthermore, the opening of ruptured post- ovulatory follicles closes in 3−4 days post-ovulation (Scott and Ankney 1983). As a result, it is often difficult to categorize follicles older than 3−5 days as having released an ovum. On the other hand, Payne (1965) was able to count post-ovulatory follicles of Michigan cowbirds for as

many as twelve days past ovulation. In the present study, the highest number of flaccid follicles of any kind (post-ovulatory, atretic, burst prior to ovulation) found in a single specimen was six.

Assuming some of these follicles were misclassified and indeed all post-ovulatory, it still represents only half the maximum number that Payne counted. Therefore, the clutch sizes I estimated may be lower than those in the literature because I conservatively included only follicles that could reliably be distinguished as post-ovulatory, and cowbirds I collected may have absorbed post-ovulatory follicles relatively more quickly.

While an average wild female cowbird lays for 3−4 days in a row, captive females have laid eggs for up to 13 consecutive days in Florida (Chapter 2), and are capable of laying 67 eggs in as many days (Holford and Roby 1993). Therefore, the method used in this study probably generally underestimates clutch size because some birds will lay for many days in a row but older post-ovulatory follicles will be difficult or impossible to identify. For birds such as Brown- headed Cowbirds that are indeterminate layers and not constrained by energetic or time demands of incubating eggs and caring for young, clutch size may be of little significance. That is, length of the laying sequences is probably less important than the daily laying rate, for example, because a female cowbird can have a small clutch with short inter-clutch interval and lay as many eggs in the same time span as a female with a large clutch. Therefore, overall fecundity is likely a more appropriate and consistent measure of Brown-headed Cowbird reproduction in an area.

Fecundity Estimates

Estimates of daily laying rate in this study were only slightly lower than those in other published studies of cowbird reproduction. Based on proportions of birds with oviducal eggs, I estimated that breeding female cowbirds in both Florida and Texas laid approximately 0.56 to

0.57 eggs per day. Other studies have estimated average daily laying rates from 0.64 to 0.73

eggs per day using similar methodologies (Scott and Ankney 1980, Fleischer et al. 1987).

Differences in daily laying rate, however, may be related to the variation in the length of the

cowbird breeding season geographically. Other studies have shown a decreasing avian breeding season length with increasing latitude (Baker 1939, Johnston 1954). Average cowbird breeding season length is longer in Florida (60 days), Texas (62 days), and Oklahoma (8−10 weeks; Payne

1976) than in Ontario (56 days; Scott and Ankney 1980) and Michigan (5−6 weeks; Payne

1976). Cowbirds breeding in areas with short seasons, whether due to abiotic (e.g., climate,

photoperiod) or biotic (e.g., host availability) factors, may have relatively higher average laying

rates to maximize overall fecundity. I assumed all birds stayed on site throughout the breeding

season and that daily laying rate was similar over the course of the breeding season. These

assumptions are likely violated because cowbirds are known to leave study areas (Woolfenden et

al. 2003) and because laying rate typically changes throughout the breeding season (Payne

1973a, Fleischer et al. 1987, Woolfenden et al. 2001, but see Scott and Ankney 1980). The

estimate represents an average over the course of the breeding season and therefore does not

consider temporal variation.

Fecundity was defined in this study as the number of eggs produced by an average female

cowbird during the breeding season. Some females that were in the study area likely laid fewer

than 35 eggs over the course of the 8-week season, and may not have bred at all. Of female

Brown-headed Cowbirds trapped in Ontario during the height of the cowbird breeding season,

only 2% of individuals were not in breeding condition (Scott 1978). However, Summers et al.

(2006) detected enlarged ovaries in only 18% of female cowbirds categorized as potential local

breeder in Texas, although the sample included birds collected outside of the breeding season. In

this study, 47 female cowbirds were collected in Florida between estimated beginning and end

dates of the cowbird breeding season in 2006 and 2007. Only three (6.4%) of these females were not in reproductive condition as they lacked pre- or post-ovulatory follicles, or enlarged ovaries

and oviducts. Therefore, the majority of female cowbirds collected during the breeding season in

this study had recently laid eggs, were ovulating, or preparing to lay oviducal eggs. It is also

possible that the three individuals found in non-reproductive condition had already laid eggs

prior to collection since they were all captured in mid- to late-June. Finally, many of the females

collected in this study may have laid more eggs than the 35 estimated for an average bird.

Therefore, it is likely that the fecundity value derived for Florida Brown-headed Cowbirds in this

study is at least a conservative estimate of the number of eggs a typical female lays during

breeding.

Cowbird Collection

Trapping of Brown-headed Cowbirds has been used extensively to control populations

where they have become problematic (Kelly and DeCapita 1982, Beezley and Rieger 1987,

Eckrich et al. 1999). Trapping has been shown to more effective than shooting at removing large

quantities of breeding female cowbirds over the course of a breeding season (Summers et al.

2006). However, it is less efficient overall than shooting, and the effectiveness of trapping and

shooting has been shown to vary over the course of the breeding season (Summers et al. 2006).

Cowbirds are easily trapped in March and April, but may become trap-shy after the onset of the

breeding season (Dufour and Weatherhead 1991). As a result, birds were collected in Florida

using a combination of trapping and shooting. Trapping was most effective at capturing birds at

the end of the breeding season when many birds were in declining reproductive condition,

although birds entered traps throughout the entire collection period. On the other hand, shooting

was effective in collecting birds during May and June when trapping efficiency was low.

Many factors may contribute to differences in the efficiency of each collection method.

For example, trapping relies on a “local enhancement” of a food resource (Turner 1964) and

conspecific attraction to decoys by females using foraging areas. Breeding females are known to

shift their diet during the breeding season from seeds to insects (Ankney and Scott 1980),

presumably for production of eggs. Therefore, birds that are attracted to grains and seeds in traps

may represent individuals that are food stressed and therefore generally in poorer condition

(Dufour and Weatherhead 1991). Body mass, oviduct mass, and ovary mass of Florida breeding

cowbirds were significantly lighter in trapped birds than shot birds. However, I detected no

differences in morphological measurements such as wing chord and tail length between trapped

and shot birds. Other research has also found similar body condition between trapped birds and

those collected using other methods (Scott and Ankney 1979, Davis 2005). Furthermore, body

mass of trapped and shot females in this study were not significantly different when the mass of

oviducal eggs was removed. Therefore, mass differences of trapped versus shot birds may not

reflect a condition bias (e.g., Weatherhead and Greenwood 1981, Dufour and Weatherhead

1991), per se, but rather a sampling bias. Indeed, shot birds were more likely to have an oviducal

egg than trapped birds (Log Odds Ratio=2.76, CI=3.63−68.41) so that trapping may bias toward

individuals in between clutches rather than toward birds in poorer condition. It is not clear why birds in between clutches would be more likely to be trapped than those that are ovulating. Egg production can be energetically costly (Carey 1996, Williams 2005) so that non-laying birds may be more attracted to easily-attainable, but nutritionally inferior food resources than laying individuals. Some cowbird populations have been shown to be promiscuous (Strausberger and

Ashley 2003), so female cowbirds preparing for another bout of egg-laying may also have been looking for extra pair copulations with male cowbird decoys in traps.

While trapping conducted in this project may have been biased toward non-laying birds, it is also possible that the shooting protocol was biased toward individuals in laying condition.

Birds were singled out and shot when foraging within and near feed lots and pastures. Collecting females by shooting individual birds foraging in these areas is likely selective toward breeding birds because they are exhibiting breeding behavior within and near host habitat (Summers et al.

2006). Furthermore, I often collected females that were foraging alone or in small flocks of a few individuals. Scott & Ankney (1980) observed that females foraging in larger flocks (groups of 10 or more) were less likely to be in laying condition. Indeed, 17 of the 20 breeding-season cowbirds in reproductive condition that were collected via shooting had oviducts containing developing eggs. If female cowbirds had been collected solely using this method over the course of the field season, average female fecundity would be estimated as 51 eggs per female per breeding season. With trapping alone, the estimate would be 22 eggs. Therefore, trapping or shooting methodology, when used alone, may bias fecundity estimates that rely on the presence of oviducal eggs and should be used in tandem when sampling cowbird populations.

Conclusions

Studies of Brown-headed Cowbird reproduction have distinguished potential fecundity from effective (Hahn et al. 1999) or realized fecundity (Alderson et al. 1999, Woolfenden et al.

2003) because some cowbird eggs are likely ejected by host adults, laid outside of nests, or placed in nests that cannot be monitored. For example, Woolfenden (2003) used genetic analysis to assign eggs found in nests to individual female cowbirds. Realized fecundity was substantially lower than potential fecundity as measured by proportion of birds with oviducal eggs, a result partially due to female cowbirds laying eggs outside of the study area. Results from the present study indicate that potential cowbird fecundity is high in both Florida and Texas populations. Therefore, fecundity is not physiologically limited in north-central Florida and does

not help explain low observed parasitism rates across the host community. However, fecundity may be ecologically limited (reduced realized fecundity) through multiple mechanisms such as egg rejection behaviors (Chapter 2), community interactions (e.g., nest predation), or the abundance and diversity of suitable host species (Chapter 4).

Table 3-1. Summary of means (N, 95% confidence interval) of morphometric data taken from breeding female Brown-headed cowbirds collected in Florida and Texas, 2005−2007 and results of statistical comparisons (independent samples t-tests). Data for wing chord length of Texas birds are taken from Summers et al. (2006). Ovary Mass Oviduct Mass Body Mass (g) Wing Chord (mm) Florida 0.41 1.20 36.0 94.98 (54, 0.35−0.46) (55, 1.11−1.29) (54, 35.14−36.86) (62, 94.82−95.14) Texas 0.39 1.38 36.61 94.95 (74, 0.33−0.45) (73, 1.29−1.46) (59, 35.95−37.26) (959, 94.34−95.56) P-value 0.422 0.023 0.268

Table 3-2. Estimate of clutch size of 2005 Texas cowbirds using number of post-ovulatory follicles (POFs) clearly identified to have a ruptured stigma. Laying birds are those with oviducts containing a developing egg while non-laying birds are individuals in breeding condition (presence of enlarged oviducts, pre-ovulatory follicles, or post- ovulatory follicles) but without an oviducal egg. Number of POFs for 10 non-laying birds without clearly identifiable POFs (−) is estimated using the proportion of POFs in all other non-laying birds (66/29=2.3 POFs/bird). Method derived from Scott & Ankney (1983). Laying Birds Non-laying Birds Number of Est. Clutch Number of No. POFs Birds Total POFs Size Birds Est. POFs 1 2 2 − 8 18 2 13 26 1 8 8 3 20 60 2 7 14 4 5 20 3 12 36 4 2 8 Total 40 108 37 84

Clutch size = [(108 x 2) – 40] + 84 = 3.38 eggs 40 + 37

Table 3-3. Estimate of clutch size of 2006−2007 Florida cowbirds using number of clearly identifiable post-ovulatory follicles (POFs). Number of POFs for non-laying birds without clearly identifiable POFs (−) is based on the proportion of 2.0 (26/23) POFs/bird in all other non-laying birds. Laying Birds Non-laying Birds Certain Number of Est. Clutch Number of Total POFs Est. POFs POFs Birds Size Birds 1 1 1 − 4 8 2 4 8 1 9 9 3 18 54 2 6 12 4 5 20 3 7 21 4 1 4 Total 28 83 27 54

Clutch size = [(83 x 2) – 28] + 54 = 3.49 eggs 28 + 27

Figure 3-1. Location of Brown-headed Cowbird collection sites from April−August, 2005−2007. Site 1 is Fort Hood, Texas, and Site 2 is north-central Florida. Site 1 included collection of female cowbirds in 2005 and Site 2 included cowbird collection in 2006 and 2007.

45 42 40

. 35 2006 30 2007 27 25 21 19 20 17 15

Females Collected 10 6 5 11 0 April May June July

Figure 3-2. Number of female cowbirds collected per month at sites in north-central Florida in 2006 and 2007. Sample sizes are given above each bar.

1.6 1.4 2006 1.2 2007 y Small & Medium Traps 1.0 Large Traps 0.8 0.6

Females/trap/da 0.4 0.2 0.0 April May June July

Figure 3-3. Mean number of female cowbirds collected per trap day at sites in north-central Florida from April to July, 2006−2007. Total trap days were 146 and 463 in 2006 and 2007, respectively. Total trap days were 392 and 217 for small/medium and large traps, respectively. Trap efficiency data presented by trap size includes both 2006 and 2007 samples.

25 Shot 1 20 10 Trapped 10 15

10 20

5 9 6 7 Number of Females Collected 0 April May June July

Figure 3-4. Number of female cowbirds collected per month in Florida by trap and by pellet gun from April−July 2007. Sample sizes are given for each collection method.

0.70 Florida 5 0.60 2 18 Texas 6 7 3 0.50 1 0.40 3 8 8 19 11 0.30 8 Mass (g) 7 2 7 0.20

0.10 2 3 18 21 12 5 34 0.00 4/15 4/29 5/13 5/27 6/10 6/24 7/8 Week Beginning

Figure 3-5. Mean weekly mass (± SE) of ovaries of all Florida and Texas female cowbirds collected between 15 April and 15 July, 2005−2007. Samples sizes for each week are given next to each point.

2.00 1.80 5 (FL) 9 19 1.60 3 6 1 3 1.40 1 18 11 1.20 6 7 4 1.00 2 6 8 Mass (g) 0.80 8 Florida 0.60 5 2 Texas 0.40 18 21 34 0.20 2 0.00 4/15 4/29 5/13 5/27 6/10 6/24 7/8 Week Beginning

Figure 3-6. Mean weekly mass (± SE) of oviducts of all Florida and Texas female cowbirds collected between 15 April and 15 July, 2005−2007. Sample sizes for each week are given next to each point.

0.80 0.70 3 4 0.60 2 6 5 3 0.50 3 2 0.40 1

Mass (g) 0.30 2 4 1 0.20 Laying 5 5 1 0.10 Non-laying 3 0.00 4/15 4/29 5/13 5/27 6/10 6/24 7/8 Week Beginning

Figure 3-7. Mean weekly mass (± SE) of ovaries of breeding Brown-headed Cowbirds collected in Florida, 2006−2007. Laying birds are those with an oviducal egg while non-laying birds are in reproductive condition but lack an oviducal egg. Sample sizes for each collection week are given next to each point.

2.00 1.80 3 4 1.60 63 5 1 1.40 3 2 1.20 1 2 1.00 2

Mass (g) 0.80 4 0.60 5 Laying 3 5 0.40 1 0.20 Non-laying 0.00 4/15 4/29 5/13 5/27 6/10 6/24 7/8 Week Beginning

Figure 3-8. Mean weekly mass (± SE) of oviducts of breeding Brown-headed Cowbirds collected in Florida, 2006−2007. Laying birds are those with an oviducal egg while non-laying birds are in reproductive condition but lack an oviducal egg. Sample sizes for each collection week are given next to each point.

0.70 Laying 8 0.60 6 Non-laying 1 3 8 3 0.50 1 3 2 0.40 8 0.30 2 Mass (g) 0.20 4 6 4 0.10 2 2 11 11 0.00 4/15 4/29 5/13 5/27 6/10 6/24 7/8 Week Beginning

Figure 3-9. Mean weekly mass (± SE) of ovaries of breeding Brown-headed Cowbirds collected in Texas, 2005. Laying birds are those with an oviducal egg while non-laying birds are in reproductive condition but lack an oviducal egg. Sample sizes for each collection week are given next to each point.

2.00 3 1.80 6 10 1.60 1 3 1 4 8 1.40 3 1.20 2 3 8 2 3 1.00 1 6 1

Mass (g) 0.80 10 0.60 Laying 0.40 Non-laying 0.20 0.00 4/15 4/29 5/13 5/27 6/10 6/24 7/8 Week Beginning

Figure 3-10. Mean weekly mass (± SE) of oviducts of breeding Brown-headed Cowbirds collected in Texas, 2005. Laying birds are those with an oviducal egg while non- laying birds are in reproductive condition but lack an oviducal egg. Sample sizes for each week are given next to each point.

45.0

) 40.0

35.0 Female Mass (g 30.0 All Birds Breeding Birds 25.0 4/15 4/29 5/13 5/27 6/10 6/24 7/8 7/22

Figure 3-11. Mass of female Brown-headed Cowbirds collected in Florida between 23 April and 12 July, 2006−2007. Mass of all birds decreased significantly over time (hashed line, R2=0.16, P<0.001, N=110). Mass of breeding birds did not significantly change over the course of the breeding season (solid line, R2=0.024, P=0.263, N=53).

50.0 All Birds 45.0 Breeding Birds )

40.0

35.0 Female Mass (g

30.0

25.0 4/15 4/29 5/13 5/27 6/10 6/24 7/8

Figure 3-12. Mass of female Brown-headed Cowbirds collected in Texas between 19 April and 6 July, 2005. Mass of all birds (hashed line, R2=0.001, P=0.826, N=79) and of breeding birds (solid line, R2<0.001, P=0.990, N=58) did not change significantly over time.

30.0 * 25.0 Texas 40 )

2 Florida 20.0 26 * 15.0 34 *

10.0 25 18 POF (mm Area 3 5.0 22 4 0.0 -1 -2 -3 -4 Collection Day

Figure 3-13. Mean area (± SE) of post-ovulatory follicles (POFs) of Brown-headed Cowbirds collected in Texas and Florida from April to June, 2005−2007. Sample presented includes only birds with oviducal eggs. Day -1 represents POFs associated with oviducal eggs and which released yolk one day prior to being collected. Asterisks indicate comparisons between Florida and Texas that are statistically different (t-tests, P<0.01). Sample sizes are given with each bar.

1 5/5 Florida 0.8 Texas 9/13

. 5/8 9/14 0.6 10/20 7/14 20/39 2/5 0.4 Laying Rate Rate Laying 0.2

0 4/26-5/10 5/11-5/25 5/26-6/9 6/10-End*

Figure 3-14. Estimated laying rate (eggs produced per day) of breeding cowbirds collected in Florida and Texas during four periods of the cowbird breeding season. Laying rate is based on the proportion of reproductively active birds that have oviducal eggs. Proportions are next to each bar. *End of cowbird breeding season for Florida and Texas are 24 June and 26 June, respectively.

CHAPTER 4 ALTERNATIVE HYPOTHESES FOR THE “SLOW” COWBIRD INVASION OF FLORIDA

Introduction

Brood parasitism by Brown-headed Cowbirds is thought to be a significant threat to

particular songbird populations and communities, especially in areas where cowbirds have only

recently invaded. Research on cowbird expansion to new areas has typically focused on effects

of cowbirds on breeding avian species and communities (e.g., Mayfield 1965, Braden et al. 1997,

Strausberger and Ashley 1997, Payne and Payne 1998), but not much attention has been paid to

the reverse. That is, cowbirds have been depicted as “agents of extermination” (Mayfield 1977)

that steamroll into new areas wreaking havoc on native bird populations. Yet, regional and local

dynamics of the host community have the potential to significantly influence patterns of cowbird

range expansion through their effect on reproductive success. Cowbird fecundity, for example,

has been estimated to be extremely high in some regions where an average female lays 40 eggs

per breeding season (Scott and Ankney 1980). Potential fecundity (total number of eggs that are

laid), however, may not reflect actual fecundity (total number of eggs incubated or young

surviving; Alderson et al. 1999, Hahn et al. 1999, Woolfenden et al. 2003). Multiple local factors may reduce effective fecundity and the overall reproductive success of cowbirds. Host adults, for example, may eject cowbird eggs from their nests or abandon parasitized nests, thereby reducing the percent of parasitism detected and the number of cowbird young that are produced.

The arrival of two cowbird species to Florida was thought to herald a conservation crisis

for many species of potential hosts. Shiny Cowbirds may have reached Florida as early as 1971

(Ogden 1971), but were officially recorded in south Florida in the mid-1980s (Smith and Sprunt

1987). By the mid 1990s Shiny Cowbirds were recorded in numerous localities (primarily

coastal) throughout Florida (Cruz et al. 2000). Arrival of the Shiny Cowbird to Florida was of

particular concern because of impacts on songbirds on recently colonized Caribbean islands and

because Shiny Cowbirds are known to colonize cavity nesters that are rarely if ever parasitized

by the Brown-headed Cowbird (Wiley 1985, Wiley et al. 1991). Shiny Cowbirds also often

prefer only one host (Post and Wiley 1977) so that populations of Black-whiskered Vireos (Vireo

altiloquus), Cuban Yellow Warblers and Florida Prairie Warblers confined to south Florida were

thought to be at risk to extinction through brood parasitism (Atherton and Atherton 1988). So

far, however, these dire predictions have not come to pass and there are only two cases

suggesting breeding by Shiny Cowbirds in Florida (Smith and Sprunt 1987, see Chapter 3).

Brown-headed Cowbirds were first documented breeding in Florida in the mid-1950s (Monroe

1957) and have since spread through most of the state where they are found at moderate levels of

abundance relative to other regions of North America (Sauer et al. 2006). Surveys of Brown-

headed Cowbird parasitism of host nests and family groups feeding fledglings show unusually

low frequencies both for individual species and for the entire community of potential hosts in

north-central Florida (Chapter 1).

In previous chapters of this dissertation, I explored hypotheses that might explain the

seemingly slow invasion of Florida by cowbirds. First, I tested the hypothesis that the low frequency of Brown-headed Cowbird parasitism observed in north-central Florida resulted from host defenses, i.e., cowbird eggs being removed by adults or nests being abandoned before the active nest could be monitored (Chapter 2). I monitored over 1100 nests of 32 potential host species and noted no cases of natural cowbird egg ejection. Furthermore, parasitism experiments revealed that parasitism rejection behaviors do not appear to be prevalent in many common

Florida hosts and are certainly no more prevalent in Florida than in areas where cowbird invasion

proceeded more rapidly (Rothstein 1994). Second, reproductive assays of female Brown-headed

Cowbirds breeding in north-central Florida revealed that potential fecundity is very high, with an

average female laying 35 eggs per breeding season (Chapter 3). Therefore, these hypothesized

limitations to cowbird reproduction do not appear to have influenced the observed rates of

cowbird parasitism in the region. In this chapter, I explore two more hypotheses that may

explain either observed low rates of parasitism in north-central Florida or the seemingly slow

invasion of Florida by cowbirds: host community composition and nest predation. The first

hypothesis is that the community composition of hosts in Florida slows invasion because they are

dominated by poor cowbird hosts that either have defenses against parasitism or are poor at

raising cowbird young. I test this hypothesis by comparing community composition of hosts in

areas where cowbirds have been successful and in Florida where the invasion is ongoing. The

second hypothesis is that host communities in Florida are themselves less productive than those

elsewhere in the cowbirds’ range as a result of high rates of nest predation. I test these data

using simple models for source-sink dynamics and data from two regions where data exist on cowbird productivity and host nest predation rates.

Host Community Composition

The occurrence and abundance of Brown-headed Cowbirds over local landscape scales has

been shown to be influenced by several factors, including the diversity (Chace 2004, Purcell

2006), abundance, and population density of hosts (Johnson and Temple 1990, Robinson and

Wilcove 1994, Tewksbury et al. 1999, Jensen and Cully 2005a, but see Evans and Gates 1997).

Abundance of Brown-headed Cowbirds in three vegetation types in western Montana, for

example, was positively correlated with the density of potential hosts (Tewksbury et al. 1999,

Robinson et al. 2000). Levels of parasitism may also be influenced by the availability of suitable

host species (Barber and Martin 1997, Brodhead et al. 2007, but see Jensen and Cully 2005b).

Parasitism of Black-capped Vireos, for example, was lower in areas with lower cumulative

density of four other potential host species (Barber and Martin 1997). Therefore, cowbird

abundance and distribution may be affected by the distribution and abundance of host species at

local scales.

Patterns of cowbird and host occurrence are more difficult to discern at larger (e.g.,

regional) scales because many factors can affect species distributions. Research has shown a

strong trend of diminishing faunal diversity (Means and Simberloff 1987) and density (Emlen

1978) from base to tip along Florida’s peninsula. Several mechanisms have been proposed to

explain this general pattern, including changes in immigration and extinction rates (Cox 2006),

past climate change and sea-level fluctuations (Robertson and Kushlan 1974), and differences in

availability of different habitats (Emlen 1978, Wamer 1978) that would provide suitable

resources. One likely limiting factor on cowbird reproductive success should be the availability

of suitable hosts to raise cowbird young. Breeding Brown-headed Cowbirds are distributed

throughout Florida but are primarily located in the northern third of the state (Figure 4-1). Of the

ca. 50 host species known to breed in Florida that have raised cowbird young (Table 4-1), only

19 species have statewide distributions whereas 28 of the remaining 31 species are either found only in north Florida (Cox 2006) or have distributions primarily in the northern half of the state

(FFWCC 2007).

Taken together, these data suggest that the spread of the cowbird into south Florida may be

limited by the availability of quality hosts. To test this hypothesis, I categorized species breeding in Florida into one three categories based on their quality as cowbird hosts (Table 4-1) based on criteria in Grzybowski and Pease (1999), Robinson et al. (unpubl. data) and data from the literature. High quality host species are those that are often highly parasitized elsewhere and

that raise cowbird young to fledging. Marginal quality species are those that accept cowbird

eggs but are rarely parasitized and raise few cowbird young. Poor quality hosts represent cowbird egg sinks due to rejection of parasitism or life-history traits (e.g., nestling diet) incompatible with brood parasitism. In Florida, the majority of species that are distributed statewide are abundant but are probably marginal hosts (e.g., Northern Cardinal), or poor hosts

(e.g., Mourning Dove, Eastern Meadowlark) for a variety of reasons (Table 4-1). On the other hand, cowbird distribution roughly coincides with the highest diversity and abundance of good quality hosts (see Figure 1). These data are generally consistent with the hypothesis that cowbirds are most abundant in areas where good hosts are also found.

If Brown-headed Cowbirds are located in areas in Florida with the highest diversity and

abundance of suitable hosts, why are observed parasitism rates in north-central Florida so low?

To qualitatively address whether the composition of the host community contributes to this

pattern, I examined Breeding Bird Survey (BBS) data from 1997−2006 at routes within 75km of

three study areas where parasitism rates have been recorded. Study areas included (1) north-

central Florida (city of Gainesville, N=10 routes), (2) southwestern South Carolina (Savannah

River Site near Aiken, N=8), and (3) southern Illinois (city of Anna, N=7). BBS routes in each

region incorporated numerous habitat types but primarily forest, pasture, row crops, grassland

and wetland habitats (Sauer et al. 2006). Routes for southern Illinois included more pasture and

row crops (59%) on average than routes near Florida (14%) and South Carolina sites (37%).

Illinois routes included more deciduous forest (21%) than South Carolina (12%) and Florida

(0%), but routes in these latter areas also included significant evergreen forest (24−38%).

Documented rates of parasitism are lowest in Florida (Chapter 1), low to moderate in South

Carolina (Whitehead et al. 2002, Kilgo and Moorman 2003), and high in Illinois (Robinson

1992, Strausberger and Ashley 1997, Hoover 2003). Species breeding in each study area were categorized as good, marginal, or poor based on their suitability as cowbird hosts. Total abundance of species in each host category was compared among study areas using one-way

ANOVA and Dunnett’s post-hoc comparisons assuming unequal variances. As in Gryzbowksi and Pease (1999), I excluded species that are known to be cowbird hosts, but have various traits that may limit parasitism of their nests (e.g., parids, bluebirds, doves).

In both Florida and South Carolina, the mean abundance of good quality hosts constitutes a relatively higher proportion of the overall bird abundance than in Illinois (Figure 4-2). This may suggest that Florida has a high availability of suitable hosts. However, the diversity of all potential hosts is 91 and 121% higher in South Carolina and Illinois, where there are approximately two times as many quality host species as in north-central Florida (Figure 4-3).

Furthermore, there were significant differences among study areas in the abundances of good

(F=33.73, P<0.001), marginal (F=391.26, P<0.001), poor (F=160.25, P<0.001), and all

(F=341.52, P<0.001) host species. The abundances of all three classes of hosts as well as total host abundance are significantly lower in Florida than in both other regions (Dunnett’s post-hoc comparisons, P<0.001; Figure 4-4). Kilgo and Moorman (2003) reported low-to-moderate rates of nest parasitism for 24 species breeding in the southeastern coastal plain of South Carolina and

Georgia. Among the species with the highest average parasitism rates were Prairie Warbler

(13.6%), Hooded Warbler (14.4%), Yellow-breasted Chat (18.6%), Blue Grosbeak (18.8%) and

Indigo Bunting (16.5%). Relative abundances of these species are between 1.6 and 6.4 times higher in South Carolina than in Florida, where most of them reach their southern range limit

(Sauer et al. 2006).

The Breeding Bird Survey (BBS) data summarized above are based on surveys conducted

primarily along roadsides, which may not adequately sample forested habitat (Bart et al. 1995,

Betts et al. 2007) that cowbirds may prefer (Hahn and Hatfield 1995). As a result, BBS surveys

may not adequately represent forest-interior songbirds that may be preferred hosts of cowbirds. I

compared census data obtained for forested habitats in Illinois (Robinson and Robinson 1999)

and Florida (S.K. Robinson unpubl. data) to better estimate differences in abundance of quality

forest hosts between regions. Illinois censuses were conducted in the Trail of Tears State Forest

from May−July, 1989−1992 and Florida censuses were conducted at various locations primarily

within 100km of Gainesville from May−July in 2004 (Stracey and Robinson 2008). Abundance

data obtained from 50-m radius point counts in Illinois were transformed to compare with 100-m radius count data obtained in Florida. Only data from species considered quality cowbird hosts

were included.

The abundance of quality forest hosts was much higher in Illinois than in Florida.

Abundance of quality canopy and understory hosts was 5.2 and 3.2 times higher in Illinois than

Florida, respectively (Figure 4-5) although the number of species in each host type was similar between regions. Rates of parasitism in these same Illinois forests are very high (Robinson and

Robinson 2001) and have more than 10 times as many cowbirds as Florida forests (Figure 4-5).

Cowbird numbers, however, may not be solely related to the overall abundance of the host community but may depend primarily on abundances of a small number of common species that act as good hosts (Rothstein and Robinson 2000). As a result, monitoring of high quality host species with abundant Florida populations such as Red-eyed Vireo, Northern Parula and Pine

Warbler may be needed to determine how they are contributing to population growth of cowbirds

in Florida. Thus, BBS data and forest census data, in particular, suggest that quality hosts are

actually less abundant in Florida than they are in areas where cowbirds invaded long ago and

where cowbird populations have stabilized or even declined.

Based on these crude and correlative analyses, Brown-headed Cowbirds breeding in

Florida may suffer from a reduced availability of hosts. Lack of suitable hosts may in turn

reduce effective fecundity of cowbirds. If cowbirds are unable to locate suitable hosts when eggs

are ready to be laid, females may lay eggs outside of nests or in those of inappropriate hosts,

which are often not monitored in studies of brood parasitism. From 2004−2006, I monitored

approximately 1100 nests of 35 potential host species breeding in Florida (Chapter 1), many of

which are inappropriate hosts because of life-history traits incompatible with cowbird parasitism

(e.g., Mourning Dove, Great-crested Flycatcher, Blue Jay). Of the limited cases of nest

parasitism observed in this study, however, none included nests of these inappropriate hosts. If

nests of preferred hosts are not available, females may also resorb the developing egg before it

enters the oviduct (Payne 1998). Indeed, Payne (1965) suggested that atresia of developing

ovarian follicles may be due to a shortage of suitable nests, although captive cowbirds do not

appear to limit laying based on availability of artificial nests (Holford and Roby 1993). Finally,

Hahn et al. (1999) suggested that a female cowbird unable to find an appropriate nest for her egg

may consume it to retain important nutrients.

Finally, host suitability is based on multiple factors such as nestling diet, breeding

phenology, nest-site choice, nest defense and rejection behavior. While many of these traits may

be retained among populations over their entire distribution, regional differences in the

expression of these traits may alter the suitability of some populations as cowbird hosts. For example, some populations of Warbling Vireos (Vireo gilvus) accept cowbird parasitism of their nests whereas females in other populations puncture-eject cowbird eggs (Sealy 1996). Data for

Florida, however, are lacking for estimating reproductive parameters influencing cowbird population growth (e.g., cowbird fledging success, proportion of eggs accepted).

Nest Predation

Brood parasitism and nest predation are the two primary causes for reduced reproduction in many songbird populations (Mayfield 1977, Wilcove 1985, Martin 1992, Robinson et al.

1995, Arcese and Smith 1999). Nest predation, in particular, can account for >90% of nest losses in some populations also exposed to brood parasitism (Møller 1988, Hoover et al. 1995,

Martin 1998) and significantly limit annual reproductive success of songbird populations

(Robinson 1992, Pease and Grzybowski 1995). Schmidt & Whelan (1999) developed a model to examine the relative contributions of nest predation and parasitism to songbird seasonal fecundity. Results indicated that nest predation had a greater influence on reproductive success than parasitism. Furthermore, high rates of nest predation under some circumstances could lead to sink populations even with re-nesting and double-brooding. The authors suggest that management practices that reduce nest predation may be more important in some populations than management of brood parasitism through cowbird control.

While effects of nest predation (and parasitism) on host communities have been commonly studied, less attention has been paid to general effects of nest predation on reproduction of cowbirds. Winfree (2004) examined the possible effects of host community and habitat type on the population growth of Brown-headed Cowbirds breeding in southern Illinois. Cowbird offspring survival rates were 2−3 times lower in old fields than in forested habitats, the latter of which represents a more recently invaded habitat type and are characterized by generally lower rates of nest predation. These results suggest that invasion success of Brown-headed Cowbirds may be facilitated by habitat types that reduce nest predation pressures that could slow cowbird

population growth. Winfree et al. (2006) even speculated that high nest predation rates in the

scrubbier habitats that cover much of Florida may be slowing their rate of spread because such

habitats tend to be associated with very high rates of nest predation.

Nest failure rates of select understory nesting species breeding in north-central Florida are

very high. For example, Mayfield daily survival rates for Northern Cardinal and Northern

Mockingbird in north-central Florida are as low as 0.9187 (Reetz et al. unpubl. data) and 0.8932

(C.M. Stracey pers. comm.), respectively. Host populations may compensate for high nest

failure rates through re-nesting or multiple broods (Pease and Grzybowski 1995) over long breeding seasons. Indeed, both Northern Cardinals and Northern Mockingbirds have very short re-nesting intervals following nest failure (Zaias and Breitwisch 1989, Filliater et al. 1994) and begin breeding as early as February (M. Reetz pers. obs), which may alleviate high predation pressures on these species. Because cowbirds only breed for a part of these long seasons, however, high rates of re-nesting may not actually increase the availability of hosts to parasitize

(e.g., Morrison and Bolger 2002).

By modifying a well-known source-sink equation (Pulliam 1988: Equation 5a), Winfree

(2004) calculated upper and lower values (Tcrit) for egg-to-fledgling transition success of

cowbirds that would be required for a stable population. Values derived were based on published data for adult female cowbird survival (Woolfenden et al. 2001), juvenile survival

(Ricklefs 1973, Greenberg 1980, Smith et al. 2002), and female cowbird fecundity (Scott and

Ankney 1980, Woolfenden et al. 2003). Lower and upper bounds of cowbird egg-to-fledgling transition success needed to achieve cowbird replacement were calculated as 0.03−0.22. In north-central Florida, an average female cowbird is estimated to produce 34 eggs per season

(Chapter 3) resulting in modified Tcrit values of 0.04−0.11. To generate T values derived from

field data, the following equation can be used (Winfree 2004), T = U x V x W, where U is the probability of egg acceptance by the host, V is the probability of survival over the nesting interval, and W is the probability that the cowbird will survive to fledging in a successful nest.

Based on this equation, Winfree determined that certain habitats and species generate T values below that needed to sustain cowbird populations. Using field data on cowbird egg acceptance

(Chapter 2) and nest interval survival rates (Reetz et al. unpubl. data, C.M. Stracey pers. comm.) for three species breeding in Florida, values for U and V would be 0.85 and 0.20 for Northern

Mockingbird, 0.12 and 0.16 for Brown Thrasher, and 0.95 and 0.15 for Northern Cardinal.

Assuming an average V value of 0.79 based on those determined by Winfree, T estimates of cowbird egg-to-fledgling transition success for these three species in Florida are 0.13, 0.02, and

0.11. Two of these values are near the higher estimate of Tcrit (0.11) needed to sustain cowbird populations and one (Brown Thrasher) is below the lower estimate (0.04). Thus, even with liberal estimates of some of these values, these hosts could not sustain cowbird populations.

These species, however, are probably not the most suitable hosts in the songbird community in north-central Florida. Indeed, a large sample of nests (N=826) of these three species was monitored with only six cases of parasitism recorded. Nevertheless, the rate of nest parasitism over the entire sample of species monitored was also low. It is possible that parasitism frequency was affected by nest predation occurring prior to monitoring of nests. This would require partial nest predation where the cowbird egg(s) were removed and one or more host eggs remained to be found and monitored, an unlikely event as there were only a few cases of partial clutch loss in all unparasitized nests monitored.

The most heavily parasitized species in northern Florida are likely those that nest in the forest canopy. Family groups of these species were far more likely to contain a cowbird young

than family groups of understory species. Percent of family groups of canopy nesting species with cowbird young were 39% for Blue-gray Gnatcatcher and 33% for Northern Parula, which

would be considered moderate as parasitism rates of nests. These estimates of parasitism

frequency are based on the assumption of complete survival of cowbird young in nests that

fledge host young, which is very unlikely. If we use the same V value (cowbird survival in a

successful nest) of 0.79 for canopy species and assume cowbirds lay one egg per nest, as many

as 49% and 42% of nests of these two species may be parasitized. Furthermore, cowbird parasitism may facilitate or increase nest predation (Dearborn 1999, Burhans et al. 2002, Zanette et al. 2007) perhaps due to loud begging vocalizations of cowbird young. This would result in family group parasitism rates that also underestimate actual parasitism of nests. Therefore, rates of nest parasitism on midstory and canopy nesting species may be higher than family group surveys indicate.

If Brown-headed Cowbirds are preferentially laying the majority of their eggs in nests of preferred host species that are difficult to find or monitor (e.g., canopy nesting species), rates of nest failure combined with behaviors that reduce the number of eggs that are incubated (e.g., abandonment) in these species would be more important in affecting overall cowbird production.

For example, Red-eyed Vireos are common in Florida (Sauer et al. 2006) and have been recorded to abandon 34% of parasitized nests (Graber et al. 1985). If the probability of cowbird egg acceptance (U) is 0.66 and cowbird survival in a successful nest (V) is 0.81 (Winfree 2004), the interval survival rate would need to be 0.21 (daily survival of 0.937) to result in net positive cowbird reproduction in Florida (Tcrti=0.11). This rate is lower than most data from the literature

for this species (daily survival of 0.932-0.963: Burke and Nol 2000, Duguay et al. 2001, Winfree

2004), which suggests that they may be a “source” host where they are found in sufficient

numbers. Despite logistical difficulties in collecting the data, comprehensive nest surveys of midstory and canopy nesting species are required to determine if both parasitism frequencies and rates of nest failure. Some evidence also suggests that variation in cowbird parasitism is negatively correlated with interspecific differences in nest predation, indicating that the probability of nest failure can influence host choice by cowbirds (Avilés et al. 2006). High nest- loss rates in the limited subset of preferred Florida host species (as well as across the community of alternative lower quality hosts), however, would primarily limit their capacity to act as cowbird “generators” and would partially explain why Brown-headed Cowbirds have not invaded Florida as successfully as they have other parts of North America. Alternatively, the abundance of ideal “source” hosts, those that accept cowbird eggs and do not suffer high rates of nest predation, may also determine the rate at which cowbirds can spread.

Figure 4-1. Distribution of Brown-headed Cowbirds in Florida. Redrawn from Breeding Bird Survey abundance map (1994−2003). Legend indicates count of birds per route.

Figure 4-2. Proportion of total abundance composed of species of varying quality as cowbird hosts. Classifications roughly based on criteria in Grzybowski and Pease (1999). Data are summarized for Breeding Bird Survey routes within 75km of study sites in three regions.

60 Florida 50 South Carolina Illinois 40

No. Species No. 20

0 Good Marginal Poor All

Figure 4-3. Total number of host species of three types at locations in three regions.

600 Florida C 500 South Carolina . 400 Illinois B 300 C

200 A B B Total Abundance Total A B C 100 A B A 0 Good Marginal Poor All

Figure 4-4. Mean (± SE) abundance of three types of Brown-headed Cowbird hosts in three regions. Bars within each host category that do not share a letter are significantly different using average abundance values from 1997−2006 (One-way ANOVA, Dunnett’s post-hoc test assuming unequal variances, P<0.05).

14 11 12 Illinois Florida . 10

6 6 Birds per count count per Birds 4 8 2 6

0 Canopy Understory Cowbird

Figure 4-5. Mean (±SD) abundance of forest species that represent good quality cowbird hosts in Illinois and Florida. Birds per count represents abundance of birds within 100-m radius point counts. Values above bars indicate number of species recorded for each host type. Data are from Robinson and Robinson (2000) for Illinois, and from unpublished data (S.K. Robinson). Cowbird abundance includes individuals of both sexes.

Table 4-1. Host species documented as breeders in Florida (FFWCC 2007) also known to have fledged Brown-headed Cowbirds. Species are arranged in descending order of relative abundance (Sauer et al. 2006). Florida distribution category based on Cox (2006) and Florida Breeding Bird Atlas (FFWCC 2007). Abbreviations indicate that species occur, SW: Statewide, N: only in northern half, S: only in southern half, PN: primarily in northern half. Suitability is based on criteria in Grzybowski and Pease (1999) and Robinson et al. (unpubl. data). G: good quality hosts; M: marginal hosts, P: poor hosts. Florida Relative Common Name Scientific Name Distribution Suitability Abundance Red-winged Blackbird Agelaius phoeniceus SW M 49.94 Northern Mockingbird Mimus polyglottos SW P 49.11 Northern Cardinal Cardinalis cardinalis SW M 44.03 Mourning Dove Zenaida macroura SW P 38.74 Eastern Towhee Pipilo erythrophthalmus alleni SW G 30.93 Carolina Wren Thryothorus ludovicianus SW P 25.49 Eastern Meadowlark Sturnella magna SW P 24.04 Common Yellowthroat Geothylpis trichas SW G 13.18 White-eyed Vireo Vireo griseus SW G 12.36 European Starling Sturnus vulgaris SW P 11.64 House Sparrow Passer domesticus SW P 9.74 Northern Parula Parula americana PN G 6.40 Pine Warbler Dendroica pinus SW G 6.37 Loggerhead Shrike Lanius ludovicianus SW P 4.60 Brown Thrasher Toxostoma rufum SW P 4.08 Eurasian Collared Dove Streptopelia decaocto SW P 4.07 Summer Tanager Piranga rubra PN G 3.92 Eastern Bluebird Sialia sialis SW P 3.76 Carolina Chickadee Poecile carolinensis N P 3.52 Indigo Bunting Passerina cyanea N G 3.42 Blue Grosbeak Guiraca caerulea N GM 3.26 Orchard Oriole Icterus spurious N G 2.99 Blue-gray Gnatcatcher Polioptila caerulea SW GM 2.91 Eastern Kingbird Tyrannus tyrannus SW P 2.79 Yellow-breasted Chat Icteria virens N G 2.35

100

Table 4-1. Continued. Florida Relative Common Name Scientific Name Distribution Suitability Abundance Red-eyed Vireo Vireo olivaceus PN G 1.82 Bachman’s Sparrow Aimophila aestivalis SW GM 1.62 Wood Thrush Hylocichla mustelina N G 1.39 Prothonotary Warbler Protonotaria citrea PN G 1.30 Yellow-throated Vireo Vireo flavifrons N G 1.19 Barn Swallow Hirundo rustica PN P 1.16 House Finch Carpodacus mexicanus PN P 0.91 Hooded Warbler Wilsonia citrina N G 0.85 Florida Prairie Warbler Dendroica discolor paludicola S GM 0.84 Acadian Flycatcher Empidonax virescens N G 0.73 Yellow-throated Warbler Dendroica dominica N G 0.48 Eastern Wood-Pewee Contopus virens N G 0.40 Field Sparrow Spizella pusilla N M 0.27 101 Painted Bunting Passerina ciris N G 0.17 Kentucky Warbler Oporornis formosus N G 0.13 Swainson’s Warbler Limnothylpis swainsonii N G 0.10 Gray Catbird Dumetella carolinensis PN P 0.04 Yellow Warbler Dendroica petechia S GP 0.01 American Robin Turdus migratorius N P <0.01 Eastern Phoebe Sayornis phoebe N G <0.01 Louisiana Waterthrush Seiurus motacilla N G <0.01 Worm-eating Warbler Helmitheros vermivorus N G <0.01 American Redstart Setophaga ruticilla N G <0.01 Grasshopper Sparrow Ammodramus savannarum S G <0.01 Chipping Sparrow Spizella passerina N GM <0.01

APPENDIX A STUDY SITE DESCRIPTION AND HOST SPECIES SURVEYED

These tables include general descriptions of study sites (Table A-1) and the number of

nests and family groups of host species monitored at each site (Table A-2) during Brown-headed

Cowbird parasitism surveys in north-central Florida from March-August, 2004-2006.

Approximate area (hectares) of each site includes only locations where field work was conducted. Coordinates represent the approximate center of each field site. Minimum and maximum distances (meters) from nests at each site to cowbird feeding areas (livestock pastures) were determined using plant community and landcover GIS data (Kautz et al. 1993) and satellite imagery. Distance values in parentheses are to urban areas that provide cowbird feeding opportunities. Gainesville Public Areas include city parks, cemeteries, and parking lots and

Gainesville Suburban Areas include private homes and neighborhoods in the city of Gainesville,

Florida. General habitat categories where nests were located at each site were determined using

GIS vegetation data. Habitat categories generally follow Kautz et al. (2007) and are 1 = Scrub, 2

= Sandhill, 3 = Dry Prairie, 4 = Upland Forest, 5 = Pineland, 6 = Freshwater Marsh, 7 = Shrub

Swamp, 8 = Forested Wetland, 9 = Shrub and Brushland, 10 = Grassland or Pasture, 11 =

Disturbed/Transitional, 12 = Urban/Developed.

102

Table A-1. Description of study sites surveyed for cowbird parasitism in north-central Florida, 2004-2006. Minimum Maximum Site Area Latitude Longitude Distance Distance Habitats Bolen Bluff State Park 160 29°31’49”N 82°17’02”W 200 1000 4, 7, 10 Gainesville Public Areas 400 29°39’07”N 82°20’21”W (0) (50-100) 4, 5, 9, 10, 12 Gainesville Suburban Areas 300 29°39’07”N 82°20’21”W (0) (50-100) 4, 5, 12 Gum Root Nature Park 124 29°40’48”N 82°14’18”W <100 650 4, 5, 6, 8 Loblolly Woods Nature Park 14 29°39’34”N 82°21’53”W <100 500 4, 5, 6, 8 Myakka River State Park 1880 27°14’37”N 82°15’48” W 200 4500 1, 3, 4, 5, 6, 7, 8 Newnan’s Lake 44 29°37’23”N 82°15’01”W 900 1400 6, 7, 8 Ordway-Swisher Biological Station 1400 29°41’57”N 81°58’32”W 1500 4400 2, 4, 5, 6, 7, 8, 9, 10, 11 Payne’s Prairie State Preserve 1000 29°31’49”N 82°17’02”W <100 1400 4, 5, 6, 7, 8, 10 San Felasco Hammock State Park 3800 29°44’00”N 82°27’06”W 200 2200 2, 4, 5, 6, 8, 9, 10 103 Split Rock/Sugarfoot Nature Preserves 122 29°37’57”N 82°24’16”W 800 1500 4, 5, 6, 7, 8 University of Florida Campus 670 29°38’47”N 82°20’45”W (50) (200) 4, 5, 6, 8, 9, 10, 12 USDA National Wildlife Research Center 11 29°39’13”N 82°17’14”W (<50) (250) 4, 5, 9

Table A-2. Number of nests and family groups of host species monitored at study sites in north-central Florida, 2004-2006. Nests Families Study Site Species Total Nests Parasitized Total Families Parasitized Bolen Bluff State Park White-eyed Vireo 1 − 2 − Blue-Gray Gnatcatcher − − 1 − Brown Thrasher 1 − − − Northern Parula − − 5 3 Eastern Towhee 1 − − − Northern Cardinal 26 1 14 − Gainesville Public Areas Mourning Dove 2 − − − Loggerhead Shrike 4 − 1 − Carolina Wren − − 1 − Eastern Bluebird 2 − 1 − 104 Northern Mockingbird 113 − 6 − Brown Thrasher 13 − 5 − Northern Cardinal 9 − 6 − Gainesville Suburban Areas Mourning Dove 8 − − − Eastern Tufted Titmouse 1 − − − Carolina Wren 2 − − − Northern Mockingbird 131 − 14 − Brown Thrasher 8 − 1 − Northern Parula − − 1 1 Summer Tanager − − 2 2 Northern Cardinal 5 − 12 1 Gum Root Nature Park White-eyed Vireo 1 − − − Red-eyed Vireo − − 1 −

Table A-2. Continued. Nests Families Study Site Species Total Nests Parasitized Total Families Parasitized Northern Mockingbird 1 − − − Brown Thrasher 1 − − − Northern Cardinal 5 − − − Loblolly Woods Nature Park Carolina Wren 1 − 1 − Northern Parula − − 1 − Northern Cardinal 4 − 1 − Myakka River State Park Mourning Dove 19 − − − Great-crested Flycatcher 12 − − − Blue Jay 8 − − − Eastern Bluebird 7 − − − 105 Northern Mockingbird 18 − − − Common Yellowthroat 2 1 − − Eastern Towhee 43 3 − − Bachman’s Sparrow 11 1 − − Eastern Meadowlark 3 − − − Newnan’s Lake Red-winged Blackbird 61 − − − Ordway-Swisher Biological Station Mourning Dove 2 − − − White-eyed Vireo 4 − 8 − Yellow-throated Vireo − − 5 − Blue Jay 1 − − − Eastern Tufted Titmouse 1 − − − Blue-Gray Gnatcatcher 2 − 5 1 Eastern Bluebird 42 − 1 −

Table A-2. Continued. Nests Families Study Site Species Total Nests Parasitized Total Families Parasitized Northern Mockingbird 1 − − − Brown Thrasher 6 − − − Northern Parula − − 4 − Pine Warbler 3 − 13 1 Common Yellowthroat − − 1 − Summer Tanager − − 3 − Eastern Towhee 6 − 1 − Northern Cardinal 32 − 8 − Payne’s Prairie State Preserve Mourning Dove 3 − − − White-eyed Vireo 3 − 7 − 106 Carolina Chickadee − − 1 − Carolina Wren − − 3 − Blue-gray Gnatcatcher 2 1 3 1 Brown Thrasher 5 − − − Northern Parula − − 8 3 Pine Warbler − − 1 − Prothonotary Warbler 1 − − − Common Yellowthroat 1 − − − Summer Tanager 1 − 1 − Eastern Towhee 5 − 2 − Northern Cardinal 20 − 14 − Blue Grosbeak 2 − − − Indigo Bunting 1 − − −

Table A-2. Continued. Nests Families Study Site Species Total Nests Parasitized Total Families Parasitized Red-winged Blackbird 9 − − − Orchard Oriole − − 1 − San Felasco Hammock State Park Mourning Dove 2 − − − Acadian Flycatcher 14 − 9 − White-eyed Vireo 4 2 1 − Red-eyed Vireo − − 3 1 Carolina Wren 2 − 2 − Blue-gray Gnatcatcher − − 1 − Eastern Bluebird − − 1 − Wood Thrush 3 − − − 107 Northern Parula − − 3 − Pine Warbler − − 1 − Hooded Warbler 5 1 7 − Summer Tanager − − 2 1 Eastern Towhee 1 − − − Northern Cardinal 27 − 11 − Blue Grosbeak 2 − − − Common Grackle 1 − − − Split Rock/Sugarfoot Preserves Mourning Dove 1 − − − White-eyed Vireo 5 − 7 − Yellow-throated Vireo 1 1 − − Red-eyed Vireo − − 1 − Carolina Wren 2 − 4 −

Table A-2. Continued. Nests Families Study Site Species Total Nests Parasitized Total Families Parasitized Blue-gray Gnatcatcher 1 − 2 1 Northern Mockingbird 1 − − − Northern Parula − − 3 − Common Yellowthroat − − 1 − Yellow-breasted Chat 1 − − − Northern Cardinal 64 1 14 − Blue Grosbeak 3 1 − − Indigo Bunting 1 − 3 − University of Florida Campus Eurasian Collared Dove 3 − − − Mourning Dove 14 − − − 108 Loggerhead Shrike 1 − − − White-eyed Vireo 1 − − − Carolina Wren 2 − 1 − Northern Mockingbird 130 − 5 − Brown Thrasher 41 − − − Northern Cardinal 44 4 11 1 Red-winged Blackbird 11 − − − Common Grackle 1 − − − Boat-tailed Grackle 2 − − − Orchard Oriole 2 − 2 − USDA Wildlife Research Center Mourning Dove 1 − − − Carolina Wren 1 − − − Northern Mockingbird 3 1 3 −

Table A-2. Continued. Nests Families Study Site Species Total Nests Parasitized Total Families Parasitized Brown Thrasher 5 − 1 − Eastern Towhee 1 − 9 − Northern Cardinal 7 − 4 −

109

LITERATURE CITED

Abrahamson, W. G., and D. C. Hartnett. 1990. Pine Flatwoods and Dry Prairies. Pages 103-149 in R. L. Myers and J. J. Ewel, editors. Ecosystems of Florida. University of Central Florida Press, Orlando.

Alderson, G. W., H. L. Gibbs, and S. G. Sealy. 1999. Determining the reproductive behaviour of individual Brown-headed Cowbirds using microsatellite DNA markers. Animal Behaviour 58:895-905.

Ankney, C. D., and D. M. Scott. 1980. Changes in nutrient reserves and diet of breeding Brown- headed Cowbirds. Auk 97:684-696.

Arcese, P., and J. N. M. Smith. 1988. Effects of population density and supplemental food on reproduction in Song Sparrows. Journal of Animal Ecology 57:119-136.

Arcese, P., and J. N. M. Smith. 1999. Impacts of nest depredation and brood parasitism on the productivity of North American passerines. Pages 2953-2966 in N. J. Adams and R. H. Slotow, editors. Proceedings of the 22nd International Ornithological Congress. University of Natal, Durban.

Arcese, P., J. N. M. Smith, and M. I. Hatch. 1996. Nest predation by cowbirds and its consequences for passerine demography. Proceedings of the National Academy of Sciences, USA 93:4608-4611.

Arguedas, N., and P. G. Parker. 2000. Seasonal migration and genetic population structure in House Wrens. Condor 102:517-528.

Arnold, T. W., J. E. Thompson, and C. D. Ankney. 1997. Using post-ovulatory follicles to determine laying histories of American Coots: implications for nutrient-reserve studies. Journal of Field Ornithology 68:19-25.

Ashmole, N. P. 1963. The regulation of numbers of tropical oceanic birds. Ibis 105:458-473.

Atherton, L. S., and B. H. Atherton. 1988. Florida region. American Birds 42:60-63.

Avilés, J. M., B. G. Stokke, and D. Parejo. 2006. Relationship between nest predation suffered by hosts and Brown-headed Cowbird parasitism: a comparative study. Evolutionary Ecology 20:97-111.

Badyaev, A. V., and C. K. Ghalambor. 2001. Evolution of life histories along elevational gradients: trade-off between parental care and fecundity. Ecology 82:2948-2960.

Baker, J. R. 1939. The relation between latitude and breeding season in birds. Proceedings of the Zoological Society of London 109A:557-582.

Baltz, M. E., and D. E. Burhans. 1998. Rejection of artificial parasite eggs by Gray Kingbirds in the Bahamas. Condor 100:566-568.

110

Barber, D. R., and T. E. Martin. 1997. Influence of alternate host densities on Brown-headed Cowbird parasitism rates in Black-capped Vireos. Condorr 95:595-604.

Barclay, R. M. R. 1994. Constraints on reproduction by flying vertebrates: energy and calcium. American Naturalist 144:1021-1031.

Bart, J., M. Hofschen, and B. G. Peterjohn. 1995. Reliability of the breeding bird survey: effects of restricting surveys to roads. Auk 112:758-761.

Bazin, R. C., and S. G. Sealy. 1993. Experiments on the responses of a rejector species to threats of predation and cowbird parasitism. Ethology 94:326-338.

Beezley, J. A., and J. P. Rieger. 1987. Least Bell's Vireo management by cowbird trapping. Western Birds 18:55-61.

Bell, C. P. 1996. The relationship between geographic variation in clutch size and migration pattern in the Yellow Wagtail. Bird Study 43:333-341.

Betts, M. G., D. Mitchell, A. W. Dlamond, and J. Bety. 2007. Uneven rates of landscape change as a source of bias in roadside wildlife surveys. Journal of Wildlife Management 71:2266-2273.

Bonifait, S., M.-A. Villard, and D. Paulin. 2006. An index of reproductive activity provides an accurate estimate of the reproductive success of Palm Warblers. Journal of Field Ornithology 77:302-309.

Boutin, S. 1990. Food supplementation experiments with terrestrial vertebrates: patterns, problems, and the future. Canadian Journal of Zoology 68:203-220.

Braden, G. T., R. L. McKernan, and S. M. Powell. 1997. Effects of nest parasitism by the Brown-headed Cowbird on nesting success of the California Gnatcatcher. Condor 99:858-865.

Breitwisch, R. 1988. Sex differences in defense of eggs and nestlings by Northern Mockingbirds, Mimus polyglottos. Animal Behaviour 36:62-72.

Briskie, J., V., S. G. Sealy, and K. A. Hobson. 1992. Behavioral defenses against avian brood parasitism in sympatric and allopatric populations. Evolution 46:334-340.

Brodhead, K. M., S. H. Stoleson, and D. M. Finch. 2007. Southwestern Willow Flycatchers (Empidonax traillii extimus) in a grazed landscape: factors influencing brood parasitism. Auk 124:1213-1228.

Brodkorb, P. 1957. New passerine birds from the Pleistocene of Reddick, Florida. Journal of Paleontology 31:129-138.

Brown, R. B., E. L. Stone, and V. W. Carlisle. 1990. Soils. Pages 35-69 in R. L. Myers and J. J. Ewel, editors. Ecosystems of Florida. University of Central Florida Press, Orlando.

111

Burgham, M. C. J., and J. Picman. 1989. Effect of Brown-headed Cowbirds on the evolution of Yellow Warbler anti-parasite strategies. Animal Behaviour 38:298-308.

Burhans, D. E., D. Dearborn, F. R. Thompson, III, and J. Faaborg. 2002. Factors affecting predation at songbird nests in old fields. Journal of Wildlife Management 66:240-249.

Burhans, D. E., F. R. Thompson, III, and J. Faaborg. 2000. Costs of parasitism incurred by two songbird species and their quality as cowbird hosts. Condor 102:364-373.

Burke, D. M., and E. Nol. 2000. Landscape and fragment size effects on reproductive success of forest-breeding birds in Ontario. Ecological Applications 10:1749-1761.

Carey, C. 1996. Avian Energetics and Nutritional Ecology. Chapman and Hall, New York.

Chace, J. F. 2004. Habitat selection by sympatric brood parasites in southeastern Arizona: the influence of landscape, vegetation, and species richness. Southwestern Naturalist 49:24- 32.

Clotfelter, E. D., and K. Yasukawa. 1999. Impact of brood parasitism by Brown-headed Cowbirds on Red-winged Blackbird reproductive success. Condor 101:105-114.

Cody, M. L. 1966. A general theory of clutch size. Evolution 20:174-184.

Coe, S. J., and J. T. Rotenberry. 2003. Water availability affects clutch size in a desert sparrow. Ecology 84:3240-3249.

Cox, J. 2006. The breeding birds of Florida. Part II: trends in breeding distributions based on Florida's Breeding Bird Atlas Project. Florida Ornithological Society Special Publication 7:71-140.

Cruz, A., T. Manolis, and J. W. Wiley. 1985. The Shiny Cowbird: a brood parasite expanding its range in the Caribbean region. Ornithological Monographs 36.

Cruz, A., W. Post, J. W. Wiley, C. P. Ortega, T. K. Nakamura, and J. W. Prather. 1998. Potential impacts of cowbird range expansion in Florida. Pages 313-336 in S. I. Rothstein and S. K. Robinson, editors. Parasitic Birds and their Hosts: Studies in Coevolution. Oxford University Press, New York.

Cruz, A., J. W. Prather, W. Post, and J. W. Wiley. 2000. The spread of Shiny and Brown-headed Cowbirds into the Florida region. Pages 47-57 in J. N. M. Smith, T. L. Cook, S. I. Rothstein, S. K. Robinson, and S. G. Sealy, editors. Ecology and Management of Cowbirds and Their Hosts: Studies in the Conservation of North American Passerine Birds. University of Texas Press, Austin.

Cruz, A., and J. W. Wiley. 1989. The decline of an adaptation in the absence of a presumed selection pressure. Evolution 43:55-62.

112

Cruz, A., J. W. Wiley, T. K. Nakamura, and W. Post. 1989. The Shiny Cowbird Molothrus bonariensis in the West Indian region: biogeographical and ecological implications. Pages 519-540 in C. A. Woods, editor. Biogeography of the West Indies: Past, Present, and Future. Sandhill Crane Press, Gainesville, FL.

Curson, D. R., and N. E. Mathews. 2003. Reproductive costs of commuting flights in Brown- headed Cowbirds. Journal of Wildlife Management 67:520-529.

Davies, N. B., and M. L. Brooke. 1989. An experimental study of co-evolution between the Cuckoo, Cuculus canorus, and its hosts. II. Host egg markings, chick discrimination and general discussion. Journal of Animal Ecology 58:225-236.

Davis, A. K. 2005. A comparison of age, size, and health of House Finches captured with two trapping methods. Journal of Field Ornithology 76:339-344.

Davis, D. E. 1942. The number of eggs laid by cowbirds. Condor 44:10-12.

Dawkins, R., and J. R. Krebs. 1979. Arms races between and within species. Proceedings of the Royal Society of London, B, Biology 205:489-511.

Dearborn, D. C. 1999. Brown-headed Cowbird nestling vocalizations and risk of nest predation. Auk 116:448-457.

Deerenberg, C., C. H. de Kogel, and G. F. J. Overkamp. 1996. Costs of reproduction in the Zebra Finch Taeniopygia guttata: manipulation of brood size in the laboratory. Journal of Avian Biology 27:321-326.

Dufour, K. W., and P. J. Weatherhead. 1991. A test of the condition-bias hypothesis using Brown-headed Cowbirds trapped during the breeding season. Canadian Journal of Zoology 69:2686-2692.

Duguay, J. P., P. B. Wood, and J. V. Nichols. 2001. Songbird abundance and avian nest survival rates in forests fragmented by different silvicultural treatments. Conservation Biology 15:1405-1415.

Dunn, P. O., K. J. Thusius, K. Kimber, and D. W. Winkler. 2000. Geographic and ecological variation in clutch size of Tree Swallows. Auk 117:215-221.

Eckerle, K. P., and R. Breitwisch. 1997. Reproductive success of the Northern Cardinal, a large host of Brown-headed Cowbirds. Condor 99:169-178.

Eckrich, G. H., T. E. Koloszar, and M. D. Goering. 1999. Effective landscape management of Brown-headed Cowbirds at Fort Hood, Texas. Studies in Avian Biology 18:267-274.

Edscorn, J. B. 1980. Florida region. American Birds 34:887-889.

Ehrlich, P. R., D. S. Dobkin, and D. Wheye. 1988. The Birder's Handbook. Simon & Schuster Inc., New York.

113

Elliott, P. F. 1978. Cowbird parasitism in the Kansas tallgrass prairie. Auk 95:161-167.

Ellison, K., S. G. Sealy, and H. L. Gibbs. 2006. Genetic elucidation of host use by individual sympatric Bronzed Cowbirds (Molothrus aeneus) and Brown-headed Cowbirds (M. ater). Canadian Journal of Zoology 84:1269-1280.

Emlen, J. T. 1978. Density anomalies and regulatory mechanisms in land bird populations on the Florida peninsula. American Naturalist 112:265-286.

Emslie, S. D. 1998. Avian community, climate and sea-level changes in the Plio-Pleistocene of the Florida peninsula. Ornithological Monographs 50.

Evans, D. R., and J. E. Gates. 1997. Cowbird selection of breeding areas: the role of habitat and bird species abundance. Wilson Bulletin 109:470-480.

Filliater, T., S., R. Breitwisch, and P. M. Nealen. 1994. Predation on Northern Cardinal nests: does choice of nest site matter? Condor 96:761-768.

Fleischer, R. C., A. P. Smyth, and S. I. Rothstein. 1987. Temporal and age-related variation in the laying rate of the parasitic Brown-headed Cowbird in the eastern Sierra Nevada, California. Canadian Journal of Zoology 65:2724-2730.

Fleischer, T. L., and G. E. Woolfenden. 2004. Florida Scrub-jays eject foreign eggs added to their nests. Journal of Field Ornithology 75:49-50.

Florida Fish and Wildlife Conservation Commission (FFWCC). 2007. Florida's breeding bird atlas: a collaborative study of Florida's birdlife. Accessed February 5, 2008. http://www.wildflorida.org/bba.

Freeman, S., D. F. Gori, and S. Rohwer. 1990. Red-Winged Blackbirds and Brown-Headed Cowbirds: some aspects of a host-parasite relationship. Condor 92:336-340.

Friedmann, H. 1963. Host relations of the parasitic cowbirds. Bulletin of the U.S. National Museum 233.

Friedmann, H., and L. F. Kiff. 1985. The parasitic cowbirds and their hosts. Proceedings of the Western Foundation of Vertebrate Zoology 2:225-304.

Friedmann, H., L. F. Kiff, and S. I. Rothstein. 1977. A further contribution to knowledge of the host relations of the parasitic cowbirds. Smithsonian Contributions to Zoology 235:1-75.

Gill, S. A., and S. G. Sealy. 1996. Nest defence by Yellow Warblers: recognition of a brood parasite and an avian nest predator. Behaviour 133:263-282.

Goertz, J. W. 1977. Additional records of Brown-headed Cowbird nest parasitism in Louisiana. Auk 94:386-389.

114

Goguen, C. B., and N. E. Mathews. 1998. Songbird community composition and nesting success in grazed and ungrazed pinyon-juniper woodlands. Journal of Wildlife Management 62:474-484.

Graber, J. W., R. R. Graber, and E. L. Kirk. 1985. Illinois birds: vireos. Illinois Natural History Survey Biological Notes 68.

Graham, D. S. 1988. Responses of five host species to cowbird parasitism. Condor 90:588-591.

Granfors, D. A., P. J. Pietz, and L. A. Joyal. 2001. Frequency of egg and nestling destruction by female Brown-headed Cowbirds at grassland nests. Auk 118:765-769.

Graveland, J. 1990. Effects of acid precipitation on reproduction in birds. Experientia 46:962- 970.

Graveland, J. 1996. Avian eggshell formation in calcium-rich and calcium-poor habitats: importance of snail shells and anthropogenic calcium sources. Canadian Journal of Zoology 74:1035-1044.

Graveland, J., and R. H. Drent. 1997. Calcium availability limits breeding success of passerines on poor soils. Journal of Animal Ecology 66:279-288.

Graveland, J., R. van der Wal, J. H. van Balen, and A. J. van Noordwijk. 1994. Poor reproduction in forest passerines from decline of snail abundance on acidified soils. Nature 368:446-448.

Graveland, J., and T. van Gijzen. 1994. Arthropods and seeds are not sufficient as calcium sources for shell formation and skeletal growth in passerines. Ardea 82:299-314.

Greenberg, R. 1980. Demographic aspects of long-distance migration. Pages 493-504 in A. Keast and E. S. Morton, editors. Migrant Birds in the Neotropics: Ecology, Behavior, Distribution, and Conservation. Smithsonian Institution Press., Washington, D.C.

Grzybowski, J. A., R. B. Clapp, and J. T. Marshall, Jr. 1986. History and current population status of the Black-capped Vireo in Oklahoma. American Birds 40:1151-1161.

Grzybowski, J. A., and C. M. Pease. 1999. A model of the dynamics of cowbirds and their host communities. Auk 116:209-222.

Haas, C. A., and K. H. Haas. 1998. Brood parasitism by Brown-headed Cowbirds on Brown Thrashers: frequency and rates of rejection. Condor 100:535-540.

Haggerty, T. M., and E. S. Morton. 1995. Carolina Wren (Thryothorus ludovicianus). Volume 188 in A. Poole and F. Gill, editors. The Birds of North America. Birds of North America, Inc., Philadelphia, PA.

Hahn, D. C., and J. S. Hatfield. 1995. Parasitism at the landscape scale: cowbirds prefer forests. Conservation Biology 9:1415-1424.

115

Hahn, D. C., J. A. Sedgwick, I. S. Painter, and N. J. Casna. 1999. A spatial and genetic analysis of cowbird host selection. Studies in Avian Biology 18:204-217.

Halkin, S. L., and S. U. Linville. 1999. Northern Cardinal (Cardinalis cardinalis). Volume 440 in A. Poole and F. Gill, editors. The Birds of North America. The Birds of North America, Inc., Philadelphia, PA.

Hamon, J. H. 1964. Osteology and paleontology of the passerine birds of the Reddick, Florida, Pleistocene. Florida Geological Survey Geology Bulletin 44.

Hannon, S. J. 1981. Postovulatory follicles as indicators of egg production in Blue Grouse. Journal of Wildlife Management 45:1045-1047.

Hatch, S. A. 1983. Nestling growth relationships of Brown-headed Cowbirds and Dickcissels. Wilson Bulletin 95:669-671.

Hoffman, W., and G. E. Woolfenden. 1986. A fledgling Brown-headed Cowbird specimen from Pinellas County. Florida Field Naturalist 14:18-20.

Holford, K. C., and D. D. Roby. 1993. Factors limiting fecundity of captive Brown-headed Cowbirds. Condor 95:536-545.

Hoover, J. P. 2003. Multiple effects of brood parasitism reduce the reproductive success of Prothonotary Warblers, Protonotaria citrea. Animal Behaviour 65:923-934.

Hoover, J. P., M. C. Brittingham, and L. J. Goodrich. 1995. Effects of forest patch size on nesting success of Wood Thrushes. Auk 112:146-155.

Hoover, J. P., and M. J. Reetz. 2006. Brood parasitism increases provisioning rate, and reduces offspring recruitment and adult return rates, in a cowbird host. Oecologia 149:165-173.

Hoover, J. P., and S. K. Robinson. 2007. Retaliatory behavior by a parasitic cowbird favors host acceptance of parasitic eggs. Proceedings of the National Academy of Sciences USA 104:4479-4483.

Hopp, S. L., A. Kirby, and C. A. Boone. 1995. White-eyed Vireo (Vireo griseus). Volume 168 in A. Poole and F. Gill, editors. The Birds of North America. Birds of North America, Inc., Philadelphia, PA.

Jarvinen, A. 1989. Clutch-size variation in the Pied Flycatcher Ficedula hypoleuca. Ibis 131:572- 577.

Jensen, W. E., and J. F. Cully. 2005a. Density-dependent habitat selection by Brown-headed Cowbirds (Molothrus ater) in tallgrass prairie. Oecologia 142:136-149.

Jensen, W. E., and J. F. Cully. 2005b. Geographic variation in Brown-headed Cowbird (Molothrus ater) parasitism on Dickcissels (Spiza americana) in great plains tallgrass prairie. Auk 122:648-660.

116

Johnson, L. S., E. Ostlind, J. L. Brubaker, S. L. Balenger, B. G. P. Johnson, and H. Golden. 2006. Changes in egg size and clutch size with elevation in a Wyoming population of mountain bluebirds. Condor 108:591-600.

Johnson, R. G., and S. A. Temple. 1990. Nest predation and brood parasitism of tallgrass prairie birds. Journal of Wildlife Management 54:106-111.

Johnston, R. F. 1954. Variation in breeding season and clutch size in Song Sparrows of the Pacific coast. Condor 56:268-273

Kautz, R., B. Stys, and R. Kawula. 2007. Florida vegetation 2003 and land use change between 1985-89 and 2003. Florida Scientist 70:12-23.

Kautz, R., D. T. Gilbert, and G. M. Mauldin. 1993. Vegetative cover in Florida based on 1985- 1989 Landsat Thematic Mapper imagery. Florida Scientist 56:135–154.

Kelly, S. T., and M. E. DeCapita. 1982. Cowbird control and its effect on Kirtland’s Warbler reproductive success. Wilson Bulletin 94:363-365.

Kennedy, E. D., P. C. Stouffer, and H. W. Power. 1989. Postovulatory follicles as a measure of clutch size and brood parasitism in European Starlings. Condor 91:471-473.

Kilgo, J. C., and C. E. Moorman. 2003. Patterns of cowbird parasitism in the Southern Atlantic Coastal Plain and Piedmont. Wilson Bulletin 115:277-284.

Klein, N. K., and K. V. Rosenberg. 1986. Feeding of Brown-headed Cowbirds (Molothrus ater) fledglings by more than one "host" species. Auk 103:213-214.

Koenig, W. D. 1984. Geographic variation in clutch size in the Northern Flicker (Colaptes auratus): support for Ashmole's hypothesis. Auk 101:698-706.

Kosciuch, K. L., T. H. Parker, and B. K. Sandercock. 2006. Nest desertion by a cowbird host: an antiparasite behavior or a response to egg loss? Behavioral Ecology 17:917-924.

Lack, D. 1954. The Natural Regulation of Animal Numbers. Clarendon Press, Oxford.

LaHaye, W. S., G. S. Zimmerman, and R. J. Gutierrez. 2004. Temporal variation in the vital rates of an insular population of Spotted Owls (Strix occidentalis occidentalis): contrasting effects of weather. Auk 121:1056-1069.

Lewis, R. A. 1975. Reproductive biology of the White-crowned Sparrow (Zonotrichia leucophrys pugetensis Grinnell). Temporal organization of reproductive and associated cycles. Condor 77:46-59.

Ligon, J. D. 1966. A Pleistocene avifauna from Haile, Florida. Bulletin of the Florida State Museum, Biological Sciences 10:127-158.

117

Lindstrom, E. B., M. W. Eichholz, and J. M. Eadie. 2006. Postovulatory follicles in Mallards: implications for estimates of breeding propensity. Condor 108:925-935.

Lokemoen, J. T., and R. R. Koford. 1996. Using candlers to determine the incubation stage of passerine eggs. Journal of Field Ornithology 67:660-668.

Lotem, A., and H. Nakamura. 1998. Evolutionary equilibria in avian brood parasitism: an alternative to the "Arms Race-Evolutionary Lag" concept. Pages 223-235 in S. I. Rothstein and S. K. Robinson, editors. Parasitic Birds and their Hosts: Studies in Coevolution. Oxford University Press, New York.

Lowther, P. E. 1993. Brown-headed Cowbird (Molothrus ater). Volume 47 in A. Poole and F. Gill, editors. The Birds of North America. Birds of North America, Inc., Philadelphia, PA.

Martin, T. E. 1992. Breeding productivity considerations: what are the appropriate habitat features for management? Pages 163-197 in J. M. Hagan, III and D. W. Johnston, editors. Ecology and Conservation of Neotropical Migrant Landbirds. Smithsonian Institute Press, Washington.

Martin, T. E. 1998. Are microhabitat preferences of coexisting species under selection and adaptive? Ecology 79:656-670.

Martin-Vivaldi, M., M. Soler, and A. P. Moller. 2002. Unrealistically high costs of rejecting artificial model eggs in Cuckoo Cuculus canorus hosts. Journal of Avian Biology 33:295- 301.

Mayfield, H. 1965. The Brown-headed Cowbird, with old and new hosts. Living Bird 4:13-28.

Mayfield, H. 1961. Vestiges of proprietary interest in nests by the Brown-headed Cowbird parasitizing Kirtland's Warbler. Auk 78:162-167.

Mayfield, H. 1977. Brown-headed Cowbird: agent of extermination? American Birds 31:107- 113.

Mayfield, H. F. 1992. Kirtland's Warbler (Dendroica kirtlandii). Volume 19 in A. Poole and F. Gill, editors. The Birds of North America. Birds of North America, Inc., Philadelphia.

McMaster, D. G., and S. G. Sealy. 1998. Short incubation periods of Brown-headed Cowbirds: how do cowbird eggs hatch before Yellow Warbler eggs? Condor 100:102-111.

Means, B. D., and D. Simberloff. 1987. The peninsula effect: habitat-correlated species decline in Florida's herpetofauna. Journal of Biogeography 14:551-568.

Meyer, R. K., C. Kabat, and I. O. Buss. 1947. Early involutionary changes in postovulatory follicles of the Ring-necked Pheasant. Journal of Wildlife Management 60:94-107.

118

Middleton, A. L. A. 1991. Failure of Brown-headed Cowbird parasitism in nests of the American Goldfinch. Journal of Field Ornithology 62:200-203.

Moksnes, A., and E. Roskaft. 1988. Responses of Fieldfares Turdus pilaris and Bramblings Fringilla montifringilla to experimental parasitism by the Cuckoo Cuculus canorus. Ibis 130:535-539.

Moksnes, A., E. Roskaft, A. T. Braa, L. Korsnes, H. M. Lampe, and H. C. Pedersen. 1991. Behavioral responses of potential hosts towards artificial cuckoo eggs and dummies. Behaviour 116:64-89.

Møller, A. P. 1988. Nest predation and nest site choice in passerine birds in habitat patches of different size: a study of magpies and blackbirds. Oikos 53:215-221.

Monroe, B. L., Jr. 1957. A breeding record of the cowbird in Florida. Florida Naturalist 30:124.

Moreau, R. 1944. Clutch-size: a comparative study, with special reference to African birds. Ibis 86:286-347.

Morrison, S. A., and D. T. Bolger. 2002. Variation in a sparrow's reproductive success with rainfall: food and predator-mediated processes. Oecologia 133:315-324.

Nager, R. G., C. Ruegger, and A. J. VanNoordwijk. 1997. Nutrient or energy limitation on egg formation: a feeding experiment in Great Tits. Journal of Animal Ecology 66:495-507.

Nealen, P. M., and R. Breitwisch. 1997. Northern Cardinal sexes defend nests equally. Wilson Bulletin 109:269-278.

Neudorf, D. L., and S. G. Sealy. 1994. Sunrise nest attentiveness in cowbird hosts. Condor 96:162-169.

Nolan, V., Jr. 1978. The ecology and behavior of the Prairie Warbler Dendroica discolor. Ornithological Monographs 26:1-350.

Norris, R. 1947. The cowbirds of Preston Frith. Wilson Bulletin 59:83-103.

Ogden, J. C. 1965. Florida region. Audubon Field Notes 19:534-537.

Ogden, J. C. 1971. Florida region. American Birds 25:846-851.

Ortega, C. P. 1991. The ecology of blackbird/cowbird interactions in Boulder County, Colorado. Dissertation. University of Colorado.

Ortega, C. P. 1998. Cowbirds and Other Brood Parasites. University of Arizona Press, Tucson.

Parks, R. A. 1950. Oriole 15:8-9.

Patten, M. A. 2007. Geographic variation in calcium and clutch size. Journal of Avian Biology 38:637-643.

119

Paul, R. T. 1988. Florida region. American Birds 42:1278-1281.

Paul, R. T. 1989. Florida region. American Birds 43:1307-1310.

Payne, R. B. 1965. Clutch size and numbers of eggs laid by Brown-headed Cowbirds. Condor 67:44-60.

Payne, R. B. 1973a. The breeding season of a parasitic bird, the Brown-headed Cowbird, in central California. Condor 75:80-99.

Payne, R. B. 1973b. Individual laying histories and the clutch size and numbers of eggs of parasitic cuckoos. Condor 75:414-438.

Payne, R. B. 1976. The clutch size and numbers of eggs of Brown-headed Cowbirds: effects of latitude and breeding season. Condor 78:337-342.

Payne, R. B. 1998. Brood parasitism in birds: strangers in the nest. Bioscience 48:377-386.

Payne, R. B., and L. L. Payne. 1998. Brood parasitism by cowbirds: risks and effects on reproductive success and survival in Indigo Buntings. Behavioral Ecology 9:64-73.

Pearson, S. F., and S. Rohwer. 1998. Determining clutch size and laying dates using ovarian follicles. Journal of Field Ornithology 69:587-594.

Pearson, T. G., C. S. Brimley, and H. H. Brimley. 1959. Birds of North Carolina. North Carolina Department of Agriculture, Raleigh.

Pease, C. M., and J. A. Grzybowski. 1995. Assessing the consequences of brood parasitism and nest predation on seasonal fecundity in passerine birds. Auk 112:343-363.

Peck, G. K., and R. D. James. 1987. Breeding Birds of Ontario: Nidiology and Distribution. Volume 2: Passerines. The Royal Ontario Museum, Toronto.

Peck, G. K., and R. D. James. 1997. Breeding birds of Ontario: Nidiology and distribution. Volume 2: Passerines (first revision - part A: flycatchers to gnatcatchers). Ontario Birds 15:95-107.

Peck, G. K., and R. D. James. 1998a. Breeding birds of Ontario: Nidiology and distribution. Volume 2: Passerines (first revision - part B: thrushes to warblers). Ontario Birds 16:11- 25.

Peck, G. K., and R. D. James. 1998b. Breeding birds of Ontario: Nidiology and distribution. Volume 2: Passerines (first revision - part C: tanagers to old world sparrows). Ontario Birds 16:111-127.

Peer, B. D., K. S. Ellison, and S. G. Sealy. 2002. Intermediate frequencies of egg ejection by Northern Mockingbirds (Mimus polyglottos) sympatric with two cowbird species. Auk 119:855-858.

120

Peer, B. D., S. I. Rothstein, K. S. Delaney, and R. C. Fleischer. 2007. Defence behaviour against brood parasitism is deeply rooted in mainland and island scrub-jays. Animal Behaviour 73:55-63.

Peer, B. D., and S. G. Sealy. 2004a. Correlates of egg rejection in hosts of the Brown-headed Cowbird. Auk 106:580-599.

Peer, B. D., and S. G. Sealy. 2004b. Fate of grackle (Quiscalus spp.) defenses in the absence of brood parasitism: implications for long-term parasite-host coevolution. Auk 121:1172- 1186.

Picman, J. 1989. Mechanism of increased puncture resistance of eggs of Brown-headed Cowbirds. Auk 106:577-583.

Post, W., T. K. Nakamura, and A. Cruz. 1990. Patterns of Shiny Cowbird parasitism in St. Lucia and southwestern Puerto Rico. Condor 92:461-469.

Post, W., and J. W. Wiley. 1977. The Shiny Cowbird in the West Indies. Condor 79:119-121.

Potter, E. F., and G. T. Whitehurst. 1981. Cowbirds in the Carolinas. Chat 45:57-68.

Prather, J. W., and A. Cruz. 1995. Breeding biology of Florida Prairie Warblers and Cuban Yellow Warblers. Wilson Bulletin 107:475-484.

Prather, J. W., and A. Cruz. 2002. Distribution, abundance, and breeding biology of potential cowbird hosts on Sanibel Island, Florida. Florida Field Naturalist 30:21-76.

Prather, J. W., and A. Cruz. 2006. Breeding biology of Red-winged Blackbirds in south Florida. Southeastern Naturalist 5:547-554.

Pulliam, H. R. 1988. Sources, sinks, and population regulation. American Naturalist 132:652- 661.

Purcell, K. L. 2006. Abundance and productivity of Warbling Vireos across and elevational gradient in the Sierra Nevada. Condor 108:315-325.

Pyle, P. 1997. Identification Guide to North American Birds, Part I. Columbidae to Ploceidae. Slate Creek, Bolinas, CA.

Reetz, M. J., E. Farley, and T. A. Contreras. 2008. Evidence for Bachman's Sparrow raising Brown-headed Cowbirds to fledging. Wilson Journal of Ornithology. In press.

Ricklefs, R. E. 1973. Fecundity, mortality, and avian demography. Pages 336-435 in D. S. Farner, editor. Breeding Biology of Birds. National Academy of Science, Philadelphia, PA.

121

Robbins, C. S., J. R. Sauer, R. Greenberg, and S. Droege. 1989. Population declines in North American birds that migrate to the Neotropics. Proceedings of the National Academy of Sciences, USA 86:7658-7662.

Robert, M., and G. Sorci. 1999. Rapid increase of host defence against brood parasites in a recently parasitized area: the case of Village Weavers in Hispaniola. Proceedings of the Royal Society of London, B, Biology 266:941-946.

Robertson, R. J., and R. F. Norman. 1976. Behavioral defense to brood parasitism by potential hosts of the Brown-headed Cowbird. Condor 78:166-173.

Robertson, W. B., Jr., and J. A. Kushlan. 1974. The southern Florida avifauna. Miami Geological Society Memoirs 2:414-452.

Robinson, S. K. 1992. Population dynamics of breeding Neotropical migrants in a fragmented Illinois landscape. Pages 408-418 in J. M. Hagan III and D. W. Johnston, editors. Ecology and Conservation of Neotropical Migrant Landbirds. Smithsonian Institution Press, Washington, D.C.

Robinson, S. K., and D. S. Wilcove. 1994. Forest fragmentation in the temperate zone and its effects on migratory songbirds. Bird Conservation International 4:233-249.

Robinson, S. K., and W. D. Robinson. 2001. Avian nesting success in a selectively harvested north temperate deciduous forest. Conservation Biology 15:1763-1771.

Robinson, S. K., S. I. Rothstein, M. C. Brittingham, L. C. Petit, and J. A. Grzybowski. 1995a. Ecology and behavior of cowbirds and their impact on host populations. Pages 428-460 in T. E. Martin and D. M. Finch, editors. Ecology and Management of Neotropical Birds. Oxford University Press, New York.

Robinson, S. K., F. R. Thompson, T. M. Donovan, D. R. Whitehead, and J. Faaborg. 1995b. Regional forest fragmentation and the nesting success of migratory birds. Science 267:1987-1990.

Robinson, S. K., J. P. Hoover, and J. R. Herkert. 2000. Cowbird parasitism in a fragmented landscape: effects of tract size, habitat, and abundance of cowbird hosts. Pages 280-297 in J. N. M. Smith, T. L. Cook, S. I. Rothstein, S. K. Robinson, and S. G. Sealy, editors. Ecology and Management of Cowbirds and Their Hosts: Studies in the Conservation of North American Passerine Birds. University of Texas Press, Austin.

Robinson, W. D., and S. K. Robinson. 1999. Effects of selective logging on forest bird populations in a fragmented landscape. Conservation Biology 13:58-66.

Rohwer, S., and C. D. Spaw. 1988. Evolutionary lag versus bill-size constraint: a comparitive study of the acceptance of cowbird eggs by old hosts. Evolutionary Ecology 2:27-36.

122

Røskaft, E., F. Takasu, A. Moksnes, and B. G. Stokke. 2006. Importance of spatial habitat structure on establishment of host defenses against brood parasitism. Behavioral Ecology 17:700-708.

Rothstein, S. I. 1975a. Evolutionary rates and host defenses against avian brood parasitism. American Naturalist 109:161-176.

Rothstein, S. I. 1975b. An experimental and teleonomic investigation of avian brood parasitism. Condor 77:250-271.

Rothstein, S. I. 1976. Experiments on defenses Cedar Waxwings use against cowbird parasitism. Auk 93:675-691.

Rothstein, S. I. 1977. Cowbird parasitism and egg recognition of the Northern Oriole. Wilson Bulletin 89:21-32.

Rothstein, S. I. 1982. Successes and failures in avian egg and nestling recognition with comments on the utility of optimality reasoning. American Zoologist 22:547-560.

Rothstein, S. I. 1986. A test of optimality: egg recognition in the Eastern Phoebe. Animal Behaviour 34:1109-1119.

Rothstein, S. I. 1990. A model system for coevolution: avian brood parasitism. Annual Review of Ecology and Systematics 21:581-508.

Rothstein, S. I. 1994. The cowbird's invasion of the far west: history, causes and consequences experienced by host species. Studies in Avian Biology 15:301-315.

Rothstein, S. I. 2001. Relic behaviours, coevolution and the retention versus loss of host defences after episodes of avian brood parasitism. Animal Behaviour 61:95-107.

Rothstein, S. I., and T. L. Cook. 2000. Cowbird management, host population limitation, and efforts to save endangered species. Introduction. Pages 323-332 in J. N. M. Smith, T. L. Cook, S. I. Rothstein, S. K. Robinson, and S. D. Sealy, editors. Ecology and Management of Cowbirds and their Hosts. University of Texas Press, Austin.

Rothstein, S. I., and S. K. Robinson. 2000. Introduction. Pages 13-19 in J. N. M. Smith, T. L. Cook, S. I. Rothstein, S. K. Robinson, and S. D. Sealy, editors. Ecology and Management of Cowbirds and their Hosts. University of Texas Press, Austin.

Rothstein, S. I., and S. K. Robinson, editors. 2002. Parasitic Birds and Their Hosts: Studies in Coevolution. Oxford University Press, New York.

Rothstein, S. I., J. Verner, and E. Stevens. 1980. Range expansion and diurnal changes in dispersion of the brown-headed cowbird in the Sierra Nevada. Auk 97:253-267.

123

Rothstein, S. I., J. Verner, E. Stevens, and L. V. Verner. 1987. Behavioral differences among sex and age classes of the Brown-headed Cowbird and their relation to the efficacy of a control program. Wilson Bulletin 99:322-337.

SAS Institute Inc. 2003. SAS/STAT User's Guide, version 9.1. SAS Institute Inc., Cary, NC.

Sauer, J. R., J. E. Hines, and J. Fallon. 2006. The North American Breeding Bird Survey, DRAFT Results and Analysis 1966-2005. Version 6.2.2006. USGS Patuxent Wildlife Research Center, Laurel, MD.

Schmidt, K. A., and C. J. Whelan. 1999. The relative impacts of nest predation and brood parasitism on seasonal fecundity in songbirds. Conservation Biology 13:46-57.

Schoech, S. J., and R. Bowman. 2003. Does differential access to protein influence differences in timing of breeding of Florida Scrub-jays (Aphelocoma coerulescens) in suburban and wildland habitats? Auk 120:1114-1127.

Scott, D. M. 1978. Using sizes of unovulated follicles to estimate laying rate of the Brown- headed Cowbird. Canadian Journal of Zoology 56:2230-2234.

Scott, D. M., and C. D. Ankney. 1979. Evaluation of a method for estimating the laying rate of Brown-headed Cowbirds. Auk 96:483-488.

Scott, D. M., and C. D. Ankney. 1980. Fecundity of the Brown-headed Cowbird in southern Ontario. Auk 97:677-683.

Scott, D. M., and C. D. Ankney. 1983. The laying cycle of Brown-headed Cowbirds: passerine chickens? Auk 100:583-592.

Scott, D. M., P. J. Weatherhead, and C. D. Ankney. 1992. Egg-eating by female Brown-headed Cowbirds. Condor 94:579-584.

Sealy, S. G. 1992. Removal of Yellow Warbler eggs in association with cowbird parasitism. Condor 94:40-54.

Sealy, S. G. 1995. Burial of cowbird eggs by parasitized Yellow Warblers: an empirical and experimental study. Animal Behaviour 49:877-889.

Sealy, S. G. 1996. Evolution of host defenses against brood parasitism: implications of puncture- ejection by a small passerine. Auk 113:346-355.

Sealy, S. G., and R. C. Bazin. 1995. Low frequency of observed cowbird parasitism on Eastern Kingbirds: host rejection, effective nest defense, or parasite avoidance? Behavioral Ecology 6:140-145.

Semel, B., and P. Sherman. 1991. Ovarian follicles do not reveal laying histories of post- incubation Wood Ducks. Wilson Bulletin 103:703-705.

124

Simkiss, K. 1975. Calcium and avian reproduction. Symposia of the Zoological Society of London 35:307-337.

Skutch, A. F. 1949. Do tropical birds rear as many young as they can nourish? Ibis 91:430-455.

Smith, J. N. M., P. Arcese, and I. G. McLean. 1984. Age, experience, and enemy recognition by wild Song Sparrows. Behavioral Ecology and Sociobiology 14:101-106.

Smith, J. N. M., and S. I. Rothstein. 2000. General Introduction. Pages 1-9 in J. N. M. Smith, T. L. Cook, S. I. Rothstein, S. K. Robinson, and S. D. Sealy, editors. Ecology and Management of Cowbirds and their Hosts. University of Texas Press, Austin.

Smith, J. N. M., M. J. Taitt, and L. Zanette. 2002. Removing Brown-headed Cowbirds increases seasonal fecundity and population growth in Song Sparrows. Ecology 83:3037-3047.

Smith, P. W., and A. Sprunt, IV. 1987. The Shiny Cowbird reachs the United States: will the scourge of the Caribbean impact Florida's avifauna too? American Birds 41:370-371.

Soler, M., J. J. Soler, and A. P. Moller. 2000. Effect of Great Spotted Cuckoo presence on Magpie rejection behaviour. Behaviour 137:213-220.

Spaw, C. D., and S. Rohwer. 1987. A comparative study of eggshell thickness in cowbirds and other passerines. Condor 89:307-318.

Sprunt, A., Jr., and E. B. Chamberlain. 1970. South Carolina Bird Life. South Carolina University Press, Columbia.

SPSS Inc. 2003. SPSS® Base 12.0 for Windows® User's Guide. SPSS Inc., Chicago.

St. Louis, V. L., and L. Breebaart. 1991. Calcium supplements in the diet of nestling Tree Swallows near acid sensitive lakes. Condor 93:286-294.

Strausberger, B. M., and M. V. Ashley. 1997. Community-wide patterns of parasitism of a host ''generalist'' brood-parasitic cowbird. Oecologia 112:254-262.

Strausberger, B. M., and M. V. Ashley. 2003. Breeding biology of brood parasitic Brown-headed Cowbirds (Molothrus ater) characterized by parent-offspring and sibling-group reconstruction. Auk 120:433-445.

Strausberger, B. M., and D. E. Burhans. 2001. Nest desertion by field sparrows and its possible influence on the evolution of cowbird behavior. Auk 118:770-776.

Stracey, C. M., and S. K. Robinson. 2008. Does nest predation shape urban bird communities? Studies in Avian Biology. in press.

Summers, S. G., R. M. Kostecke, and G. L. Norman. 2006. Efficacy of trapping and shooting in removing breeding Brown-headed Cowbirds. Wildlife Society Bulletin 34:1107-1112.

125

Tazik, D. J., J. D. Cornelius, and C. A. Abrahamson. 1993. Status of the Black-capped Vireo at Fort Hood, Texas. Volume I: Distribution and abundance. USACERL Technical Report EN-94101, Vol I. U.S. Army Construction Engineering Laboratories, Champaign, IL.

Tewksbury, J. J., T. E. Martin, S. J. Hejl, T. S. Redman, and F. J. Wheeler. 1999. Cowbirds in a western valley: effects of landscape structure, vegetation, and host density. Studies in Avian Biology 18:23-33.

Texas Parks and Wildlife Department. 2008. Cowbird "pick-up" trap construction. Accessed February 6, 2008. http://www.tpwd.state.tx.us/huntwild/wild/nuisance/cowbirds/media/pick_up_trap_instru ctions.pdf.

Tilgar, V., R. Mand, and A. Leivits. 1999. Effect of calcium availability and habitat quality on reproduction in Pied Flycatcher Ficedula hypoleuca and Great Tit Parus major. Journal of Avian Biology 30:383-391.

Trail, P. W., and L. F. Baptista. 1993. The impact of Brown-headed Cowbird parasitism on populations of the Nuttall's White-crowned Sparrow. Conservation Biology 7:309-315.

Trine, C. L., W. D. Robinson, and S. K. Robinson. 1998. Consequences of Brown-headed Cowbird parasitism for host population dynamics. Pages 273-295 in S. I. Rothstein and S. K. Robinson, editors. Parasitic Birds and their Hosts: Studies in Coevolution. Oxford University Press, New York.

Turner, E. R. A. 1964. Social feeding in birds. Behavior 24:1-46.

United States Department of Agriculture (USDA). 2002a. 2002 Census of Agriculture. County Profile: Alachua, Florida. National Agricultural Statistics Service.

United States Department of Agriculture (USDA). 2002b. 2002 Census of Agriculture. Volume 1, Chapter 2: U.S. State Level Data. National Agricultural Statistics Service. http://www.nass.usda.gov/census/census02/volume1/us/index2.htm.

United States Department of Agriculture (USDA). 2007. Livestock county estimates. Florida cattle and calves. January 1, 2007. National Agricultural Statistics Service.

United States Fish and Wildlife Service (USFWS). 1998. Draft recovery plan for the Least Bell’s Vireo. U.S. Fish and Wildlife Service, Portland, OR

United States Geological Survey (USGS). 1990. Groundwater Atlas of the United States: Alabama, Florida, Georgia, South Carolina. HA 730-G. Floridan Aquifer System. http://capp.water.usgs.gov/gwa/ch_g/G-text6.html.

Verner, J., and L. V. Ritter. 1983. Current status of the Brown-headed Cowbird in the Sierra National Forest. Auk 100:355-368.

126

Vickery, P. D., M. L. Hunter, Jr., and J. V. Wells. 1992. Use of a new reproductive index to evaluate relationship between habitat quality and breeding success. Auk 109:697-705.

Wamer, N. 1978. Avian diversity and habitat in Florida: an analysis of a peninsular diversity gradient. Dissertation. Florida State University, Tallahassee, FL.

Weatherhead, P. J., and H. Greenwood. 1981. Age and condition bias of decoy-trapped birds. Journal of Field Ornithology 52:10-15.

Whitehead, M. A., S. H. Schweitzer, and W. Post. 2000. Impact of brood parasitism on nest survival parameters and seasonal fecundity of six songbird species in southeastern old- field habitat. Condor 102:946-950.

Whitehead, M. A., S. H. Schweitzer, and W. H. Post. 2002. Cowbird/host interactions in a southeastern old-field: a recent contact area. Journal of Field Ornithology 73:379-386.

Wilcove, D. S. 1985. Nest predation in forest tracts and the decline of migratory songbirds. Ecology 66:1211-1214.

Wiley, J. W. 1985. Shiny Cowbird parasitism in two avian communities in Puerto Rico. Condor 87:165-176.

Wiley, J. W., W. Post, and A. Cruz. 1991. Conservation of the Yellow-shouldered Blackbird Agelaius xanthomus, an endangered West Indian species. Biological Conservation 55:119-138.

Williams, T. D. 2005. Mechanisms underlying the costs of egg production. Bioscience 55:39-48.

Winfree, R. 2004. High offspring survival of the Brown-headed Cowbird in an invaded habitat. Animal Conservation 7:445-453.

Winfree, R., J. Dushoff, S. K. Robinson, and D. Bengali. 2006. A Monte Carlo model for estimating the productivity of a generalist brood parasite across multiple host species. Evolutionary Ecology Research 8:213-236.

Woolfenden, B. E., H. L. Gibbs, and S. G. Sealy. 2001. Demography of Brown-headed Cowbirds at Delta Marsh, Manitoba. Auk 118:156-166.

Woolfenden, B. E., H. L. Gibbs, S. G. Sealy, and D. G. McMaster. 2003. Host use and fecundity of individual female Brown-headed Cowbirds. Animal Behaviour 66:95-106.

Young, H. 1963. Breeding success of the cowbird. Wilson Bulletin 75:115-122.

Zahavi, A. 1979. Parasitism and nest predation in parasitic cuckoos. American Naturalist 113:157-159.

Zaias, J., and R. Breitwisch. 1989. Intra-pair cooperation, fledgling care, and re-nesting by Northern Mockingbirds. Ethology 80:94-110.

127

Zanette, L., D. T. Haydon, J. N. M. Smith, M. J. Taitt, and M. Clincy. 2007. Reassessing the cowbird threat. Auk 124:210-223.

128

BIOGRAPHICAL SKETCH

Matthew J. Reetz was born on July 30, 1973 in Willoughby Hills, Ohio. From 1991 to

1995, he attended the University of Illinois in Urbana−Champaign, earning a bachelor’s degree in biology with concentration in ecology, ethology and evolution. Repeated field seasons working in southern Illinois on projects dealing with songbird nesting biology encouraged the pursuit of a higher degree. In the fall of 1997, he entered the University of Florida’s graduate program to study ecology and ornithology and earned a Master of Science degree from the

Department of Wildlife Ecology and Conservation in August of 2000. Matthew earned an

Alumni Fellowship which facilitated dissertation research resulting in a doctorate in wildlife ecology and conservation in May 2008. In August 2008, Matthew will be an Assistant Professor in the Natural Sciences Division at Franklin College where he will teach biology, mentor undergraduate projects, and conduct research in community ecology.

129