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Title:An integrative approach to testing density effects in a migratory songbird, the Prothonotary Warbler (Protonotaria citrea)
Author(s):Davros, Nicole
Director of Research:Brawn, Jeffrey D.; Hoover, Jeffrey P.
Doctoral Committee Chair(s):Brawn, Jeffrey D.
Doctoral Committee Member(s):Hoover, Jeffrey P.; Bell, Alison M.; Cáceres, Carla E.; Weatherhead, Patrick J.
Department / Program:School of Integrative Biology
Discipline:Ecol, Evol, Conservation Biol
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Prothonotary Warbler
cavity nesting
migratory songbird
conspecific density
density dependence
reproductive success
clutch size
hatching success
fledging success
nestling provisioning
body condition
food availability
food limitation
stress hormones
white blood cells
heterophil/lymphocyte (H/L) ratio
population dynamics
song rate
foraging movement
prey attack rate
Abstract:Animal populations often fluctuate around local carrying capacity through density-dependent processes such as competition, predation, and disease. Once subject to debate, ecologists now recognize density dependence as a conceptual cornerstone of population biology. Yet despite >50 years of research, key questions persist about the nature and generality of density effects for many animal populations. For my dissertation, I took an integrative approach to test for potential effects and mechanisms of density on the reproduction, behavior, and physiology of a population of cavity-nesting songbirds, the Prothonotary Warbler (Protonotaria citrea), in southern Illinois, USA. From 2008-2011, I experimentally manipulated the availability of nest boxes by establishing two low-density and two high-density subplots on each of two 40 ha sites. Previous work in this system has shown that high reproductive success leads to increased site and territory fidelity so I eliminated nest predation and removed the eggs of a common brood parasite, the Brown-headed Cowbird (Molothrus ater), to improve warbler nesting success and increase their densities. The resultant range of densities (1-27 neighbors/pair) were at or beyond that seen under natural conditions in this system. Each year, I estimated warbler reproductive output, nestling provisioning rates, and nestling body condition prior to fledging to test for effects of density on the annual fecundity of warblers. My results suggested that warbler annual fecundity was unaffected by local conspecific densities. Warbler parents in high-density neighborhoods provisioned their young at similar rates and raised a similar number of offspring with similar body condition prior to fledging compared to warblers in lower-density areas. During 2010 when neighbor densities were at their greatest observed levels, I selected focal males for behavioral observations and I estimated their song, food search movement (i.e., hops, walks, and flights), and prey attack rates in relation to density. Male behaviors varied somewhat in relation to neighbor density, particularly during specific stages. Males with more neighbors sang at a greater rate during the preincubation and incubation stages but this trend disappeared by the nestling stage as males reduced their song rates overall, presumably because they were busy providing parental care to their young. Males also increased their food search movement rates during the nestling stage when they had more neighbors but there was no concomitant increase in prey attack rates. Thus, crowded males appeared to be working harder to find food compared to males in lower density neighborhoods. Finally, I also collected blood samples from focal pairs each year to test for density effects on baseline plasma corticosterone (CORT, the primary glucocorticoid stress hormone in birds), white blood cell (WBC) counts, and WBC ratios. Male and female warblers had similar baseline CORT concentrations, WBC ratios, and WBC counts regardless of the number of neighbors surrounding them. Further, baseline CORT concentrations were weakly correlated with the indices of immune function that I estimated. Collectively, my results suggest that increased conspecific density did not lead to increased competition for food, reduced reproductive success, or altered stress physiology in this population of Prothonotary Warblers. The bottomland hardwood forests and swamps of southern Illinois represent a very productive habitat type and the interference mechanism that appears to regulate songbirds in many other systems (i.e., density-dependent competition leading to food limitation) does not appear to be important for warblers in this system. Future research should incorporate a larger spatial scale to determine if a site-dependent mechanism of density dependence is operating on this population. Evaluating juvenile recruitment and adult survival in relation to conspecific density would also be helpful. Finally, integrative work is needed to better understand how stress physiology and immunosuppression may mediate density effects in systems where density-dependent regulation has previously been documented.
Issue Date:2014-05-30
Rights Information:Copyright 2014 Nicole Davros
Date Available in IDEALS:2014-05-30
Date Deposited:2014-05

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