Year: 2011
Type: article
Abstract: Grassland birds are declining at greater rates than any other habitat guild in North America, yet conservation remains difficult due to extensive habitat loss and fragmentation throughout the urbanizing Midwest.Although urban green spaces can contribute to habitat conservation, fragmentation and high land acquisition costs typically limit these spaces to small fragments subjected to strong external pressure from humans.Human presence is often associated with non-native predators (e.g., cats) and anthropogenic food sources, which collectively can promote high densities of nest predators in urban landscapes.From a conservation perspective, high densities of nest predators are a concern because predation is the leading source of nest failure.Behavioral responses to predators may further diminish the value of urban habitats if birds avoid areas with high levels of predator activity, which could result in lower occupancy rates or densities of birds in urban habitat patches.In my research, I examined how habitat heterogeneity and variation in the predator community influenced the breeding ecology of grassland and early successional birds in urban parks.To understand these relationships, I asked two broad questions: (1) how do birds respond behaviorally (e.g.territory and nest-site selection) to abundance and activity of predators in urban natural areas?(2) to what extent is avian reproductive success linked to predator iii communities and/or activity at plot and site scales?To answer these questions, I collected data on avian density, nest placement, and reproductive success of grassland birds within 46 2-ha plots at seven urban parks (sites) near Chicago, Illinois, during 2009 and 2010.Focal species included Common Yellowthroat (Geothlypis trichas), Field Sparrow (Spizella pusilla), Song Sparrow (Melospiza melodia), Savannah Sparrow (Passerculus sandwichensis), Henslow's Sparrow (Ammodramus henslowii), Bobolink (Dolichonyx oryzivorus), Eastern Meadowlark (Sturnella magna), and Dickcissel (Spiza americana).Relative abundance and activity levels of potential nest predator species, including mesopredators (e.g., northern raccoons [Procyon lotor], domestic cats [Felis catus]), small mammals, snakes, and avian predators, were estimated for each plot during surveys and as part of a collaborative study.I found that, in general, territory densities were negatively related to predator activity within 2-ha plots, though associations varied across species.As capture rates of small mammals increased, territory densities of Field Sparrow, Common Yellowthroat, and Savannah Sparrow declined, but density of Song Sparrow rose.Mesopredator capture rates were negatively associated with Common Yellowthroat and Savannah Sparrow densities within 2-ha plots, as well as Eastern Meadowlark and total territory density at the site level.Whereas small mammal and mesopredator capture rates explained some of the observed variation in territory density, daily nest survival of both Field and Song Sparrows was best explained by numbers of snakes observed within plots.Interestingly, snake activity was positively associated with nest survival of Field Sparrows, though negatively associated with that of Song Sparrows.At large scales, iv vegetation characteristics best predicted nest survival of both species, with nest survival of Field Sparrow improving as density of groundcover increased and nest survival of Song Sparrow improving as structural complexity increased.While the structural complexity of vegetation at nest sites was not explained by predator activity, Song Sparrows selected nest sites with lower groundcover density than available as activity of Brown-headed Cowbird (Molothrus ater) increased.As a whole, these results provide evidence that breeding grassland and early successional birds respond to both habitat structure and activity of potential predators at different scales.I also found that behavioral (e.g., territory selection) and demographic (e.g., nest survival) associations with predators do not necessarily match.For example, snakes had the strongest, though sometimes counterintuitive, relationship with nest success of Field and Song Sparrows, yet appeared to elicit no response during territory or nest site selection.My results are also consistent with other studies demonstrating the importance of vegetation structure to both settlement and reproductive success.Consequently, the best management practices in urban parks will both maintain vegetation structure that promotes successful nesting and discourage activities that promote high abundances of predators.v Acknowledgements I first wish to thank my advisor, Amanda Rodewald, for taking me on as a graduate student for this unique project.Throughout several years, she has offered guidance and opportunities that will last a lifetime and will continue to open doors as I further pursue my interests in the sciences.I would also like to thank my committee members, Stanley Gehrt and Jacqueline Augustine, for contributing significantly to the development of my studies.This project was largely conceived and arranged by Dr.Gehrt, and his continuous efforts made this study possible.
Author: Jennifer Thieme
Institution: The Ohio State University
Language: English
Topic: Avian ecology and behavior
Subfield: Ecology
Field: Environmental Science
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W2343207205