Jennifer Lee Thieme
Published: 2011
Total Pages: 154
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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. 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. 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. I asked two broad questions: (1) how do birds respond behaviorally to abundance and activity of predators in urban natural areas? (2) to what extent is avian reproductive success linked to predator communities and/or activity at plot and site scales? I collected data on avian density, nest placement, and reproductive success of eight focal species of grassland birds within 46 2-ha plots at seven urban parks near Chicago, Illinois, during 2009 and 2010. Relative abundance and activity levels of potential nest predator species, including mesopredators, small mammals, snakes, and avian predators, were estimated for each plot during surveys and as part of a collaborative study. As capture rates of small mammals increased, territory densities of Field Sparrow (Spizella pusilla), Common Yellowthroat (Geothlypis trichas), and Savannah Sparrow (Passerculus sandwichensis) declined, but density of Song Sparrow (Melospiza melodia) rose. Mesopredator capture rates were negatively associated with Common Yellowthroat and Savannah Sparrow densities within 2-ha plots, as well as Eastern Meadowlark (Sturnella magna) 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, 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.
