Nest predation cannot explain homogeneous urban bird communities

Background/Question/Methods   Urbanization alters natural habitats, posing serious threats to wildlife, including birds. Urban bird communities are characterized by low diversity and high abundances of few, often exotic, species. Many hypotheses exist to explain this widespread pattern, including increased nest predation, increased resource competition, and differential habitat selection, among others. We used artificial nests to experimentally assay predation in residential (single-family homes) and wildland (county forest preserves) habitat across metropolitan Chicago, USA. All experimental sites contained a ground, low, and high shrub nest, each baited with a quail and a plasticine egg, resulting in 240 nests (120 in each habitat) containing 480 eggs. Overall predation rates were significantly lower in residential habitat than in urban wildland, with mean survival time 44% greater in residential habitat. While ground nests suffered the lowest predation rate in residential habitat, they suffered the greatest predation rate in urban wildland. Mean survival time was 97% greater for ground nests in residential habitat than in urban wildland.

Results/Conclusions  The different rates and patterns of predation suggest that birds nesting in residential habitat and urban wildland experience contrasting predation pressures. Predation was more severe in urban wildland. In addition, predation differed greatly between the habitats with respect to nest height. We conclude factors other than nest predation produce depauperate bird communities found in urban areas worldwide. Our results implicate competition, perhaps in conjunction with other processes like differential habitat selection, as the paramount factor(s) producing the homogeneous bird communities found worldwide in urban environments.