JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact firstname.lastname@example.org. abstract: Urban bird communities exhibit high population den-sities and low species diversity, yet mechanisms behind these patterns remain largely untested. We present results from experimental studies of behavioral mechanisms underlying these patterns and provide a test of foraging theory applied to urban bird communities. We mea-sured foraging decisions at artificial food patches to assess how urban habitats differ from wildlands in predation risk, missed-opportunity cost, competition, and metabolic cost. By manipulating seed trays, we compared leftover seed (giving-up density) in urban and desert habitats in Arizona. Deserts exhibited higher predation risk than urban habitats. Only desert birds quit patches earlier when increasing the missed-opportunity cost. House finches and house sparrows co-exist by trading off travel cost against foraging efficiency. In exclusion experiments, urban doves were more efficient foragers than passer-ines. Providing water decreased digestive costs only in the desert. At the population level, reduced predation and higher resource abun-dance drive the increased densities in cities. At the community level, the decline in diversity may involve exclusion of native species by highly efficient urban specialists. Competitive interactions play sig-nificant roles in structuring urban bird communities. Our results indicate the importance and potential of mechanistic approaches for future urban bird community studies.
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