Effects of urbanization on bird assemblages in three southwestern US cities

Researchers have attempted to make generalizations about how species with similar traits respond to urbanization, although results are geographically idiosyncratic. I present a comparative study in three cities in the arid southwestern US: Fresno, CA; Tucson, AZ; and Phoenix, AZ. These three cities share similar climates and biogeographical contexts, but differ in other ways, particularly in levels of urban water use. With presence-absence data, I tested how the urban avian assemblage of each city is filtered from the regional avian assemblage on the basis of traits and phylogeny, and if urbanization causes biotic homogenization among avian assemblages. Results show evidence for trait-based filtering in certain key traits, but not in phylogeny. While prior studies have suggested that urban avian assemblages are determined by species' traits, our results indicate urban occurrence is likely due to a combination of neutral and deterministic processes. I propose that similar trait-based results in the three cities result from structural similarities in surrounding natural habitats, but that differences in relative urban species richness and report frequency are results of (1) intensive agriculture and (2) a more mesic urban habitat due to urban irrigation contrasting with arid matrix habitats in Fresno. The relationship between urban habitats and the surrounding habitat matrix influences the urban environmental filtering process, and that urban habitats should be designed to closely match natural habitats in order to harbor more native species.