Homogenizing effects of cities on North American winter bird diversity

Urbanization is widespread throughout the United States and Canada. Studies at different scales have shown mixed consequences of urban areas for ecological communities and biodiversity. Here, we use geographic data on urban extent and survey data from the Christmas Bird Count to investigate the influence of urbanization on winter bird diversity over a continental scale. We compared the alpha (local richness) and beta diversity (turnover with distance) of 42 urban bird communities to nearby non-urban communities. We investigated the processes underlying beta diversity between non-urban and urban sites by comparing the frequency of occurrence in species and variability in relative abundances across sites. Alpha diversity was statistically indistinguishable between urban and non-urban sites when controlling for latitude. Community similarity decreased less rapidly over distance in urban compared to non-urban sites, indicating that spatial homogenization from urbanization results in lower beta diversity. Eighteen species of non-native or native generalists occurred across all urban sites, whereas no species occurred in all non-urban sites. Mean-variance scaling of relative abundance shows that species in urban sites maintained similar levels of community dominance across space compared to non-urban sites. The widespread presence of urban species is likely due to similarity in habitats among cities compared to nearby non-urban sites. The decreased variability in relative abundance of urban species is possibly due to urban resource subsidies (e.g., bird feeders, garbage, and irrigation) and shelter (e.g., landscaping, buildings, and microclimates). The increased occurrence of widespread species in cities and less variation in relative abundances across urban sites contribute to the homogenizing effect of cities on avian communities.