Geographic variation in urbanization filter effects on birds in China.

Previous studies, which explored the mechanisms of urbanization filtering based on species traits and assessed the roles of different traits in facilitating the adjustments of different species to urbanization, often yield inconsistent results, highlighting the need to gain a better understanding of the mechanisms underlying urbanization filtering. Here, we conducted field surveys of breeding bird communities in the capital cities of 34 Chinese provinces and categorized species as either urban utilizers or avoiders based on their occurrence rate among regional species pools. Bayesian phylogenetic generalized linear mixed model was used to investigate potential trait associations between urban utilizers and urban avoiders. To investigate the relative effects (measured by the model coefficients) of urbanization filtering across different environmental gradients, we incorporated the model coefficients of each trait as the response variables with city-specific environmental characteristics as predictors to construct linear mixed models. We found that urban utilizer birds exhibited greater behavioral innovation and broader habitat ranges than urban avoider birds. Additionally, we observed that the filtering effects of urbanization on different traits exhibited significant variations, showing gradients in latitude, altitude, and artificial night light intensity. This variation along geographic gradients could potentially elucidate the inconsistent findings reported in previous work on bird adaptation to urban environments. Our findings highlight the context-dependent nature of urbanization filtering effects and contribute to a better understanding of the impact of urbanization on urban bird communities.