Functional diversity patterns of urban and forest breeding birds across four city-forest pairs in China

Urbanisation has led to rapid declines in species richness, contributing to urban biotic homogenisation. Nonetheless, our understanding of the effects of urbanisation on functional community structures remains limited. During the 2023 breeding season, we employed a standardised point-count method to survey bird communities across four cities and their corresponding forest habitats in China, and compared the functional structural characteristics of urban and forest avian communities. Our results revealed that urban birds showed greater functional trait differences, which partially compensated for reduced species richness. While forest bird communities showed declines in species and functional richness with increasing latitude, urban communities displayed a weaker latitudinal trend, which we tentatively interpreted as a possible, but unconfirmed, buffering effect on the urban environment. However, we observed a decrease in functional redundancy and an imbalance within urban avian communities along the latitudinal gradient, suggesting potential shifts in the abundance-based community structure that merit further investigation. We caution that all latitudinal inferences are exploratory, as they are based on only four city-forest pairs, which severely limits statistical power and generalisability. Therefore, our findings should be viewed as hypothesis-generating rather than confirmatory. They provide preliminary evidence that the functional structure of urban avian communities may differ from that of forest communities; however, broader sampling across more cities and latitudes is needed to validate these patterns and draw firm conclusions about their stability.