Urbanization alters the seasonality of bird communities across China

Urbanization has profoundly reshaped biodiversity, yet its impacts on community seasonal changes remain poorly understood. Here, we used citizen science data from 839 bird species in 37 cities and their corresponding rural areas in China to assess how urbanization alters seasonal changes in bird communities. We calculated Sørensen beta dissimilarity indices (β_sor) between seasons to compare the seasonality of communities in urban and rural areas and decomposed these indices into turnover (β_sim) and nestedness (β_nes) components. We evaluated whether there are differences in the latitudinal clines in community seasonality between urban and rural areas, and explored whether environmental and socio-economic factors affect the urbanization-driven changes in community seasonality. Our results show that the overall seasonal β_sor in urban communities was 16.2% higher than in rural areas, due to a 49.5% increase in β_nes (urban: 0.22 ± 0.12 vs. rural: 0.15 ± 0.08), but there was no significant difference in β_sim. In rural areas, β_sor increased with latitude, but β_sor showed no latitudinal trend in urban communities. Human population emerges as a key predictor of urbanization-driven changes in the species turnover and nestedness components, with larger cities showing lower species turnover but higher nestedness components. We conclude that urbanization alters the seasonality of bird communities through nestedness components, decouples the relationship between community seasonality and latitude, and concentrates its impacts in densely populated cities. Future research must employ long-term monitoring to track how urbanization changes bird communities in space and time.