Urbanization buffers seasonal change in composition of bird communities: A multi-continental meta-analysis

Aim: Urbanization buffers the seasonality of climate conditions and food availability and, therefore, may cause a seasonal homogenization of animal communities. However, the global effect of urbanization on the seasonal dynamics of animal communities remains unexplored. Our aim was to study the multi-continental relationship between urbanization and the seasonal change in bird composition and explore the influence of climatic factors on the urban-induced reduction of seasonality of bird composition. Location: Multi-continental. Taxon: Birds. Methods: We performed a literature review and a meta-analysis. The risk ratio effect size (here referred to as urban seasonality index, USI) was used, considering the number of species seen either in the breeding (spring and/or summer) or the non-breeding (autumn and/or winter) season, and the number of species seen during both seasons between urban and less urbanized sites. Low USI values indicated a decrease in the seasonal change in bird composition in urban areas. A total of 38 USIs were obtained from 34 cities of six continents. Results: Multi-continentally, there are fewer differences in bird composition between seasons in urban than in less urbanized areas, indicating seasonal homogenization due to urbanization. The USI decreased in areas with the lowest maximum temperature and the highest latitude, suggesting the highest decrease in the seasonality of bird composition in urban areas located in regions near the poles and with high seasonal change in temperature. Moreover, studies in the Northern Hemisphere and those that compared suburban versus natural and urban versus rural habitats had a significant decrease in seasonality of bird community in urban areas. Main conclusions: Urbanization induces a seasonal homogenization of bird composition, and this impact seems more pronounced in temperate areas with broad annual temperature range, and located in the Northern Hemisphere. Results indicate that efforts to restore seasonal dynamics in habitats and resources should be made in urban areas of temperate regions.