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 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.
Results: A total of 38 USIs were obtained from 34 cities of six
continents. 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 of 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.