The Erosion of Seasonality in Avian Communities

ABSTRACT Aim Seasonality governs species composition at a given place and time. However, the effects of climate and land‐use change can vary by season, altering species composition. These changes can lead to a loss of distinct seasonal community composition, representing a novel form of biotic homogenisation. We ask if breeding and winter bird communities are becoming more similar over time. If so, is homogenisation occurring more rapidly in winter than in the breeding season, and has the presence of individual species changed between seasons? Location Northeastern United States. Time Period 1989–2019. Major Taxa Studied Two hundred thirty‐eight bird species. Methods We use data from The National Audubon Society's Christmas Bird Count and the North American Breeding Bird Survey to test if winter and breeding bird communities have become more similar (homogenised). We evaluate this change using the Sørensen dissimilarity index, and its components of turnover (species replacement) and nestedness (a subset of a more species rich community) and describe the mechanism in which the seasonal winter and breeding bird communities are changing. Results We found that winter and breeding bird communities are homogenising, driven by significant decrease in turnover and a marginal decrease nestedness. When viewing breeding and wintering communities separately, we observe different trends. Breeding communities are becoming more unique with decreasing turnover and nestedness. Winter communities are becoming more similar to each other, with decreasing turnover and nestedness. More breeding species are declining and species that are typically found in the winter and year‐round residents are the main contributors to the homogenisation between seasons. Main Conclusions We show for the first time homogenisation between winter and breeding bird communities over time across the northeastern United States. This insight into how individual species are faring between seasons, and how they impact community structure, can be used when implementing conservation measures for maintaining ecological functioning and integrity.