Résumé
1. Global climate change is driving species’ distributions towards the
poles and mountain tops during both non-breeding and breeding seasons,
leading to changes in the composition of natural communities. However, the
degree of season differences in climate-driven community shifts has not
been thoroughly investigated at large spatial scales. 2. We compared the
rates of change in the community composition during both winter
(non-breeding season) and summer (breeding) and their relation to
temperature changes. 3. Based on continental-scale data from Europe and
North America, we examined changes in bird community composition using the
community temperature index (CTI) approach and compared the changes with
observed regional temperature changes during 1980–2016. 4. CTI increased
faster in winter than in summer. This seasonal discrepancy is probably
because individuals are less site-faithful in winter, and can more readily
shift their wintering sites in response to weather in comparison to the
breeding season. Regional long-term changes in community composition were
positively associated with regional temperature changes during both
seasons, but the pattern was only significant during summer due to high
annual variability in winter communities. Annual changes in community
composition were positively associated with the annual temperature changes
during both seasons. 5. Our results were broadly consistent across
continents, suggesting some climate-driven restructuring in both European
and North American avian communities. Because community composition has
changed much faster during the winter than during the breeding season, it
is important to increase our knowledge about climate-driven impacts during
the less-studied non-breeding season.