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.

The data has been collected using breeding and winter bird surveys and the detailed methodologies are given in the Supplements of the article. The community temperatures are calculated using methodology of Devictor et al. 2008. 

Devictor, V., Julliard, R., Couvet, D. & Jiguet, F. (2008) Birds are tracking climate warming, but not fast enough. Proceedings of the Royal Society B, 275, 2743–2748. https://doi.org/10.1098/rspb.2008.0878

The dataset includes two files, one for annual changes and one for long term changes in community temperature index of birds and related temperature values.

The annual data includes annual (fyear) estimates (fit), the standard error of the change (se) and 95% confidence intervals (lower, upper) of community temperature indices (CTI) in 57 regions (Country) in North America (Continent) and Europe for breeding and winter seasons (Season). Annual change in CTI (dCTI) and temperature (dTemp) as well as CTI of previous year of regions are also given.

The long-term data includes long-term change (annual slope, dCTI) and the standard error of the change (CTI_se) in community temperature index (CTI) in 57 regions (Region) in North America (Continent) and Europe for breeding and winter seasons (Season).  Raw mean CTI value (mCTI_raw) and centred CTI (mCTI), coordinates (Lat, Lon), long-term change in temperature (Temp) and its standard error (TempSE), predicted CTI changes (CTITempPred), mean raw temperature (mTemp_raw) and centred mean temperature (mTempSt) of regions are also given.

In addition two R scripts to analyses the data (CTI_Annual_Analyses_20200817.R and CTI_Longterm_Analyses_20201130.R) are included.