The combined impact of climate and land-use change on biological communities is poorly understood, limiting our ability to assess past and predict future ecological consequences.  We examined changes from 1975 to 2001 in four community measures (species richness, abundance, body size, and geographic distribution size) for 444 assemblages of terrestrial winter avifauna in North America containing 228 species.  We tested predictions from a space-for-time substitution and assessed causal relationships for climate change (annual temperature) and land-use change (urban land-cover) using linear mixed models within a longitudinal study design.

Results/Conclusions

All community measures had significant positive trends over time.  Trends for species richness and abundance broadly matched space-for-time predictions; this was not the case for body size or distribution size.  Only climate change showed evidence for causal associations, with urban land-cover showing evidence of a positive interaction with climate change for distribution size.  Thus, warming annual temperatures were associated with increasing numbers of species and assemblages defined by greater average abundance, body size, and distribution size.  These findings support the conclusion that large-bodied, widespread, generalist species have responded more readily to the effects of climate change, and this outcome has been enhanced by land-use change.  Therefore, community level projections of the impact of climate change derived from spatial ecological gradients could contain incomplete short-term (or possibly long-term) predictions.