| Summary: | Global temperature increases of 0.6 oC over the past century and predicted increases of 2o to 6 oC over the next century have prompted many studies on effects of global warming on the population dynamics of plants and animals. Studies of landbirds in Europe and North America have shown effects of climatic variation on productivity and survival. However, such effects have been seen to translate to changes in population densities in only a few studies of European landbirds. Thus, the extent to which global warming has the potential to cause population declines in North American landbirds has remained unclear. In the research presented here, I model 39 years of climatic data, including indices of the North Atlantic Oscillation and El NinÞo Southern Oscillation, and distribution-wide Breeding Bird Survey data on 21 North American landbird species, to examine potential relationships between large-scale climate change and changes in avian population densities. Results of these analyses indicate geographic variation in strength of the effect of climate on population densities, with stronger effects in regions in which climate has stronger effects on local temperatures. Results also indicate that for species exhibiting long-term declines, there is a relationship between strength of the effect of local temperatures on population densities and magnitude of population decline. Additional analyses indicate that Neotropical-Nearctic migratory species are more negatively affected by warm winter temperatures than are closely-related North American residents. These results support the hypothesis that differences in climatic effects on migrants and residents may follow from greater trophic mismatch for species that are unable to time migration in response to earlier peaks in food availability. Finally, results of this work show that species are more highly affected by densities of sympatric congeners in areas in which they are also affected by climate, indicating that increased effects of climate on food resources may increase the effects of competition between congeners. Overall, results of this research indicate that North American landbird densities are affected by annual changes in large-scale climate, such that continued increases in global temperatures have the potential to affect long-term changes in landbird population densities across the continent.
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