Distribution-wide effects of climate on population densities of a declining migratory landbird.

1. Increases in global temperatures have created concern about effects of climatic variability on populations, and climate has been shown to affect population dynamics in an increasing number of species. Testing for effects of climate on population densities across a species' distribution allows for elucidation of effects of climate that would not be apparent at smaller spatial scales. 2. Using autoregressive population models, we tested for effects of the North Atlantic Oscillation (NAO) and the El Niño Southern Oscillation (ENSO) on annual population densities of a North American migratory landbird, the yellow-billed cuckoo Coccyzus americanus, across the species' breeding distribution over a 37-year period (1966-2002). 3. Our results indicate that both the NAO and ENSO have affected population densities of C. americanus across much of the species' breeding range, with the strongest effects of climate in regions in which these climate systems have the strongest effects on local temperatures. Analyses also indicate that the strength of the effect of local temperatures on C. americanus populations was predictive of long-term population decline, with populations that were more negatively affected by warm temperatures experiencing steeper declines. 4. Results of this study highlight the importance of distribution-wide analyses of climatic effects and demonstrate that increases in global temperatures have the potential to lead to additional population declines.