Extreme events are becoming more frequent as climate change increases the mean and variability of global temperatures. Despite strong evidence that temperature and precipitation affect avian distributions, we know very little about the different effects of mean and extreme temperature and precipitation on bird populations, and how responses may vary across a species’ geographic range. We used North American Breeding Bird Survey (BBS) data to ask how changes in interannual weather affected yearly population changes of 156 passerine bird species across their ranges from 1980–2016. Using daily climate surfaces, we calculated local breeding-season, post-breeding season, and late-summer temperatures and precipitation, as well as anomalies, for each year. We combined climatic data with the following year’s population and modeled the effects of climate on each species’ local population change. We hypothesized hot temperatures and high precipitation would be worse for birds in the hot or wet portion of their breeding distribution, as extreme weather here may push birds close to their physiological limits.
Results/Conclusions
Contrary to our hypotheses, breeding and post-breeding temperatures and precipitation do not show strong relationships to interannual population changes across birds’ North American distributions, potentially due to phenological timing of the breeding season across the latitudinal gradient. Breeding and post-breeding temperatures and precipitation showed little variation across the distributions of the passerines included in this study. In accordance with our hypothesis, higher temperatures during the late summer (July/August) showed a negative relationship with interannual bird populations. This relationship included an interaction with the position in the breeding distribution with warmer temperatures in the hot parts of the distribution having a significantly stronger negative impact on populations compared to the cooler portions of the distribution. Higher precipitation in the late summer had a slightly positive relationship with bird populations across the wet and dry portions of the range. As extremes become more common with continuing climate change, it is imperative to understand how species will react at both the population level and at larger spatial scales, and our results suggest birds in the warm portions of their breeding distribution may be at higher risk than their cooler counterparts.