Climate change in our backyards: The reshuffling of North America’s wintering bird communities

Background/Question/Methods

Poleward range shifts are a widely cited biological fingerprint of modern climate change, but documenting these shifts focus on individual species and little is known whether these shifts result in community-level changes. Here, we measure over twenty years of change in the composition of wintering bird communities across eastern North America. We document this change using the community temperature index (CTI). CTI is a functional index that measures the balance between low- and high-temperature dwelling species. Using data from Project Feeder Watch, an international citizen science program, we quantified spatiotemporal changes in wintering bird communities (n = 40 bird species) and tested the influence of recent changes in winter minimum temperature. We used jackknife analysis to identify which species were the most influential in driving changes at the community level and identified which population dynamics are driving these community changes (e.g., extinction vs. colonization). Given the importance of minimum temperature in constraining species distributions at northerly latitudes and recent winter warming, we hypothesized that wintering bird communities would show an increase in CTI (more warm-adapted species) and these patterns would be driven by range expansion in northerly latitudes.  

Results/Conclusions

Over the span of two decades, the wintering bird communities of eastern North America have changed significantly and became dominated by warm-adapted species. Contrary to our prediction, increases in CTI were strongest in the more southerly latitudes (+6.9 ± 1.4 x 10^-3 °C yr^-1). Regardless of latitude, however, changes in CTI were driven by local increases in the abundance and regional colonization of southerly birds. Throughout eastern North America, increasing populations of warm-adapted, small-bodied birds were responsible for positive trends in CTI. We found that climate change, in the form of rising winter temperatures, has reshuffled wintering bird assemblages throughout eastern North America and has led to an annual northward shift in communities of about 6.6 km yr^-1.