The breeding distribution of a migratory bird fluctuates with nonbreeding season rainfall over the last century

Species responses to climate change include shifts in distribution, abundance, and range limits. Predicting such shifts for migratory birds is inherently complex given the diversity of ways climate change can impact species throughout their annual cycles. For example, recent findings demonstrate that the breeding origin of a nonbreeding population of American Redstarts (Setophaga ruticilla) in Jamaica is shifting southward in response to prolonged drought on the nonbreeding grounds differentially causing lower survival in longer migrating individuals. Here, we examine how widespread this mechanism has operated over the past century and across the range of this species. We sampled tail feathers from redstart museum specimens and live birds from three time periods dating back to the turn of the 20th century from 5 regions across their nonbreeding range (Andean, Greater Antillean, Isthmian, Pacific Slope, and Yucatan) and used stable hydrogen isotopes to estimate changes in breeding origins. We used an ANCOVA to estimate the strength of the shift in mean breeding origin for each population of nonbreeding redstarts since the turn of the 20th century, and the role of nonbreeding season rainfall in driving observed shifts. Populations of redstarts on their tropical nonbreeding grounds experiencing a drying trend in rainfall showed a corresponding southward shift in their mean breeding origin (and vice versa) in subsequent years. The link between nonbreeding rainfall and mean breeding origin was most pronounced in the modern time period, where nonbreeding rainfall has decreased across most of the redstart nonbreeding range. Our findings illustrate how complex mechanistic drivers operate over space and time to help shape breeding range dynamics for a migratory bird, and emphasize how climate impacts species distributions throughout the annual cycle.