Identifying general responses to warming temperatures can help transform global change biology into a more predictive science. While much emphasis has been placed on shifts in range and phenology as major axes of adaptation to climate change, considerably less is known about the role of morphological adaptation. To better understand the role of morphological change in species responses to global warming, we ask three questions: 1) are there consistent morphological responses to warming temperatures, 2) does morphological change interact with shifts in phenology, and 3) are there species-specific attributes that predict rates of morphological change in response to shifting climatic conditions?
To answer these questions, we take advantage of a unique 40-year series of >70,000 bird specimens from 52 species of North American migratory birds that were collected after individuals collided with windows in Chicago, IL, USA during their spring and fall migrations. We find that there have been universal declines in size across these species, and that these size declines are associated with increasing temperatures on the breeding grounds. At the same time that birds have gotten smaller, their wing lengths have increased, representing a change in shape that is expected to increase flight efficiency. While more efficient flight may enable more rapid migration, advancing migratory phenology in these species has occurred independently from increases in wing length. Finally, while the direction of change in size and wing length has been remarkably consistent across species, the rates of change among species have been varied, with smaller species experiencing more rapid rates of morphological change. Surprisingly, body size explains more of this inter-specific variation in rates of morphological change than generation length. Together, our results suggest morphological change is a pervasive response to warming temperatures that is, in many ways, predictable. Understanding the consequences of these changes, and how they interact with shifts in range and phenology, will improve understanding of the impacts of climate change on natural systems.