Differential winter and breeding range shifts: Implications for avian migration distances

Aim For many migratory avian species, winter and breeding habitats occur at geographically distinct locations. Disparate magnitudes and direction of shifts in wintering and breeding locations could lead to altered migration distances. We investigated how shifts in the centre of abundance (COA) of winter and breeding ranges have changed for 77 species of short‐distance migratory birds. We addressed whether species tracked their historical average temperature and precipitation conditions at their winter and breeding COA, using data from 1990 to 2015. Location North America. Methods We calculated the COA for winter and breeding ranges from the National Audubon Society's Christmas Bird Count and the North American Breeding Bird Survey. We regressed the annual change in distance (km) between the two annual COAs of each species as a proxy for change in migration distance. We constructed a series of generalized linear mixed models (GLMMs) to evaluate changes in average temperature and precipitation at the wintering and breeding COAs. Results Winter shifts in COA were predominantly northward. For most species, average temperature and precipitation that species experienced had not changed. Breeding shifts in COA varied in direction. For breeding season COAs, average temperature warmed, but average precipitation had not changed. Thirty‐one species significantly decreased their migration distances, mainly driven by northward shifts in the winter range. Ten species increased their migration distances. Main conclusions Winter and breeding range shifts in COA have not occurred at the same magnitude and direction and have therefore impacted migration distance. Our results suggest that wintering and breeding range shifts occur independently and under different climate pressures.