Abstract: Species’ distributions are shifting in response to climate change. Many distributional shifts are predictably poleward or higher in elevation, but heterogeneity in the rate and direction of these shifts both within and between species appears to be common. We found heterogeneity in the trajectory of winter range shifts for 65 species of birds across eastern North America and in the different trait combinations and interactions associated with these shifts in the spatial scales we examined. We used data from the Christmas Bird Count to quantify the trajectory of and variation in winter latitudinal center of abundance range shifts over four decades (1980-2019) for 65 species of songbirds and woodpeckers, both across eastern North America (ENA) as a whole and for the Atlantic (ATL) and Mississippi (MISS) flyways separately. We then used linear models and AICc to test whether species traits could explain any of the variation in range shifts or flyway discrepancies. Across ENA, most species showed northward latitudinal range shifts, but some showed no apparent latitudinal shift while others shifted southwards. Amongst ATL and MISS, we documented both within- and between-species differences in the rate and direction of latitudinal shifts, complicating the results from across ENA. No single trait emerged as a dominant driver of range shift differences across scales. Migration strategy interacted with winter diet to explain variation at the largest spatial scale (ENA), whereas species with medium levels of frugivory and those with a medium mean winter latitude shifted at higher rates than other species in ATL and MISS, respectively. Exploring heterogeneity in range shifts within and between species, and in the associations between range shifts and life history traits, will help us both better understand the mechanisms that mediate differing responses to environmental change and predict which species will be better able to adapt to those changes.
