Winter range shifts and their associations with species traits are heterogeneous in eastern North American birds

Many 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 shifts both
within and between species appears to be common. We found high
heterogeneity in the trajectory of winter range shifts for 65 species of
birds across eastern North America and in the different traits and trait
interactions associated with these shifts across the spatial scales we
examined. We used data from the Christmas Bird Count to quantify the
trajectory of winter latitudinal center of abundance range shifts over
four decades (1980–2019) for 65 species of songbirds and woodpeckers in
North America, 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 model selection to test whether species traits
could explain variation in range shifts or flyway discrepancies. Across
ENA, most species showed northward latitudinal range shifts, but some
showed no 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 at the ENA and flyway scales. Migration strategy
interacted with insectivory to explain variation at the largest spatial
scale (ENA), whereas frugivory and mean winter latitude explained much of
the variation 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 better understand the
mechanisms that mediate differing responses to environmental change and
predict which species will be better able to adapt to those changes.