Ecological and life-history traits explain recent boundary shifts in elevation and latitude of western North American songbirds
Species are expected to move uphill or poleward in response to climate change, yet their distributions show idiosyncratic responses; many species are moving in the predicted direction, but others are not shifting at all or are shifting downhill or towards the equator. Fundamental questions remain about the causes of interspecific variation in range responses and whether shifts along elevational and latitudinal gradients are correlated. We examined whether shifts in northern-latitude and upper-elevation boundaries of western North American songbirds over a 35-year period were correlated and whether species ecological and life-history traits explained interspecific variation in observed shifts.
We used data from the North American Breeding Bird Survey to determine shifts in northern-latitude and upper-elevation boundaries of 40 North American songbird species between two time periods, 1977–81 and 2006–11. We used an analysis of covariance approach that controlled for species population trends and changes in survey effort to test whether: (1) songbirds shifted in elevation, latitude or both; (2) shifts in elevation and latitude were correlated; and (3) responses could be explained by species-level traits including life history, ecological generalization and dispersal capability.
The majority of species shifted uphill and poleward during this period, but there was no correlation between the distances that species range boundaries shifted in elevation and latitude. Species with smaller clutch sizes and narrower diet breadths exhibited greater northward shifts, while species with larger clutch sizes and narrower diet breadths exhibited greater uphill shifts.
Shifts in latitude and elevation were not correlated. However, a common set of species-level traits explained differential responses among species to climate change. Consideration of shifts in both elevation and latitude is needed to understand the full extent to which species are tracking changing climates.