LAUSR

Aim Many studies postulate that physiological tolerance of climatic variables imposes the primary limit on species geographical distributions, that tolerances are constant through time, that climate has warmed and that geographical distributions shift to maintain species in their thermal niches when climate changes. However, recent studies present evidence that is inconsistent with each of these propositions. Here we ask: how strongly did avian species entire geographical distributions (as opposed to their latitudinal extremes) in North America track temperature changes between 1979 and 2010? Location The continental United States (excluding Alaska) and southern Canada. Methods We examined changes from 1979 to 2010 in the geographical distributions, and the realized temperature niches, of 21 species of passerine birds whose entire breeding ranges fall within the area well sampled by the North American Breeding Bird Survey. We related changes in breeding distributions to the concomitant changes in breeding season temperature. Results The median temperature increased within the breeding ranges of most, but not all, species. Temperature on the coolest 2.5% of routes increased significantly for only 8 of 21 species. Most species' distributions shifted geographically, but the most frequent shift was westward, not northward. Most species' realized temperature niches changed detectably through time, mainly as a result of changing temperature (versus geographical shifts). Where shifts in geographical distribution occurred, in most cases they did not result in smaller changes in species realized temperature niche than species would have experienced by not moving at all. There is little suggestion of a lagged response to climate change. Main conclusions We find only slight evidence that the geographical distributions of North American passeriform birds, considered in their entirety (as opposed to their latitudinal extremes), tracked temperature change. Of the factors that have driven recent shifts in the geographical distributions of North American avian species temperature change is probably only a minor one.