Are North American bird species' geographic ranges mainly determined by climate?

Aim It is commonly asserted that climate presents the primary constraint on species’ geographic distributions, and that species’ distributions should therefore shift to track changing climate. However, the evidence is surprisingly mixed about the causal link between species’ distributions and climate. Correlations between distributions and climate may be indirect, reflecting influences of other spatially structured habitat variables (e.g., land cover) and/or population processes (e.g., dispersal). Here, we ask if species’ geographic distributions are more strongly related to climate, or to other spatially structured variables. Location The contiguous United States and southern Canada. Time period 1990–2000. Major taxa studied North American passerine birds. Methods We used the breeding‐season distributions of 19 widespread species of passerine birds whose breeding‐season distributions fall entirely within the area sampled by the North American Breeding Bird Survey. We related these distributions to temperature and precipitation, geographic coordinates, and the degree of occupation of neighboring sites by conspecifics. Two spatial scales were examined: the geographic location of species’ ranges within North America, and site occupancy within species' ranges. We examine these relationships using generalized linear models, structural equation modeling, random forest models and spatial correlograms. Results On average, geographic coordinates and a model of neighborhood occupancy outperform a simple climatic model. After controlling for geographic coordinates, species occupancy is poorly related to climate. Neighborhood occupancy accounts for the majority of variance captured by geographic coordinates within ranges, and more for the continental placement of ranges. On average, species’ distributions are more strongly and more directly related to spatial coordinates, and neighborhood occupancy, than to climate. Main conclusions Our results are inconsistent with the hypothesis that climate is the primary, direct determinant of species’ geographic distributions. Rather, other spatially structured factors appear to be stronger determinants of both continental range placement and within‐range distributions of this sample of North American birds.