Spatial Autocorrelation and Local Disappearances in Wintering North American Birds

I examined the degree to which population sizes of North American wintering birds exhibit spatial synchrony (autocorrelation) and local disappearances based on 30 yr of Christmas Bird Counts. The primary goal of these analyses was to determine whether processes expected in metapopulations, including low spatial synchrony, frequent local disappearances, and colonization leading to population rescue, play important roles in species that are not necessarily subdivided into discrete patches. In general, spatial autocorrelation was less than expected based on environmental factors, with only one-third of species significantly spatially autocorrelated between sites up to 100 km apart and only three (1%) spatially autocorrelated on a continental scale. Spatial autocorrelation was significantly lower for primarily aquatic compared to terrestrial species and tended to be higher among small, widely distributed, and relatively abundant species with more northern winter distributions and low relative population variability. Local disappearances (years when no individuals were detected) were also common and tended to last longer in aquatic, migratory species high in the food chain. When controlling for confounding factors, there was a significant positive relationship between spatial synchrony and the length of disappearances among resident species, as expected if population rescue contributes to interspecific variation in these parameters. These results support the hypothesis that processes expected in metapopulations may be important in many resident bird species. They also provide insight as to which species are more vulnerable to global extinction as well as those for which monitoring efforts aimed at detecting large-scale population declines may be performed efficiently by extrapolating from data acquired at relatively few sites.