Change in Avian Abundance Across the Geographic Range in Response to Environmental Change

The abundance changes of three North American passerine bird species after the harsh winters of the late 1970s were analyzed to determine relationships between proportional abundance change and prewinter abundance, position within the geographic range, and winter severity. Analyses were made between both the prewinter period and a period of declining abundance immediately after the winters and between this decline period and a later period of recovery in which abundance increased. The proportional declines in abundance showed no relationship to prewinter abundances. Proportional abundance change was greatest at sites closer to the range edge and, for two species, at sites with a more severe winter. Proportional abundance increases of these two species between the decline and recovery periods similarly were greatest at sites near the range edges and with previously more severe winters. Abundance increases were greater at sites with the lowest predecline abundance. Geographic range structures showed changes parallel to the abundance changes, including overall contraction and expansion. In addition, there was evidence of fine-scale range dynamics, including extinction and colonization events toward the range margins and a roughening of the range boundary with abundance decrease followed by a smoothing of the boundary with abundance increase. Periods of major environmental change, such as these severe winters, may provide insight into how populations of organisms will respond to global climate change. In particular, this study suggests that population declines will be accompanied by a contraction toward core areas of the range that formerly had the highest abundance. However, more investigation is clearly needed into the precise nature of broad-scale population dynamics following environmental perturbations that cause abundance change.