Temperature and The Northern Distributions of Wintering Birds

Abstract

Root (1988b) proposed that a physiological limit on metabolic rate constrains the northern distributions of wintering birds. Thermoregulation requires that metabolic rate increase as temperature declines, and hence a ceiling on metabolism that is reached before food or other factors become limiting would set the northern extent of a species distribution. I compared data with two predictions stemming from the proposal. First, there should be close correspondence between the northern range boundaries of species and temperature isotherms. To test this, I re—evaluated the data and found that only 8—42% of the species that Root classified as limited by temperature had northern range boundaries that corresponded to particular temperature isotherms. The second prediction examined was new: Species' northern limits should scale to body mass, because the ceiling on metabolism is stated as a multiple of basal metabolic rate, which itself scales to body mass. Large species should extend farther north than small species. A test of the prediction using Root's data showed little evidence that a constraint strongly affects the distributions of birds. In a second test, I analyzed additional recent Christmas Bird Counts that include areas farther north than were available to Root. I found that species of all sizes occur at the lowest temperatures, indicating that a ceiling on metabolism does not generally constrain the distributions of birds wintering in North America. My analysis turned up one other unexpected result: Few large passerines in North America have northern distribution boundaries that occur in warm, southerly areas, whereas small species have northern distribution boundaries in southern as well as in northern areas. The pattern might reflect the relationship between body size and the size of a species' geographic range coupled with the geography of North America. Large species tend to have only large geographic ranges, whereas small species are not so restricted. The avifauna of North America north of Mexico is bounded by a barrier formed by the Gulf of Mexico and a turnover in avifauna between the United States and Mexico. To the north of such a barrier, the northern distribution boundaries of large species should be well away from the barrier because of their large size.