Geographic Variation in Winter Fat of Dark‐Eyed Juncos: Displacement to a Common Environment

Abstract

Main small temperature—zone birds fatten in winter and use this reserve when food becomes unavailable. The winter body mass of migratory Dark—eyed Juncos (Junco hyemalis) usually is greater where severe weather is more frequent, i.e., at higher latitudes or altitudes. The mechanism responsible for this geographic variation is unknown, and we asked whether geographically separated populations are composed of birds with different capacities or tendencies to fatten. We sampled the lipid index (LI, grams of lipid per gram of lean dry mass) of free—living juncos at three latitudes in two winters and found geographic variation. However, the pattern of variation in LI at the three latitudes was not consistent, either within or between winters. We also displaced wintering juncos from northern, intermediate, and southern sites and exposed them to a common environment in Indiana. One group of displaced birds was released in natural junco habitat (during three winters), and another was held outdoors in captivity (during two winters). Many released birds remained nearby, and when we recaptured some of them in late winter the LIs of the three geographic subgroups were statistically indistinguishable. It was also indistinguishable from the LI of juncos sampled at that time from the free—living Indiana population. Among the displaced birds that we held captive, we found no differences among the geographic subgroups either in LI or total body mass. In one of the years, late—winter LI and body mass of the captives did not differ from values for the released group or from values of newly caught Indiana juncos, but in the second year values for the captives were lower. Our results indicate the geographic variation in winter fattening of juncos is not a population—specific trait that persists independently of current environmental information and that it probably reflects a response to local environmental conditions. To our knowledge, this is the first experiment using free—living birds to test the role of the environment in maintaining geographic variation in a life history trait in the nonbreeding period.