Winter fattening in the dark-eyed junco: plasticity and possible interaction with migration trade-offs.

Although fat often supplies the major source of metabolic fuel during winter fasts of birds, this critical life-history trait is little studied by ecologists. In the dark-eyed junco Junco hyemalis, we have in a series of studies investigated the extent of plasticity in the winter fat reserve. Earlier (Rogers et al. 1993), we reported (1) a highly variable pattern of geographic variation in the winter fat reserve of junco populations in eastern North America, (2) disappearance of statistically significant interpopulation variation after experimental displacement to a common latitude, and (3) post-displacement temporal variation in the fat reserve. In analyses reported here, recent temperature, recent snowfall (a measure of short-term predictability of resources), season (perhaps reflecting continued exposure to unpredictable resources) and daylength explained spatial variation in the fat store. Recent temperature explained temporal variation in the fat reserves of groups of displaced juncos. These results suggest that platticity in a life-history trait has evolved in an uncertain winter environment. Through environment-dependent fattening, the costs of fat can be avoided during warm periods and at locations where fat confers little benefit, whereas benefits of fat can be quickly gained if weather conditions become harsh and snowfall might restrict food. Three types of winter fatteners probably exist among birds: responders (fatten in response to the proximate environment), predictors (fatten in anticipation of long-term environmental conditions), and responder-predictors (combination of both types of regulation). Because dark-eyed juncos select different winter latitudes as they age, we hypothesize that the nonbreeding component of the life-history of juncos includes the co-adapted plastic traits of winter fattening and post-breeding migration. Life-history theory can apparently explain important traits related to fitness in the nonbreeding period.