Integrating stress physiology, environmental change, and behavior in free-living sparrows.

As weather deteriorates, breeding animals have a diverse array of options to ensure survival. Because of their mobility, birds can easily abandon territories to seek out benign conditions away from the breeding site. The timing of abandonment, however, may have repercussions for territory size, mate quality, reproductive success, and survival. There is a large body of evidence indicating that the adrenocortical response to stress plays a role in mediating the onset and maintenance of this behavioral switch. Here we develop a model describing the interactions of weather, food availability, body condition, and stress physiology in initiating departure from the breeding site. We tested the model using a population of white-crowned sparrows breeding at high elevation in the Sierra Nevada, where severe weather at the beginning of the breeding season often induces temporary abandonment of breeding territories and facultative altitudinal migration to lower elevation refugia. The data show that (1). during inclement weather, exogenous corticosterone delays return to the breeding site after territory abandonment; (2). during good weather, exogenous corticosterone alone does not induce territory abandonment, but does increase activity range around the breeding site; and (3). the magnitude of the corticosteroid response to stress is inversely related to body condition of the sparrow.