Thesis (Ph. D.)--University of Washington, 2006.Glucocorticoid hormones have multiple purposes. At low levels, they function to maintain basic energy and salt balance. At high levels, they are key hormones in the emergency response to noxious stimuli. In the short term, glucocorticoids help the individual cope with a given challenge. Chronic exposure to high glucocorticoid levels can be detrimental to health and may increase mortality. In this dissertation, I explore how corticosterone (CORT) is modulated with life stage, season, habitat, and perturbations in song birds.In rufous-collared sparrows (Zonotrichia capensis costaricensis), I studied the effects of season, breeding, and molt on baseline CORT levels and the CORT response to stress. Month explained most of the variation in the CORT data, with higher CORT levels occurring in the spring. Breeding individuals had a higher HPA axis response to stress than non-breeding individuals, most likely to favor self-maintenance over a current reproductive attempt. Unlike past research, we found that CORT levels did not change or were higher in molting birds.I measured CORT levels, body condition, behavior, and hematocrit in song wrens (Cyphorhinus phaeocephalus) along a rainfall-induced habitat gradient. Birds living in drier habitat had lower body condition and were more likely to have an abnormally low hematocrit score. The relationship between rainfall and baseline CORT was not significant, but birds with the highest baseline CORT levels lived at the dry edge of the range. Birds in better body condition and with lower baseline CORT levels were captured more quickly. Our results indicate that physiology and behavior can change with an environmental gradient.To better understand the consequences of living in a disturbed environment, I studied the effects of repeated, acute pulses of CORT on the HPA axis and body condition in captive, wild Gambel's white-crowned sparrow (Zonotrichia leucophrys gambelii). CORT-treated birds had higher endogenous baseline CORT levels and failed to increase CORT levels with exposure to stress. Body mass, flight muscle, and food intake all declined with CORT treatment. CORT-treated birds expressed migratory restlessness but delayed the onset of molt. We conclude that frequent, acute CORT administration can create a chronic stress phenotype
