In a changing environment, individuals must have the ability to physiologically cope with unpredictable stressors. The endocrine response to stressors is one mechanism vertebrates can use. The corticosterone (CORT)-fitness hypothesis predicts a negative relationship between acute stress induced levels of CORT and fitness. I examined the effectiveness of hypothalamo-pituitary-adrenal (HPA) axis regulation through exogenous hormone stimulation and plasma CORT levels as a predictor for various fitness measures in free-living song sparrows (Melospiza melodia). CORT levels at 30 minutes and in response to dexamethasone injection were predictive of lay date in this population. However, no relationships were observed between HPA regulation and other measures of fitness, including clutch size, proportion of eggs hatched, and adult survival (return rate). Thus, although there was no support for the narrowly defined CORT-fitness hypothesis, this study in combination with prior work suggests that the relationship between HPA-regulation and fitness can vary with life-histoiy stage and between year
