Abstract
The glucocorticoid stress response mediates a suite of physiological and behavioural changes that allow vertebrates to cope with transient stressors. Chronically elevated glucocorticoid levels are known to result in a variety of organismal costs, but relatively little is known about the downstream effects of mounting a series of brief, acute spikes in circulating glucocorticoids. Conceptual models of stress suggest that repeated acute stressors might produce 'wear-and-tear' on the stress-response system when encountered in sequence. We used a novel technique to experimentally induce acute corticosterone spikes on either three or six consecutive days in incubating tree swallows. Consistent with the 'wear-and-tear' hypothesis, we found that (i) a sequence of corticosterone spikes produced cumulative effects on corticosterone regulation, (ii) treatment frequency predicted the severity of consequences, and (iii) individual variation in the ability to terminate the stress response through negative feedback predicted the duration of physiological disruption in the group that experienced the most frequent challenges. Our results illustrate the importance of assessing multiple aspects of the hormonal stress response and have implications for understanding both individual and population resilience to repeated transient stressors.