Summary 1.Stress has pervasive consequences for the well-being of animals. Currently, understanding how individuals cope with stressors is typically accomplished via short-term quantification of blood glucocorticoids released after activation of the hypothalamic-pituitary-adrenal (HPA) axis. 2. We investigated whether the amount of corticosterone (CORT) deposited in growing feathers provides a long-term, integrated measure of HPA activity in birds using captive red-legged partridges Alectoris rufa as a model species. 3. We examined CORT levels in primary feathers induced to grow at the same time as stress series were performed with a capture and restraint protocol. Plasma CORT titres after stress-induced stimulation, but not baseline values, correlated with feather CORT. Feather levels showed the same pattern as plasma of decline across the breeding season, but more severely. 4. For females, CORT in naturally moulted flank feathers was highly and positively correlated with the number of eggs laid in the previous few months, but not clutch size of the following year. For males, the amount of black on a feather, known to be a social signal, was positively correlated with its CORT level. 5. The analysis of feather CORT is a novel methodology that allows for meaningful interpretations of how individuals respond to environmental perturbations and adjust to life-history stages. 6. The analysis of feather hormones has the unique advantages of allowing for experimentation and sampling at any time of the year with minimal investigator-induced impacts and artefacts, and shows the HPA activity of an individual with a flexible time frame from days to months depending on the length of time taken to grow the feather. As this technique can be applied to living or dead birds, or feathers picked up after moult, it provides the ultimate non-invasive physiological measure of considerable benefit in terms of animal welfare and sampling effort.

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Short-term elevation of circulating glucocorticosteroids (GCs) in vertebrates facilitates the adoption of a distinct emergency life history state, which allows individuals to cope with perturbations and recover homeostasis at the expense of temporarily suppressing nonessential activities. Although GC responses are viewed as a major evolutionary mechanism to maximize fitness through stress management, phenotypic variability exists within animal populations, and it remains unclear whether interindividual differences in stress physiology can explain variance in unequivocal components of fitness. We show that the magnitude of the adrenocortical response to a standardized perturbation during development is negatively related to survival and recruitment in a wild population of long lived birds. Our results provide empirical evidence for a link between stress response, not exposure to stressors, and fitness in a vertebrate under natural conditions. Recent studies suggest that variability in the adrenocortical response to stress may be maintained if high and low GC responders represent alternative coping strategies, with differential adaptive value depending on environmental conditions. Increased fitness among low GC responders, having a proactive personality, is predicted under elevated population density and availability of food resources, conditions that characterize our study population.animal personality ͉ corticosterone ͉ glucocorticosteroids ͉ reproduction ͉ survival E xposure to environmental perturbations constitutes a major selective force in natural populations. Animals have evolved behavioral and physiological strategies to avoid the deleterious effects of stressors, and among vertebrates, the adrenocortical response is one of the most conserved physiological mechanisms aimed at this end (1-3). In response to modifying factors (e.g., decreased food resources, predation, harsh weather), vertebrates activate the hypothalamous-pituitary-adrenal axis, which triggers a rapid release of glucocorticosteroids (GCs) from the adrenal glands into the bloodstream (4). Elevations of circulating GCs, in turn, redirect individuals into a distinct emergency life history state (3) with changes in physiology and behavior (e.g., increased gluconeogenesis and mobilization of fat stores, suppressed territorial and reproductive behavior, irruptive migration; refs. 2 and 4), aimed at coping with the perturbation and recovering homeostasis at the expense of temporarily suppressing nonessential activities. The quantification of circulating GC titers has become a useful tool in psychology, animal husbandry, and conservation biology because elevations of plasma levels constitute a physiological marker of exposure to stress (e.g., refs. 5-7), and the latter has deleterious effects on fitness. However, even within animal populations exposed to constant environments there is a strong interindividual variability in the adrenocortical response to standardized stressors (8-10), and it remains unknown whether such natural variability exerts ...