Age and the timing of breeding in a long-lived bird: a role for stress hormones?

P>1. Young birds often reproduce later in the season than older ones, with poorer breeding success, but the proximate mechanisms involved in such a pattern remain poorly studied, especially in long-lived species. One possible mechanism is the endocrine stress response which is accompanied by the release of corticosterone. Elevated corticosterone levels can trigger physiological and behavioural responses that may delay or even suppress reproduction. 2. We tested the hypothesis that the delayed timing of breeding of young birds may be related to a greater susceptibility to stress compared with older ones during the pre-laying period of the breeding season. For this purpose, known-age (7-44 years old) pre-laying snow petrels, Pagodroma nivea, were monitored for baseline and acute stress-induced corticosterone levels. We examined whether baseline and stress-induced corticosterone levels were related to age, and whether they could influence the decision to breed and egg-laying date. 3. Young snow petrels were more likely to skip the breeding season and to breed later than middle-aged birds. In addition, the oldest birds bred later than middle-aged ones, suggesting a possible senescence on laying dates. Baseline corticosterone levels were independent of age but young and very old birds were more sensitive to stress than middle-aged ones. However, there was no effect of stress-induced corticosterone levels on breeding decision and egg-laying date. Elevated baseline corticosterone levels during the pre-laying period were associated with a higher probability of skipping breeding in females and a delayed timing of egg-laying in both sexes. 4. These results suggest that the greater susceptibility of young breeders to stress was not the functional mechanism explaining their delayed timing of breeding. Baseline corticosterone levels, although independent of age, appear to be a more likely mediator of breeding decision and egg-laying date. In long-lived birds, the relationship between age and timing of breeding may be rather indirect, as other age-related factors such as breeding experience or foraging skills may have a deeper impact on baseline corticosterone than age per se.