Development of the corticosterone stress response in young northern mockingbirds (Mimus polyglottos)

In birds, additional adrenocortical secretion in response to stressors often redirects an individual's ongoing activities toward immediate life-saving activities, usually by facilitating an increase in food searching and food intake needed to meet periods of increased energy demand. We asked whether young birds, who are entirely dependent on parents for food acquisition and therefore unable to manipulate their own food intake, fail to show an adult-like adrenocortical response to the acute stress of capture and handling. In 1998, plasma profiles of acute corticosterone secretion (e.g., samples taken at the time of capture and 30 min later) were compared across seven age classes of Northern Mockingbirds (Mimus polyglottos) representing various age-related stages of foraging ability and opportunity. As predicted, young birds less able, or entirely unable, to readjust their own foraging effort exhibited significantly lower stress responses compared to adults. The magnitude of the stress response (at 30 min postcapture) increased and approached that of adults as young birds approached independence. Energetic condition was not correlated with the magnitude of the stress response at any age, suggesting that variation in its expression was most likely due to age alone. We also investigated at what level within the hypothalamic-pituitary-adrenal (HPA) axis the corticosterone response may be controlled in young birds. In 1999, baseline corticosterone samples were taken in 8-day-old nestlings and were immediately followed by intrajugular injections of adrenocorticotropic hormone (ACTH) or saline. While plasma corticosterone concentrations did not change in saline-injected nestlings, ACTH-injected nestlings showed a significant increase in plasma corticosterone concentrations similar to 30-min samples taken from adults. These results indicate that, while young birds do not normally show the corticosterone response, the adrenocortical tissue has the capacity to do so, and the control appears to be within the hypothalamic-pituitary component of the HPA axis. Collectively, our results indicate that the expression of the corticosterone stress response develops in concert with a young, altricial bird's ability to utilize it as it approaches independence; the reduced corticosterone secretion may also allow young, rapidly growing birds to avoid potential deleterious exposure to elevated glucocorticosteroid concentrations.