Development of the corticosterone stress response in young Northern Mockingbirds (Mimus polyglottos)

In birds, additional adrenocortical secretion in response to stressors ofte n 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 an d therefore unable to manipulate their own food intake, fail to show an adu ltlike 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 a ge classes of Northern Mockingbirds (Mimus polyglottos) representing variou s age-related stages of foraging ability and opportunity. As predicted, you ng birds less able, or entirely unable, to readjust their own foraging effo rt exhibited significantly lower stress responses compared to adults. The m agnitude of the stress response (at 30 min postcapture) increased and appro ached that of adults as young birds approached independence. Energetic cond ition was not correlated with the magnitude of the stress response at any a ge, 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 nes tlings and were immediately followed by intrajugular injections of adrenoco rticotropic hormone (ACTH) or saline. While plasma corticosterone concentra tions did not change in saline-injected nestlings, ACTH-injected nestlings showed a significant increase in plasma corticosterone concentrations simil ar to 30-min samples taken from adults. These results indicate that, while young birds do not normally show the corticosterone response, the adrenocor tical tissue has the capacity to do so, and the control appears to be withi n the hypothalamic-pituitary component of the HPA axis. Collectively, our r esults indicate that the expression of the corticosterone stress response d evelops in concert with a young, altricial bird's ability to utilize it as it approaches independence; the reduced corticosterone secretion may also a llow young, rapidly growing birds to avoid potential deleterious exposure t o elevated glucocorticosteroid concentrations, (C) 2000 Academic Press.