Disturbance history alters the development of the HPA axis in altricial nestling birds

Glucocorticoid (GC) hormones regulate the vertebrate stress response and are secreted by the hypothalamic-pituitary-adrenal (HPA) axis. Acute elevation of GCs is thought be adaptive because it promotes physiological and behavioural changes that allow animals to cope with disturbances. In contrast, chronic elevation of GCs is associated with reduced body condition, immune function, reproductive success, and survival. In adult birds, the effects of chronic stress have been well documented, including human-related disturbances. In contrast, the effects of chronic stress on nestlings have rarely been addressed. This is of interest, as many ecological or monitoring studies of wild birds require nestlings to be regularly handled. However, the consequences of repeated handling of nestlings on HPA axis function and body mass in wild birds remain poorly quantified. We examined whether daily exposure to handling stress increased corticosterone (the dominant avian glucocorticoid) secretion and reduced pre-fledging body mass relative to undisturbed control nestlings of the noisy miner (Manorina melanocephala), a native Australian passerine bird. Daily handling resulted in an elevated baseline and attenuated stress-induced corticosterone levels in disturbed 14-day nestlings, in comparison with control nestlings handled for the first time. Despite this, disturbed and control nestlings fledged at a similar body mass. Baseline and stress-induced corticosterone increased with nestling age but remained independent of nestling sex and hatching order. Our findings are some of the first to suggest that chronic handling stress causes physiological alterations to the development of the HPA axis in nestling birds, and our data suggest that researchers should minimise or account for handling stress in their experiments. These data also raise the possibility that other chronic stressors may have long term physiological consequences for the development of the HPA axis in nestling birds.