Neonatal handling alters the development of the adrenocortical response to stress in a wild songbird (eastern bluebird, Sialia sialis).

Neonatal handling of captive vertebrates can shape the development of their hypothalamo-pituitary-adrenal (HPA) axis and alter their ability to respond to stressful stimuli later in life. However, the long-term effects of such handling on this endocrine axis in free-living species are not well understood. We investigated the effects of age and neonatal handling on corticosterone secretion in response to restraint in eastern bluebird (Sialia sialis) chicks. We found that unhandled ("naïve") and handled ("experienced") chicks exhibited no corticosterone response to handling early in development. Thereafter, naïve individuals exhibited the progressive development of a corticosterone response with age, and by day 12 post-hatch, the response resembled that of adult bluebirds. Experienced nestlings, which were handled every other day from the day of hatch, showed a similar pattern of HPA development until day 12 post-hatch, when their corticosterone response was significantly reduced compared to that of naïve nestlings. In contrast, chicks that were handled only once, when 10days old, did not show a reduced corticosterone response at 12days old. Taken together, our data suggest that a certain threshold of accumulated neonatal handling episodes is necessary to depress corticosterone secretion, and/or that the cumulative effects of several handling episodes only manifest themselves once the HPA axis is fully developed. Our findings, in concert with studies on two other wild species, indicate that routine handling of nestlings in the field can alter their responses to stress in a species-specific manner, potentially leading to important fitness consequences.

A single exposure to an acute stressor has lasting consequences for the hypothalamo-pituitary-adrenal response to stress in free-living birds.

In vertebrates, activation of the hypothalamo-pituitary-adrenal (HPA) axis in response to unpredictable events results in elevated glucocorticoid secretion. Repeated exposure to stressors alters subsequent glucocorticoid secretion, either by inducing chronic stress or as a result of habituation. However, most studies of repeated stress focus on the impacts of multiple and frequent exposures to acute stressors, and few have been carried out in free-living animals. We investigated whether a single exposure to a novel stressor was sufficient to produce long-lasting alterations in HPA function in free-living eastern bluebirds (Sialia sialis). We subjected adult females to a capture/restraint protocol in which we collected serial blood samples over an hour of restraint to be analyzed for corticosterone. We administered this protocol to three groups of females during the nestling phase of their first and/or second brood of the season: Repeaters (sampled during brood 1 and brood 2), Naïve-Brood 1 (sampled only during brood 1), and Naïve-Brood 2 (sampled only during brood 2). Repeaters had attenuated corticosterone responses to the second restraint bout compared to the first, and in brood 2, Repeaters had lower responses than Naïve-Brood 2 females. However, Naïve-Brood 1 and Naïve-Brood 2 birds did not differ in their responses to restraint. Thus, as little as one prior experience with an acute stressor was sufficient to alter subsequent HPA responsiveness, and this effect was not due to a natural change in HPA responsiveness as the breeding season progressed. These data may have important implications for understanding how acute stressors can alter a free-living animal's ability to cope in the face of subsequent stressors, and for longitudinal field studies in which individuals are repeatedly sampled for glucocorticoid responsiveness.