Ecologically Relevant Cooling Early in Life Alters Prefledging Adrenocortical Response in Free-Living Songbirds

In vertebrates, exposure to stressful stimuli early in development may alter the activity of the hypothalamo-pituitary-adrenal (HPA) axis, with the potential for fitness consequences later in life. For altricial species, whose young rely on their parents for food, warmth, and protection from predators, adult behavior can modify the impact of some stressors on their offspring after birth or hatching. We have shown that single bouts of cooling that normally occur when brooding females leave the nest elevate corticosterone secretion in very young free-living eastern bluebird (Sialia sialis) chicks. Thus, natural variation in maternal brooding patterns can result in differential exposure of offspring to cooling, and also to glucocorticoids, very early in development. We tested the hypothesis that exposure to repeated bouts of cooling (mimicking those that occur normally when females leave the nest) would alter the activity of the chicks' HPA axis later in life. We exposed free-living chicks to either four 18-min bouts of cooling or brooding temperatures (control) during the first week after hatching. Then, just before fledging (i.e., at least 7 d after the cooling treatments had ceased), we assessed their corticosterone responses to restraint. Repeatedly cooled chicks had a significantly lower corticosterone response to restraint than did control chicks but did not differ from controls in other measures of growth and development. Our data suggest that natural variation in maternal brooding patterns, and hence natural variation in the chicks' body temperature, can alter the activity of the HPA axis well beyond the brooding period.