Age-related variation in the adrenocortical response to stress in nestling white storks (Ciconia ciconia) supports the developmental hypothesis

Abstract

The post-natal development of the adrenocortical response to stress was investigated in European white storks. Sixty wild nestlings aged 24–59 days old were subjected to a standardized capture and restraint protocol, and the time-course pattern of the response to stress was assessed through determination of circulating corticosterone in blood samples collected at Wve Wxed times during the 45-min period following capture. The time course of the response was best Wt to a third-order function of handling time, and showed a strong eVect of age. Although age did not aVect baseline titers and all birds showed a positive post-capture increase in circulating corticosterone, age had a positive eVect on the relative increase from baseline titer, the recorded time to reach maximum level, and the acute concentration after 10 min following capture and restraint. While young nestlings displayed very little response to capture, the response near Xedging resem- bled the typical adrenocortical pattern widely reported in fully developed birds. Our results concur with those found in altricial and semi- altricial species, and suggest that non-precocial birds follow a similar mode of development of the hypothalamic–pituitary–adrenal (HPA) axis. The fact that HPA sensitivity to stress is functional suggests that young storks gradually develop emergency responses of adaptive value and are able to overcome acute perturbations in spite of their parental dependence, at least during the last two-thirds of post-natal development. According to the Developmental Hypothesis, such gradual changes would allow nestlings to respond to pertur- bations as a function of the speciWc behavioral and physiological abilities of their age. The potential sources of stress that nestlings have to face during development (i.e., weather conditions, dietary restrictions, and social competition) are discussed according to developmen- tal changes in behavioral and physiological abilities