Stress in the nest: Causes and consequences of adrenocortical secretion in developing birds

Abstract

During the last decade, a vast number of studies have considerably increased our understanding of the interplay between the ecology and the endocrinology of the stress axis in wild avian populations. A theoretical framework known as the ‘Emergency Life History Stage’ has been developed to explore the ecological bases of stress and underlying endocrine mechanisms. Despite a growing body of literature on the role of hypothalamic-pituitary-adrenal (HPA) function in adult birds, little research has been performed in birds during development, possibly due to the wealth of variability in developmental strategies within the Class Aves, and the lack of evidence that non-precocial species have functional stress responses before parental independence. A series of investigations on HPA function in nestling white storks (Ciconia ciconia) under natural conditions have provided novel insights into the proximate causes and ultimate fitness consequences of adrenocortical responses to stress in developing birds. The aim of this review is to provide an integrated summary of recent research on endogenous and exogenous factors affecting adrenocortical function using stork chicks as a study model. Among the endogenous factors that explain differences in HPA function, nestling age constitutes a prime source of variability, which is discussed here within the context of the Developmental Hypothesis. Superimposed on this ontogenic pattern, exogenous factors of ecological origin such as food availability, habitat and parental quality generate interindividual differences in baseline corticosterone (cort) secretion. These findings support the notion that non- precocial nestlings can activate emergency responses to environmental perturbations despite their parental dependence for food and shelter, and suggest a role of environmental stressors in the preparation for fledging and independence. A second set of exogenous factors of concern for conservation biologists is the exposure of wild avian populations to environmental pollutants. Maximum levels of cort following capture and restraint were positively related to storks´ blood lead levels. This relationship was stronger in single nestlings than in birds from multiple-chick broods, suggesting a greater impact of pollutants on individuals additionally exposed to ecological stressors such as poor parental quality. With the aim of assessing the ultimate fitness consequences of individual variability in stress responses, the probability of survival and recruitment was modelled using information of a long-term study that accounted for all the factors described above. Survival and recruitment were negatively related to the magnitude of the adrenocortical response during development, providing the first empirical evidence for a link between stress- response early in life and subsequent fitness in a wild, long-lived vertebrate. In the light of these findings, here we review the published literature on this topic and discuss the current state of knowledge regarding adrenocortical function in developing birds, highlighting study gaps and suggesting future research directions.