Chapter 33 - Stress in Birds

Abstract

Birds respond to the perception of potentially noxious stimuli by activating the adrenocortical response to stress. The resulting elevation of circulating corticosterone (the predominant avian “stress hormone”) promotes highly adaptive physiological and behavioral changes aimed at coping with stress, redirecting the individual into a survival mode while suppressing non-essential activities. However, corticosterone is also required at low levels in the circulation for basic physiological processes, and it plays a fundamental role in the regulation of body energy balance regardless of exposure to stress. Furthermore, high and prolonged elevations like those occurring during chronic adverse conditions exert deleterious effects on critical body systems. The ambiguity of the concept of stress, and the fact that the same hormone exerts opposing (“good and bad”) actions depending on its level, has led to the recent development of novel conceptual frameworks. These are aimed at integrating the energy requirements of birds across their lifecycles with the adrenocortical response, and also at explaining the role of corticosterone in “allostasis”, the maintenance of homeostasis through change, a term gradually replacing the word “stress”. The Allostasis Model and the Reactive Scope Model are recent paradigms to understand allostasis in wild animals, with a vast potential for testable predictions in wild birds. The two models will be explained in the first section of this chapter, with an updated classification of the nomenclature describing corticosterone levels and actions. Avian endocrine and behavioral responses are dramatically different when facing predictable versus unpredictable environmental change (also called perturbations or stressors). Why, when, and how environmental conditions promote an activation of the hypothalamus-pituitary-adrenal axis will be addressed in a second section, providing an updated classification of the types of perturbations. This classification takes into account the duration of the unpredictable stimuli, the associated adrenocortical response, and the resulting effects on the normal progression of an individual’s lifecycles. Avian lifecycles can be temporarily or permanently disrupted in response to perturbations. Although short-term elevations of circulating corticosterone activate a highly adaptive “emergency life history stage”, longer exposures disrupt lifecycles, potentially leading to individual deaths and population extinction. However, the adrenocortical response shows all the features of a trait subjected to natural selection (high individual variation, repeatability, and a genetic basis), and individuals display a strong phenotypic plasticity that may allow differential survival of stress-tolerant individuals. Several examples of the adaptive variability in the adrenocortical response will be provided in a third section. For example, avian developmental modes range across a spectrum of altricial and precocial species, and the adrenocortical response at hatching and during growth increases across this gradient. This pattern has likely evolved to balance the costs and benefits of corticosterone actions with the ability of young birds to cope with perturbations without parental support. Corticosterone can be also transferred from mothers to offspring. This process has been termed “maternal programming”, and has the potential to translate ecological and environmental conditions into permanent offspring responses, resulting in phenotypes better adapted to cope with perturbations. Adult birds of many avian species have been shown to modulate their adrenocortical response during reproduction and according to the value of their brood. These findings suggest that modulation of corticosterone secretion allows a trade-off of energy and resources between current reproductive investment and survival, providing a proximate (corticosterone-mediated) mechanism for life history evolution. Many of these findings and hypotheses result from field research using wild avian species, which imposes important challenges compared to laboratory studies. These will be addressed in the last section, with a description of common methodological tools for assessing adrenocortical function in wild birds.