Development of the corticosterone stress response differs among passerine species.

Glucocorticoids are steroid hormones which increase dramatically in response to a physical or perceived stressor. However, developing young of altricial species typically have a damped glucocorticoid stress response. The developmental hypothesis posits that the physiological stress response should develop concurrently with an individual's ability to respond to a challenge. The dampened response may benefit an organism, as chronic exposure to glucocorticoids can have short- and long-term detrimental effects, and altricial young are unable to escape most stressors. However, we do not know if or why species with similar ontogeny vary in their development of the physiological stress response. We assessed levels of baseline and stress-induced corticosterone (the main avian glucocorticoid) in six passerine species with varying life-history strategies, including a brood parasite, the brown-headed cowbird (Molothrus ater). Circulating baseline corticosterone levels increased with nestling age for all species. Stress-induced corticosterone levels sampled at 15-min post-capture significantly increased with nestling age at a similar rate and magnitude in brown-headed cowbirds, eastern phoebes (Sayornis phoebe), hooded warblers (Setophaga citrina), red-winged blackbirds (Agelaius phoeniceus), and song sparrows (Melospiza melodia). However, gray catbird (Dumetella carolinensis) nestlings showed an extremely dampened elevation in corticosterone in response to stress, even near fledge. Gray catbirds are unusual among songbirds, as they are open-ended song learners. Stress during development can profoundly influence avian song learning and performance abilities. However, further study is necessary to determine if there is a relationship between nestling adrenocortical activity and open- versus closed-ended song learning.