Activation of the adrenocortical response to stress during development can have fundamental consequences over the lifetime of the organism; as such, many organisms are less responsive to stress during critical developmental periods. In this study, we evaluated stress reactivity in nestling white-crowned sparrows, examining corticosterone and binding globulin levels in response to both restraint stress and ACTH challenge. Restraint stress induced a significant corticosterone response in both 4 to 6- and 7 to 9-day-old nestlings, but not in the youngest group (1-3 days); this non-significant increase in corticosterone in the youngest birds resembles the mammalian hyporesponsive period, wherein young animals are resistant to most stressors. Binding globulin levels appear to extend this period of low reactivity: when free corticosterone levels were calculated, only the oldest age group (7-9 day) showed a significant response to restraint. ACTH challenge data revealed that all ages of white-crowned sparrow nestlings responded to exogenous ACTH treatment with significant elevation of corticosterone, although early-stage nestlings did not reach adult levels of response.