Nestling corticosterone response to microclimate in an altricial bird

Although altricial young are dependent on their parents during early life, they must respond to environmental variation to maintain homeostasis. The hormone corticosterone (CORT) may be an important link between environment and phenotype during early life; however, no previous study has experimentally assessed the sensitivity of CORT to nest microclimate in altricial birds beginning to thermoregulate. We tested the hypothesis that microclimate influences CORT by cross-fostering nestling Tree Swallows (Tachycineta bicolor (Vieillot, 1808)) between thicker-walled “aspen” nest boxes and thinner-walled “plywood” nest boxes. Quantification of CORT in nestling feathers allowed us to consider hormone secreted over days, rather than instantaneously from blood samples. In agreement with our hypothesis, we detected significant positive relationships between feather CORT and increased temperature variability and maximum, but not minimum, nest box temperatures. This could reflect the energetic challenge of warmer temperatures or positive developmental effects on the nestling hypothalamic–pituitary–adrenal axis. Feather CORT was significantly lower in chicks hatched in aspen nest boxes compared with plywood ones, but cross-fostering did not influence nestling CORT. This suggests that the influence of natal nest box environment on feather CORT was likely greater than the influence of the foster nest box environment. The relationships we detected highlight the sensitivity of feather CORT to environmental variation and contribute insight into nestling responses to environmental change.