The recently introduced technique of measuring corticosterone in feathers currently provides the longest-term measure of corticosterone in birds. This review examines the strengths, weaknesses, and unresolved technical issues of the feather corticosterone technique. Feather corticosterone's major strengths are that it provides: a retrospective assessment of corticosterone physiology, including information from absent (unseen) or dead (e.g. museum specimens) individuals; a long-term measure of corticosterone exposure over the period of feather growth (days-weeks), integrating both baseline and responses to stressors; and flexible, minimally-invasive, sampling. However, researchers considering this technique should be aware of its limitations. Feather corticosterone only reflects hormone exposure during feather growth and, when sampling during molt, corticosterone titers and ecological conditions may not be representative of the majority of the annual cycle. Synchronization of molt is often unknown for a population, requiring assumptions when making inter-individual comparisons. Additionally, unresolved technical issues include: assessing whether corticosterone is the only hormone measured by assays; determining deposition dynamics to fully understand connections between feather and plasma corticosterone titers; studying the longevity and stability of corticosterone in the feather; establishing the impact of feather size and color on corticosterone deposition; and understanding the causes and implications of corticosterone variation along the length of the feather. Notwithstanding the above limitations and technical challenges, determining corticosterone titers in feathers is proving to be a useful technique for exploring some ecological and physiological correlates in individual birds. Given the unique perspective that feather corticosterone offers, we suggest that this measure complement, not replace, plasma measurements.

SUMMARYIntegrated measures of corticosterone (CORT), such as from feathers (CORT f ), have intuitive appeal because they incorporate both the duration and amplitude of glucocorticoid secretion. An association between CORT f and plasma CORT has never been shown in wild birds, and it is unclear as to when and whether these measures should be correlated, given that they are fundamentally different yet related measures of physiology. We hypothesized that CORT f should correlate with instantaneous measurements of plasma CORT when the latter reflect sustained changes in the activity of the hypothalamic-pituitary-adrenal (HPA) axis. To test this, we experimentally manipulated levels of plasma CORT in wild nestling tree swallows (Tachycineta bicolor) using 5day time-release CORT pellets, and measured plasma CORT and growth parameters before, during and at the end of hormone manipulation (days 7, 9 and 11 post-hatch, respectively). CORT f and plasma CORT were significantly positively related only when the latter was at its highest and most variable among individuals (day 9). A similar relationship was expected at day 11, but plasma CORT had returned to near-original levels. Nestlings with higher CORT f were smaller, lighter and less likely to fledge, but we did not detect seasonal effects on CORT f . Our results clearly demonstrate that CORT f from free-living birds can reflect plasma CORT, but correlations may not always be expected, especially if elevations in plasma CORT are relatively modest and of short duration. Our work suggests that CORT f is best used to study the activity of the HPA axis over relatively long time frames and can be used effectively to advance avian ecology. Supplementary material available online at

Enrichment is widely used as tool for managing fearfulness, undesirable behaviors, and stress in captive animals, and for studying exploration and personality. Inconsistencies in previous studies of physiological and behavioral responses to enrichment led us to hypothesize that enrichment and its removal are stressful environmental changes to which the hormone corticosterone and fearfulness, activity, and exploration behaviors ought to be sensitive. We conducted two experiments with a captive population of wild-caught Clark's nutcrackers (Nucifraga columbiana) to assess responses to short- (10-d) and long-term (3-mo) enrichment, their removal, and the influence of novelty, within the same animal. Variation in an integrated measure of corticosterone from feathers, combined with video recordings of behaviors, suggests that how individuals perceive enrichment and its removal depends on the duration of exposure. Short- and long-term enrichment elicited different physiological responses, with the former acting as a stressor and birds exhibiting acclimation to the latter. Non-novel enrichment evoked the strongest corticosterone responses of all the treatments, suggesting that the second exposure to the same objects acted as a physiological cue, and that acclimation was overridden by negative past experience. Birds showed weak behavioral responses that were not related to corticosterone. By demonstrating that an integrated measure of glucocorticoid physiology varies significantly with changes to enrichment in the absence of agonistic interactions, our study sheds light on potential mechanisms driving physiological and behavioral responses to environmental change.

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.