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Synopsis The glucocorticoid mediated stress response plays a major role in coping with both gradual and rapid changes in environmental conditions and may be especially important when conditions depart from expectations. Conceptual models of endocrine flexibility suggest that individual flexibility, measured using reaction norms along an environmental gradient, might predict differences in the ability to cope with challenges. For example, differences in the speed or scope of acute endocrine responses might underpin coping ability. However, empirical results have been limited by the inability to accurately measure individual level endocrine reaction norms. Here, we took advantage of a database of corticosterone measures in 1727 individuals of 99 bird species sampled around the world to extend the concept of endocrine reaction norms to species differences. We first describe a global reaction norm for birds and then demonstrate species-specific differences in reaction norms for baseline corticosterone, maximum corticosterone, and the speed of corticosterone increase to both absolute temperature and to the difference between current and expected temperature. Overall, we found that in addition to changes in absolute corticosterone, the speed of the acute response increased when minimum daily temperature dropped below 0°C–2°C. In contrast, we found little evidence for increases at higher temperatures. We found a similar pattern when temperature was colder than expected given the location and date regardless of absolute temperature, but this effect was only seen for baseline corticosterone. Our models also consistently indicated that species differed in the shape of their corticosterone reaction norm to absolute temperature and temperature deviations. However, we did not have adequate data to fully characterize species-specific reaction norms. We suggest that the endocrine flexibility and reaction norm framework applied in a comparative context could help predict species sensitivity to changing climate, but that additional field data will be needed to fully test this idea. 

Hormones mediate physiological and behavioral changes in adults as they transition into reproduction. In this study, we characterize the circulating levels of five key hormones involved in reproduction in rock doves ( Columba livia ): corticosterone, progesterone, estradiol, testosterone, and prolactin using univariate and multivariate approaches. We show similar patterns as previous studies in the overall patterns in circulating levels of these hormones, i.e., testosterone (males) and estradiol (females) high during nest-building or egg-laying, prolactin increasing at mid-incubation and peaking at hatching (both sexes), and elevated corticosterone levels in later incubation and early nestling development. In our investigation of hormone co-variation, we find a strong correlation between prolactin and corticosterone across sampling stages and similarities in earlier (early to mid-incubation) compared to later (late incubation to nestling d9) sampling stages in males and females. Finally, we utilized experimental manipulations to simulate nest loss or altered caregiving lengths to test whether external cues, internal timing, or a combination of these factors contributed most to hormone variation. Following nest loss, we found that both males and females responded to the external cue. Males generally responded quickly following nest loss by increasing circulating testosterone, but this response was muted when nest loss occurred early in reproduction. Similar treatment type, e.g., removal of eggs, clustered similarly in hormone space. These results suggest internal drivers limited male response early in reproduction to nest loss. In contrast, circulating levels of these hormones in females either did not change or decreased following nest manipulation suggesting responsiveness to external drivers, but unlike males, this result suggests that reproductive processes were decreasing.