Data from: An experimental analysis of the heritability of variation in glucocorticoid concentrations in a wild avian population

Glucocorticoid hormones (CORT) are predicted to promote adaptation to variable environments, yet little is known about the potential for CORT secretion patterns to respond to selection in free-living populations. We assessed the heritable variation underlying differences in hormonal phenotypes using a cross-foster experimental design with a quantitative genetics approach to partition variance in baseline and stress-induced CORT as a function of both genetic and environmental features in nestling North American barn swallows (Hirundo rustica erythrogaster). A bivariate animal model analysis estimated phenotypic variation due to pedigree and rearing environment and indicated a low heritability (0.152) of variation in baseline CORT that was best explained by rearing environment. Variation in stress-induced CORT was contributed to a combination of both heritable (0.343) and environmental components. Further, we did not detect a genetic correlation between these two hormonal traits. Although environment appears to play an important role in the secretion of both types of CORT, our results suggest that stress-induced CORT levels are underlain by greater additive genetic variance compared to baseline CORT levels. Accordingly, we infer that the glucocorticoid response to stress has a greater potential for evolutionary change in response to selection compared to the physiological processes mediating baseline CORT secretion.