Glucocorticoid hormones are crucial mediators of behavioral flexibility, enabling organisms to shift behavior in accordance with changing environments. Because of their role as mediators of behavior and other phenotypic traits, selection operating on endocrine traits could enable organisms to rapidly adapt to novel environments. Yet while selection requires heritable variation on which to act, very little is known about the heritability of variation in circulating glucocorticoids and other components of the neuroendocrine system in natural populations. Recent brood swap experiments we have conducted in free-living songbirds (tree swallows: Tachycineta bicolor) indicate that circulating glucocorticoids under both baseline and stress-induced conditions are significantly heritable, at low to moderate levels. Thus, selection can operate on these traits to produce evolutionary change. Furthermore, a significant amount of the phenotypic variance in these traits is attributable to nest environment, suggesting that developmental environment and experience may also contribute substantially to rapid shifts in phenotype in changing environments.