Heritable variation in circulating glucocorticoids and endocrine flexibility in a free-living songbird

Phenotypic flexibility is a central way that organisms cope with challenging and changing environments. As endocrine signals mediate many phenotypic traits, heritable variation in hormone levels, or their context-dependent flexibility, could present an important target for selection. Several studies have estimated the heritability of circulating glucocorticoid levels under acute stress conditions, but little is known about the potential for either baseline hormone levels or rapid endocrine flexibility to evolve. Here we assessed the potential for selection to operate on the elevation (circulating hormone levels) and flexibility of glucocorticoid reaction norms to acute restraint stress. Multivariate animal models revealed low but significant heritability in baseline (h2=0.13-0.14) and stress-induced glucocorticoids (h2=0.18), and moderate heritability in glucocorticoid flexibility in response to acute stress (h2=0.38) in free-living juvenile tree swallows (Tachycineta bicolor; n=408). B...