Acutely stressful experiences can have profound and persistent effects on phenotype. Across taxa, individuals differ remarkably in their susceptibility to stress. However, the mechanistic causes of enduring stress effects, and of individual differences in stress susceptibility, are poorly understood. Here, we tested whether brief, acute increases in glucocorticoid hormones have persistent effects on phenotype, and whether effects differ according to the magnitude or duration of elevation. We used a novel method to non-invasively manipulate hormone levels on short time scales: the application of corticosterone gel to a model egg secured in the nest. Free-living female tree swallows () exposed to several brief corticosterone increases during incubation showed dose-dependent differences in behaviour throughout the reproductive period. Birds receiving treatments that simulated higher or longer acute stress responses later provisioned larger broods at lower rates; the resulting offspring were smaller in size. Treatment did not influence female body condition, oxidative stress, reproductive success or inter-annual survival, but exposed females maintained higher baseline corticosterone after treatments ceased. Overall, these results indicate that brief, acute elevations in glucocorticoids in adulthood can have long-term consequences. Furthermore, individuals that mount a greater or longer acute stress response may be more likely to experience lingering effects of stress.

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 (h = 0.13-0.14) and stress-induced glucocorticoids (h = 0.18), and moderate heritability in glucocorticoid flexibility in response to acute stress (h = 0.38) in free-living juvenile tree swallows (Tachycineta bicolor; n = 408). Baseline glucocorticoids were not genetically correlated with either stress-induced glucocorticoids or glucocorticoid flexibility. These findings indicate that baseline glucocorticoids and the acute stress response are distinct traits that can be independently shaped by selection. Microevolutionary changes that influence the expression or flexibility of these endocrine mediators of phenotype may be an important way that populations adapt to changing environments and novel threats.