Why are some individuals, and some species, better able to cope with stress than others? When faced with an acute challenge – the attack of a predator, a severe storm, a major injury – hormones mediate changes in an enormous diversity of behavioral and physiological traits. Variation in these key mechanisms of phenotype can influence the ability to survive and reproduce. In this talk I will discuss how integrative research in free-living birds is providing fundamental insight into the capacity of individuals, populations, and species to persist in changing environments, and how selection shapes endocrine mediators of phenotype. [more]