Virtually all environments vary in space and time. Therefore, individuals should express a phenotype, within the population reaction norm, that is best suited for the conditions it encounters (Via and Lande 1985; Kawecki and Stearns 1993). However, because most environments change markedly with season, many individuals actually move from one environment to another (i.e., they migrate). Thus, the reaction norm must include many adjustments in morphology, physiology, and behavior so that a given phenotype can track changes in its environment by maintaining homeostasis at each point in its life cycle. This raises a number of fascinating questions about control mechanisms. We have put forward the concept of life-history stages (LHSs) and finite-state machine theory to describe the problems faced by individuals in changing environments, including potential consequences for hormonal control mechanisms (Wingfield and Jacobs 1999; Jacobs and Wingfield 2000). Superimposed on the predictable life cycle are unpredictable, and potentially stressful, events that have a completely separate repertoire of physiological and behavioral traits, each with its own characteristic controls (Wingfield et al. 1998; Wingfield and Romero 2000). Here, we will focus on migratory behavior in the face of environmental perturbations (see also Wingfield and Silverin 2002).