Behavioral and physiological conflicts in migrants: the transition between migration and breeding

Progression of the vernal migratory life history stage to breeding presents a number of apparent behavioral and physiological conflicts. Features that characterize the migratory stage include: high mobility, sociality, repetitive cycles of feeding (hyperphagia and fattening) and migratory flight. Breeding comprises: sedentary, territorial and reproductive behaviors, an initial decline in hyperphagia and reduction of fuel stores. Because morphology, physiology and behavior change, the transition between stages cannot be instantaneous. In many species development of the reproductive system actually occurs during migration, but in others gonadal development may not commence until later. This variation in degree of overlap of migration and reproductive functions is not well understood, but may be related to migratory distance and length of the breeding season, which tends to be shorter at higher latitudes and altitudes. In these habitats, migrants may arrive at their breeding sites to find unpredictable conditions that cannot support breeding. At this juncture, migrants may retreat to refugia and prolong maintenance of facultative migratory functions, termed arrival biology, until conditions improve sufficiently to initiate breeding. In this review, we focus on the Pacific races of the white-crowned sparrow, Zonototrichia leucophyrs, in which the entire spectrum of migratory strategies are represented from resident to long distance migrants and about which much is known. This species presents a unique view of the appearance and variations in arrival biology. Focusing on the juncture between migration and breeding, we discuss the diversity of responses of congeners to a spectrum of environmental conditions that favor survival and reproductive success.