Recent declines in Neotropical—Nearctic migrant songbird populations are often attributed to events during the nonbreeding season, such as tropical habitat conversion and drought. Support for this hypothesis in most species, however, is largely anecdotal or conjectural. There is a dearth of demographic information about migrants on their Neotropical winter grounds. Such data are needed to identify specific ecological factors influencing survival, dispersal, and, ultimately, population abundances aggregated over multiple habitats at regional spatial scales. In this paper, we review several lines of evidence, emphasizing results of our research on paruline warblers in Jamaica, which indicate that migrant passerines often compete intraspecifically in winter for preferred quality habitats and that their populations may be limited at least in part by ecological conditions in winter. The demographic and ecological evidence supporting this hypothesis for migrant passerines includes: (1) differing densities among habitats, suggesting variation in habitat suitability; (2) strong territoriality, site attachment, and site fidelity; (3) experimental demonstrations of habitat saturation; (4) nonrandom distributions of sex and age classes among habitats; (5) overwinter decline of body mass by individuals occupying the most drought—stressed habitats; and (6) different residence times among habitats, suggesting differences in survival or dispersal. We review ecological and behavioral explanations for these demographic patterns, and make conservation recommendations based on our understanding of how local demographic circumstances affect broader scale population processes.