WINTER HABITAT QUALITY, POPULATION LIMITATION, AND CONSERVATION OF NEOTROPICAL NEARCTIC MIGRANT BIRDS

Recent declines in Neotropical-Nearctic migrant songbird populations a re often attributed to events during the nonbreeding season, such as t ropical 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 Neotropica l 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 scale s. In this paper, we review several lines of evidence, emphasizing res ults of our research on paruline warblers in Jamaica, which indicate t hat migrant passerines often compete intraspecifically in winter for p referred 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 variatio n in habitat suitability; (2) strong territoriality, site attachment, and site fidelity; (3) experimental demonstrations of habitat saturati on; (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 ha bitats, suggesting differences in survival or dispersal. We review eco logical and behavioral explanations for these demographic patterns, an d make conservation recommendations based on our understanding of how local demographic circumstances affect broader scale population proces ses.