Non-breeding competition between migrant American Redstarts (Setophaga ruticilla) and resident Adelaide's warblers (Dendroica adelaidae) in the Guanica Biosphere Reserve, southwest Puerto Rico

Migratory animals must adapt to multiple ecosystems throughout their annual cycle, each with a different suite of competitors a nd vailable food resources. In isolation, each community would induce a different set of adaptations in a migratory species. However, these ecological pressures may c onflict, so that migrants may not be able to minimize competition in one or more of thes e communities. Behavioral adaptations can reduce the resulting interspecific competition through mechanisms such as interspecific territoriality, niche or habitat p artitioning, and use of competition refuges. I outline an approach that determines whether compe tition is occurring between migrant and resident species and the form that it takes. I also argue that studies of competition should be made at the level of individuals whenever possible, rather than at the level of populations or species. Finally, I outline the bas is for my hypothesis that resident Adelaide’s Warblers ( Dendroica adelaidae) compete for food with migratory American Redstarts ( Setophaga ruticilla) during the non-breeding season in the dry forests of southwest Puerto Rico.