Population structure and habitat use by overwintering neotropical migrants on a remote oceanic island

Abstract

We studied habitat use by migrant birds on the island of San Salvador using mist nets during January of 4 years (1997–2000). Information on capture rates, body mass, age-sex class distribution, frequency of recapture, and return rate were used to determine preferences for early successional habitat, mangrove, scrub or secondary forest. We captured 23 species (19 parulid warblers), but focused our attention on the 14 most abundant species (750 individually marked birds). A group of four abundant species exhibited equal sex ratios, but a female bias existed among common and uncommon species [e.g. black-throated blue warblers ( Dendroica caerulescens) and American redstarts ( Setophaga ruticilla)]. Twelve of 14 species used habitats unevenly. Habitat use also varied with age–sex class in prairie warblers ( D. discolor) and redstarts: in both, males used secondary forests and females used early successional habitat. Palm warblers also showed high variability in habitat use among years. Body mass did not vary with habitat in most species, but more stressful conditions late in the dry season might reveal differences. Return rates to the island were significantly lower than estimates obtained for the same species elsewhere in the Caribbean and Central America, and were of little use in identifying preferred habitats. However, most recaptures (within and between seasons) were made in the habitat where capture rates were highest. Interspecific differences in capture rate were thus indicative of habitat preferences. Based on this criterion, six of the 14 species preferred early successional habitat. Every habitat, however, was used heavily by at least three species. These latter findings, and the sex- and age-specific patterns of habitat segregation, emphasize the importance of maintaining habitat diversity for the conservation of neotropical migrants.