Stage of Taxon Cycle, Habitat Distribution, and Population Density in the Avifauna of the West Indies

The number of habitats occupied and relative abundance of birds on the West Indian islands of Jamaica, St. Lucia, and St. Kitts were compared for species in different stages of the taxon cycle. Each species was assigned to one of four taxon cycle categories: (I) expanding and undifferentiated, (II) widespread but differentiated, (III) with a fragmented distribution, and (IV) endemic. Nonpasserines exhibit no distinct trends in relation to stage of taxon cycle. Among passerines, species tend to have more restricted habitat distributions, which are shifted toward tall forest and montane habitats, and reduced population densities as they progress through the taxon cycle. These trends are well marked on Jamaica, with a fauna of 35 passerine species, but they are difficult to detect on St. Kitts, with 13 species, none of which are endemic. Patterns of ecological release and density compensation evident within the islands are due mostly to the absence of late-stage species with low abundance on small islands with few species. On Jamaica, several stage IV passerines exhibit uncharacteristically broad habitat breadths representing, perhaps, incipient cycles of expansion. Furthermore, stage IV species in genera endemic to the Caribbean had broader habitat occupancy than stage IV species belonging to wide-ranging genera. This suggests that with continued persistence in the islands, populations may become sufficiently differentiated that new colonists do not compete strongly with them. This study reinforces the general conclusion that the geographical and taxonomic patterns on which the taxon cycle is based are paralleled by changes in habitat breadth and abundance in individual islands. The mechanism that drives the cycle remains to be described.