Ecological Analysis of the Cayman Island Avifauna

Abstract

Ecological studies of the avifauna carried out over the past eight years in the three Cayman Islands of the Caribbean Sea revealed the presence of 151 species, mostly transients. Grand Cayman has 39 species of breeding birds (26 terrestrial), Little Cayman 29 (17 terrestrial), and Cayman Brac 30 (20 terrestrial). Each island also supports an additional 40 North American migrants in winter. Seasonal occurrences and relative abundances of birds are described, particularly for seven upland ecological formations in which the greatest breeding species diversities occur in logwood-thatch palm-red birch and limestone forests. In one sere, from pastureland through limestone forest, bird species diversity and total abundance increase with age of the community. High species diversity and density in the limestone forest are associated with some semblance of stratification but more so with a greater number of feeding niches. For the most part, the land birds breed in May and June at the onset of a rainy season. In the past 30-40 years, two bird species have become extinct on Grand Cayman, whereas two others have at least attempted colonization. The problems of inter- island distributional patterns are discussed in an attempt to explain the presence and absence of different species on different islands. No evidence was found to support the idea that the absence of one species on an island permitted another one on that same island to broaden its ecological niche; rather, the absence of woodpeckers, a tanager, flycatcher, dove, finch, and others on the two small islands strongly suggests vacant niches on those islands. Competition for environmental resources among the resident terrestrial birds was analyzed, especially between congeners and between species of similar ecologies. The four (or five) sympatric dove and pigeon species are separated chiefly by habitat and food differences; the two woodpeckers by food; four flycatchers by combinations of bill size, habitat, feeding height, and food type; the vireos by body size and bill size; and two Dendroica warblers by habitat and feeding height. With the exception of the scarce Barn Owl, no significant vertebrate predators on birds occur on these islands. The largely ornithophagous owl probably represents the greatest single biological control of the avifauna. Feeding ecologies, habitat distribution, and taxon cycles are specified, where known, for all the resident terrestrial species. Taxon cycles appear to be similar to those of other insular avifaunas, despite the absence of montane refugia on the Caymans. Species in Stage I (evidently the recent colonizers) are more common in early seral stages ("marginal habitats"), whereas the endemic subspecies (Stage IV) are more abundant in the mature forests. Because of the relatively large number of available habitats, Grand Cayman has 3.8 habitats per species, an exceptionally high figure when compared with mainland and other insular populations. This value plus other data indicate a remarkable generalization for the Cayman Island birds. The large number of wintering species appears to influence the residents very little because the wintering forms usually occupy feeding niches different from the residents. It is suggested that the winter is characterized by an abundant food supply (chiefly insects) that is incompletely exploited by the resident avifauna.