Divergence in the Foraging Responses of Birds on Two Bahama Islands

Abstract

Pine Warblers and Blue—gray Gnatcatchers forage extensively on bark in the pine forests on Andros Island, while on neighboring Grand Bahama Island they forage largely on foilage. The two islands are very similar in physiography, climate, and vegetation, but differ in the number of insect—gleaning species sharing the arboreal foraging substrates. Andros has only the two species mentioned above; Grand Bahama has five, and to of the additional species are bark specialists. Summed densities and consuming biomasses (masses adjusted for size variation in metabolic rate) of the birds in these arboreal insectivore communities were found to be roughly equal on the two islands, but the distribution of total foraging activity as measured in these same units was clearly unequal; foraging biomass on bark was 2.6 times as great on Andros as on Grand Bahama, on foliage it was 1.7 times as great on Grand Bahama as on Andros. When food (insect) abundance and distribution between bark and foliage was found to be similar on the two islands, I concluded that food abundance was probably not the deciding proximal factor behind the dissimilar bird distributions. Alternatively I speculated that some form of interference competition from the bark specialists on Grand Bahama could be inhibiting Pine Warbelers and gnatcatchers from visiting the bark on that island, while in the absence of bark specialists on Andros the birds were free to forage on both substrates. Since bark avoidance is characteristics of Pine Warblers and gnatcatchers throughout Grand Bahama, however, even where the presumptive competitors are scare or absent, I tentatively ruled out interference as the overall proximal factor. With the two most obvious extrinsic factors essentially eliminated, I propose that the proximal or immediate determinant of substrate distribution in these birds is substrate preference–an innate response tendency to select foliage and avoid bark in the one case, and to accept a balance of foilage and bark in the other. Further, since bypassing a rich and readily accessible food source would be clearly maladaptive under ordinary circumstances, I suggest that populations are currently maintained below carrying capacity and that the foliage preference trait of the Grand Bahama population arose under special circumstances of intensified interference competition during an environmental crisis or series of crises in the past. Individuals which avoid the competition from bark specialists at such times by preferring substrates other than bark should theoretically be favored in natural section and emerge numerically dominant from the evolutionary bottleneck created by the crisis. Similar food crises on Andros would, in the absence of bark—gleaning specialists, produce no comparable selective pressures and permit the resident populations to retain broad general responses to all profitable substrates.