Predation, seed size partitioning and the evolution of body size in seed-eating finches

Abstract

I compare the relationship between bill size (depth) and body size among different taxa of seed-eating finches to test the hypothesis (Schluter, 1988a) that in habitats where seed-eating finches are vulnerable to predators, finches have larger bodies relative to their bill size. In support of this hypothesis, ground-foraging finches on continents (Emberizidae, Passeridae, Fringillidae), where predators are more common, have larger bodies relative to their bill sizes than do ground-foraging finches on islands (Emberizidae, Fringillidae). Ground-feeding finches on continents may also be more vulnerable to predators than tree- and shrub-foraging finches. As predicted, in North America, ground-feeding finches (Emberizinae) have larger bodies relative to their bill size than do tree- and shrub-foraging finches (Carduelinae). As a consequence of increased body size relative to bill size, both the range of possible bill sizes and potential seed sizes that can be eaten are reduced. Moreover, increased metabolic demands caused by larger body size may lessen the ability to specialize on a few seed types. These two factors reduce the potential for seed size partitioning. Consequently, vulnerability to predators may limit, and is inversely correlated with, seed size partitioning in seed-eating finch communities. The extent to which predation has influenced other bird communities may be considerable, and the patterns found by Schluter (1988a) and in this study indicate that future ecomorphological studies, especially on species that spend much of the day foraging, might profit by considering predator vulnerability as well as foraging behaviour.