Bill Size, Body Size, and the Ecological Adaptations of Bird Species to Competitive Situations on Islands

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Species of birds sympatric on islands frequently differ from each other in bill length by a larger amount than do their mainland relatives. Differences between similar species in body size, as indicated by wing length, are smaller than differences in bill length. This suggests that a major adaptation of island birds to potential competitors is a change in feeding habitats. An alternative explanation of the bill features of island birds, that they have been selected for a species recognition function, may be applicable in some instances. But since pairs of species are usually different in plumage features, as well as song and behaviour, species recognition is probably achieved most frequently by these means and not by bill size or shape. Some data show that homogeneric species differ in numbers by a greater amount on islands than on the mainland. Differences in numbers and bill characteristics support the suggestion that for sustained coexistence the requirement of mutual ecological independence of closely related species is often greater on islands than on a mainland. This results from the restricted variety of resources and area of islands.