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Feeding Habits and Bill Polymorphism in Hook-Billed Kites

Thomas Bates Smith and Stanley A. Temple
The Auk
Vol. 99, No. 2 (Apr., 1982), pp. 197-207
Stable URL: http://www.jstor.org/stable/4085968
Page Count: 11
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Feeding Habits and Bill Polymorphism in Hook-Billed Kites
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Abstract

The Hook-billed Kite (Chondrohierax uncinatus), a neotropical, snail-eating raptor, exhibits extraordinary intraspecific variation in bill size, which is unrelated to sex and age and is largely independent of geographic origin. Bill size varies much more than overall body size and is bimodally distributed in many parts of the kite's range. Plumage is also highly variable; sex and age differences, individual variation, and a distinct melanistic phase exist. Variations in bill size and plumage are unrelated. Variation in bill size is best interpreted as a polymorphism evolved and maintained by disruptive selection, although the possibility that there might be two species of Hook-billed Kites cannot be completely ruled out. Kites of different bill size feed differentially on snails of different sizes by using a unique snail-extraction procedure not previously described. The distribution of bill sizes within regional populations of kites is correlated strongly with the sizes of terrestrial snails found in the region. Except for cases of sexual dimorphism, there are few, if any, other examples of a morphometric triat being polymorphic in birds. The ecological, evolutionary, and taxonomic implications of bill polymorphism are discussed.

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