FEEDING HABITS AND BILL POLYMORPHISM IN HOOK-BILLED KITES

Hook-billed Kite (Chondrohierax uncinatus) Science Article 1

abstract

The Hook-billed Kite (Chondrohieraxu ncinatus),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 melanisticphase exist. Variations in bill size and plumage are unrelated. Variation in bill size is best interpreted as a polymorphism evolved and maintained by disruptives election, althoughthe possibility that there might be two species of Hook-billed Kites cannot be completelyruled 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 trait being polymorphic in birds. The ecological, evolutionary, and taxonomic implications of bill polymorphism are discussed.

THOMAS BATES SMITH AND STANLEY A. TEMPLE, The Auk 99:197-207 April 1982

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