Nature 329, 717 - 719 (22 October 1987); doi:10.1038/329717a0

Bill size polymorphism and intraspecific niche utilization in an African finch

Thomas Bates Smith

Museum of Vertebrate Zoology, University of California, Berkeley, California 94720, USA

The African estrildid finch, Pyrenestes ostrinus, shows a non-sex-linked polymorphism of bill size. Within P. ostrinus and its congeners, P. sanguineus and P. minor, variation in bill size is extreme, bimodally distributed and not determined by sex, age, body size or geographical origin. P. ostrinus appears to mate randomly with respect to bill size. Bill morphs differ with respect to diet and feeding performance on hard and soft seeds of sedges, Scleria spp. (Cyperaceae), the bird's principal food. The novel pattern of intraspecific divergence in trophic structure contrasts sharply with patterns of morphological variation in Darwin's finches (Geospiza) and other avian species not showing bill size polymorphisms. Coefficients of variation (CVs) for bill size are extraordinarily large while CVs for body size are small and typical of other passerine species. Differences in bill size between morphs exceed those reported for several sympatric species of Darwin's finches believed to show character displacement1, and like many co-occurring congeneric species the morphs differ in diet. Discrete intrapopulation variation in the trophic structures of birds usually consists either of differences between the sexes (sexual dimorphism)², or of morphological asymmetries, such as bill crossing^3,4. Here I describe a third pattern of discrete variation previously hypothesized to occur in the hook-billed kite (Chondrohierax uncinatus)⁵, and found only in a few species of other vertebrates^6–10, in which size differences in trophic apparatus are not determined mainly by sex. Polymorphisms of trophic apparatus have been correlated with differential resource utilization and, despite the scarcity of reports, have been postulated to be important in reducing intraspecific competition and in promoting speciation⁶.

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