Resource Use by Bill Morphs of An African Finch: Evidence for Intraspecific Competition

The African estrildid finch Pyrenestes ostrinus exhibits a polymorphism in bill size that is not related to sex. This paper examines the feeding ecology, sessonal changes in abundance, and diet overlap of the two bill morphs on a study are in south—central Cameroon. The morphs fed on two major sedge species, hard and soft. The diets of the small— and large—billed morphs were influenced by their feeding efficiencies on the sedges and seed abundance. When both major sedges were abundant, diet overlap between morphs was high, but overlap declined during the major dry season when overall food abundance was low. During the dry season, large—billed morphs specialized on the hard—seeded sedge species while small—billed morphs broadened their diet to include other foods. This pattern of resource use is consistent with increased intraspecific compeition for food and similar to patterns of resource use between distinct species during food—lean times. A model for the evolution of the polymorphism is presented.