Adaptive significance of the mega-billed form in the polymorphic black-bellied seedcracker Pyrenestes ostrinus

The polymorphic African estrildid finch, the Black-bellied Seedcracker Pyrenestes ostrinus, exhibits extreme large-billed variants in transition zones between forest and savanna. Using field data collected between 1983 and 1995 at six sites in Cameroon, I examined the adaptive significance of bill size in this mega-billed form. In comparison with small and large morphs, body size increased more rapidly in the mega-billed form, a trend that is consistent with some other species of finches subject to avian predators in open habitats and that suggests aerodynamic constraints imposed by larger bills are compensated by increased body size. Although additional research is required, preliminary results indicate mega-billed birds may interbreed with other bill morphs and may therefore represent a third distinct morph rather than a distinct species.