Patterns of morphological and geographic variation in trophic bill morphs of the African finch Pyrenestes

Abstract

Intra- and inter-population variability was studied in three species of tropical African estrildid finches comprising the genus Pyrenestes. Eleven characters were measured on P. ostrinus captured on a study area in Cameroon. Most of these same characters were also measured on museum specimens of this species and P. saguineus and P. minor. Data were analysed using univariate and multivariate statistical methods in order to characterize variation within and between populations and species. Variation in all three species of Pyrenestes is greatest in bill size, resulting from an exceptional non-sex-linked polymorphism. Bill size differences between morphs are as high as between congeneric species, with extremely large coefficients of variation, while other body characters show comparatively little variation. Sexual dimorphisms and differences in size due to age occur, but contribute little to overall size variation. Distributions of bill characters in each age and sex class are bimodal or greatly skewed, and in some geographical regions tend to be trimodal. Distributions of other body characters tend not to be significantly different from normal. Bill morphs differ in both shape and size and may be separated using principal component analysis. Static allometries of bill morphs differ significantly: relative to body size, bill size increases more rapidly in the large morph. Bill size and shape also vary geographically. The three species differ in mean size but show much overlap. Bill size is negatively correlated with total annual rainfall. In regions characterized by ecotonal transition zones between forest and savanna, tentative evidence suggests that a third, yet larger bill mode occurs. This third mode apparently results from the presence of a distinct larger species of hard-seeded sedge found only in these regions. The taxonomic implications of the polymorphism are discussed.