Sexing monomorphic western mountain greenbuls on Mount Cameroon using morphometric measurements

Abstract

The western mountain greenbul, Andropadus tephrolaemus, is a poorly-known monomorphic forest greenbul. We investigated whether morphological measurements could be used to discriminate between sexes of this species, with a sample of birds captured in two forest types on Mt Cameroon. We used discriminant function analysis based on a combination of traits to develop models to discriminate between sexes. The sex of birds predicted from these models was compared with molecular sexing of the same individuals. Discriminant analysis revealed that a discriminant function incorporating wing length, tail length and bill height allowed 74% of western mountain greenbuls to be sexed correctly based on morphometric measurements alone. Sex ratios varied between montane and lowland forests, with the former having a higher proportion of males (70.5 and 54.6%, respectively), but the difference was not statistically significant. Cloacal sizes did not differ between males and females, and males do not develop large cloacal protuberances while breeding. This is in agreement with low predicted levels of sperm competition in this species. We show that sexing based on morphology has its limitations, but can still provide some insight if DNA analysis is not available.

Key words: morphology, Andropadus tephrolaemus, sex ratio, discriminant function analysis, tropical Africa.