Abstract
Sexual monomorphism is common in avian species. However, for many so-called monomorphic
species, there may be subtle differences between males and females in plumage color and size.
The sexes of the Taiwan Barbet (Psilopogon nuchalis) are hard to distinguish and there is a lack of
a deciding statement on its monomorphism. We used a genetic-based approach to determine the sex
of male and female Taiwan Barbets and examined the degree of plumage and size differences between
males and females. It was found that adults of both sexes had a red patch on the upper back.
The red patch on the Taiwan Barbet’s upper back may be the most distinct character differentiating
this species from other taxonomically closely related species. We found the number of feathers
with ≧ 50% coverage of the red patch on the upper back was larger in males. Strictly speaking,
the Taiwan Barbet is not entirely monomorphic. Since the size of the red patch on the bird’s upper
back is fairly variable, it was suggested that it be used with caution as a reference for distinguishing
males and females. We suggest that sexual selection might be responsible for the different size of
the red patch on the birds’ upper back. No significant difference was found in bill length, tarsus
length, wing length, or mass between male and female Taiwan Barbets. In addition, resource partitioning
between the sexes has not been observed in the field. We suggest that morphological adaptation
from intersexual competition for food might not have occurred in the measured morphology
of the Taiwan Barbet. The only sexual size dimorphism in the Taiwan Barbet we found was in tail
length. However, sexual selection unlikely explains the variation in the Taiwan Barbet’s tail length.
To hypothesize that resource division had forced the sexes to variate their tail length, further work
on comparing ecological habits between the sexes is warranted.