Abstract
Previous work suggests that sexual dimorphism in the bill shape of tits is related to the relative importance of intra- and interspecific competition experienced by a species' population. Thus socially dominant species differ in bill size more due to sex than do subdominant species. We tested the hypothesis that competitive release could lead to greater sexual dimorphism in bill shape by comparing measurements of the Coal Tit from Wytham, U.K., where it is subdominant, with those from S.W. Ireland, where it has fewer competitors. Bill variation was indeed significantly greater in the Killarney population, and this was due to a significant sexual dimorphism in bill shape, which was not found in Wytham. The populations also differed in the relationships among bill and other biometric variables. Winter feeding height data from the two populations suggested that the differences in sexual dimorphism corresponded with differences in the degree to which the sexes segregated by feeding station.
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