Sexual Selection and Dimorphism in Birds

Abstract

Darwin introduced the thoery of sexual selection to account for certain characters and patterns of variation that did not appear to be explicable in terms of natural selection for adaptations promoting success in the "struggle for existence." Sexual dimorphism is of interest in that it shows that there is a class of characters differentially expressed in the sexes because of sexual variation in ways of maximizing fitness. Both natural selection and sexual selection can promote dimorphism in the feeding apparatus, and it is often difficult to distinguish their separate effects. Only when the trophic structures alone are modified can we conclude that the dimorphism results primarily or wholly from selection for differential niche utilization. This is so in the huia and also in certain woodpeckers and other feeding specialists. Some of the ways in which behavioral interactions between pair members are affected by dimorphism in plumage color are shown by W. C. Dilger's study of the monogamous Agapornis parrots.