Sexual Size Dimorphism in the Great Tit Parus major in Relation to the Number of Coexisting Congeners

Sexual size dimorphism in five morphological characters in the great tit was analyzed in three local populations inhabiting coniferous plantations. In one plantation the great tit coexisted with three competing congeners and in the other two plantations with one congener. Concerning characters related to food-gathering there was greater sexual size dimorphism in plantations where the great tit coexisted with only one congener, while this was not true for wing length dimorphism. It is argued that sexual size dimorphism in trophic characters is caused by natural selection for ecological displacement between the sexes to reduce competition between them, and that high intensity of interspecific competition greatly restricts the opportunity for evolution of sexual size dimorphism.