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We report the findings of a phylogenetic comparative analysis examining patterns and frequency of occurrence of same-sex courtship and mounting behavior in birds. Our analysis has shown associations between same-sex sexual behavior and both mating system and degree of precociousness at hatching. The patterns of expression and frequency of occurrence of same-sex sexual behavior differed markedly for males and females. Patterns of same-sex sexual expression reflected the competitive sexes that actively solicit sexual interactions in heterosexual encounters. Male–male (MM) sexual behavior occurred across all mating systems, but MM mounting was significantly more prevalent in those species with facultative polygamy. The frequency of MM sexual behavior increased with degree of polygamy. Female–female (FF) sexual behavior (especially courtship) occurred most frequently in socially monogamous species and rarely occurred in species that display obligate polygamy (predominantly polygynous species). Both expression and frequency of FF sexual behavior was strongly related to the precocial state of development at hatching. FF sexual behavior is more likely to occur in species in which monogamy occurs together with the production of precocial offspring; that is, in monogamous species that are exceptions to the more common altricial mode of development. We suggest that requirement of biparental care in monogamous species may influence the greater expression of FF sexual behavior and longer term associations. Both spatial and behavioral dispersion of females and engagement in uniparental care may be important in explaining the lower incidence of FF sexual behavior in polygynous species. Social contexts where males congregate at communal leks or display areas may influence the greater expression and frequency of MM sexual behavior in polygynous species. [ABSTRACT FROM AUTHOR]