Dominance and mating success in male boat-tailed grackles

Abstract

A study of boat-tailed grackles, Quiscalus major, shows how variation in male mating success is related to dominance rank and to competition for concentrations of breeding females. Males were organized in an absolute dominance hierarchy that was stable over a 4-year period. Position in the hierarchy was positively correlated with age, but not with size. Before the spring arrival of females, individual males dispalayed at many potential colony sites, but during the breeding period concentrated at those sites that had been colonized by females. Male attendance and display rates were correlated with numbers of fertilizable females. A mean of 4·7 males was present for each such female. A male's position in the dominance hierarchy determined its access to females. Most matings occurred at the nest, and as nests were close together and as nesting was asynchronous, first-ranking males were able to monopolize receptive females: 80% of copulations were by these males. During the first-ranking male's absence, other highranking males landed at nests, and were able to mate. Mid-ranking males remained at the periphery of colonies, and moved between different locations, apparently in response to the presence of fertilizable females. Low-ranking males rarely associated with colonies, but gathered in separate areas, where they attempted unsuccessfully to copulate with foraging females.