Sexual selection and body size in male red-winged blackbirds.

TLDR: The sexual selection hypothesis explains the relationship between pronounced sexual dimorphism in size and nonmonogamous mating systems in birds and reduces competition between males and females of the same species.

Abstract: In most species of birds adult males are larger than adult females. The two major hypotheses proposed to explain this pattern of sexual size dimorphism are appropriately termed the sexual selection and competition avoidance hypotheses. The sexual selection hypothesis states that large size gives males an advantage in intrasexual competition for females or in epigamic displays, activities not engaged in by females. Males will thus be larger than females, and the difference should be greater among polygynous and promiscuous species where there is greater variability in male reproductive success and thus stronger sexual selection (Darwin, 1871; Huxley, 1938; Fisher, 1958). The sexual selection hypothesis, then, explains the relationship between pronounced sexual dimorphism in size and nonmonogamous mating systems in birds (Darwin, 1871; Selander, 1958, 1972; Amadon, 1959; Orians, 1961), a relationship particularly evident among the Icteridae (Selander, 1958). The competition avoidance hypothesis states that sexual size dimorphism is favored because it reduces competition between males and females of the same species (Selander and Giller, 1963; Selander, 1966, 1972; Johnston and Selander, 1973). If size affects the range of resources used (e.g., larger individuals eat larger food items), then those males differing most in size from females would overlap in resource utilization with the fewest individuals and thus have the most resources available. This hypothesis may be especially applicable to monogamous, ter-