Sexual size dimorphism in shorebirds, gulls, and alcids: The influence of sexual and natural selection

Charadrii (shorebirds, gulls, and alcids) have an unusual diversity in thei r sexual size dimorphism, ranging from monomorphism to either male-biased o r female-biased dimorphism. We use comparative analyses to investigate whet her this variation relates to sexual selection through competition for mate s or natural selection through different use of resources by males and fema les. As predicted by sexual selection theory, we found that in taxa with so cially polygynous mating systems, males were relatively larger than females compared with less polygynous species. Furthermore, evolution toward socia lly polyandrous mating systems was correlated with decreases in relative ma le size. These patterns depend on the kinds of courtship displays performed by males. In taxa with acrobatic flight displays, males are relatively sma ller than in taxa in which courtship involves simple flights or displays fr om the ground. This result remains significant when the relationship with m ating system is controlled statistically, thereby explaining the enigma of why males are often smaller than females in socially monogamous species. We did not find evidence that evolutionary changes in sexual dimorphism relat e to niche division on the breeding grounds. In particular, biparental spec ies did nor have greater dimorphism in bill lengths than uniparental specie s, contrary to the hypothesis that selection for ecological divergence on t he breeding grounds has been important as a general explanation for pattern s of bill dimorphism. Taken together, these results strongly suggest that s exual selection has had a major influence on sexual size dimorphism in Char adrii, whereas divergence in the use of feeding resources while breeding wa s not supported by our analyses.