Abstract. Charadrii (shorebirds, gulls, and alcids) have an unusual diversity in their sexual size dimorphism, ranging from monomorphism to either male-biased or female-biased dimorphism. We use comparative analyses to investigate whether this variation relates to sexual selection through competition for mates or natural selection through different use of resources by males and females. As predicted by sexual selection theory, we found that in taxa with socially polygynous mating systems, males were relatively larger than females compared with less polygynous species. Furthermore, evolution toward socially polyandrous mating systems was correlated with decreases in relative male size. These patterns depend on the kinds of courtship displays performed by males. In taxa with acrobatic flight displays, males are relatively smaller than in taxa in which courtship involves simple flights or displays from the ground. This result remains significant when the relationship with mating 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 relate to niche division on the breeding grounds. In particular, biparental species did not have greater dimorphism in bill lengths than uniparental species, contrary to the hypothesis that selection for ecological divergence on the breeding grounds has been important as a general explanation for patterns of bill dimorphism. Taken together, these results strongly suggest that sexual selection has had a major influence on sexual size dimorphism in Charadrii, whereas divergence in the use of feeding resources while breeding was not supported by our analyses.Key words. Charadrii, display behavior, niche division, sexual selection, size dimorphism, waders.Received May 25, 1999. Accepted March 7, 2000 Differences between males and females in body size are striking in many animals. Several hypotheses have been advanced to explain sexual size dimorphism (reviewed by Hedrick and Temeles 1989;Shine 1989;Mueller 1990;Andersson 1994). Sexual selection was proposed originally by Darwin (1871). In particular, if competition between members of one sex (typically males) is more intense than between members of the other sex, then sexual selection is expected to lead to larger size in the more competitive sex. The selective advantage of body size, however, may depend on whether the competition occurs on the ground or in the air (Payne 1984;Jehl and Murray 1986;Hö glund 1989). In particular, if males fight on the ground, then large size may be selectively advantageous, whereas if competition occurs in the air, then small males may benefit from enhanced maneuverability (Andersson and Norberg 1981). This may help males compete for territories, and females may prefer the most acrobatic males (Hedenströ m and Møller 1992;Grønstøl 1996;Hakkarainen et al. 1996).Natural selection due to ecological differences between the sexes is an alternative to sexual ...