Melanin-based plumage coloration and flight displays in plovers and allies

Plovers and their allies exhibit an impressive diversity of melanin-based plumage patterns ranging from non-melanized to completely melanized species. We use phylogenetic comparative methods to test whether melanization has evolved in relation to sexual selection for attracting mates, to selection for signalling territory defence, or to natural selection for camouflage. First, according to sexual-selection theory, melanized plumage has evolved to amplify the courtship displays of males. As predicted by this hypothesis, we found that males with aerial displays had more melanized plumage than males of ground-displaying species. In addition, sexual dimorphism in melanization was greater in species with display flights than in species with ground displays. Second, melanization may have evolved through social interactions to signal competitive ability in territory defence. We did not find evidence for this hypothesis, since breeding density was unrelated to the melanization of either sex. Finally, melanized plumage may camouflage the incubating parent. The latter hypothesis was not supported, since melanization was unrelated either to the darkness of nest substrate or the extent of vegetation cover. Taken together, our results are most consistent with the sexual-selection hypothesis, and suggest that melanized plumage has evolved to enhance the aerial displays of male plovers.