Sexual dimorphism in a feeding apparatus is driven by mate choice and not niche partitioning

The evolutionary origins of sexual dimorphism are credited to both natural and sexual selection. Sexual dimorphism in feeding structures, however, provides some of the clearest examples of ecologically driven dimorphism. Studies of bird bills have significantly aided these claims, but bird bills are also commonly used in pair formation behaviors, and thus their morphology could be subject to sexual selection. We tested 4 hypotheses of the evolution of sexual dimorphism using the feeding structure of a sexually dimorphic and a nondimorphic subspecies of the swamp sparrow, Melospiza georgiana. The increased bill volume of males was not explained by simple allometric relationships, ecological niche divergence between the sexes, or correlations with territory defense. Male bill volume was positively selected by female mate choice, as relative male bill volume predicted both the presence of and degree of cuckoldry. Further, male bill volume increased with age, and females may thus receive benefits by choosing larger billed males for social (direct benefits) or extrapair (indirect benefits) mates. It is clear from this example that sexual selection can play a role in the evolution of sexually dimorphic feeding structures, even in bird bills, which are a classic system for ecologically driven sexual dimorphism.