Sexually dimorphic leading-edge serrations evolved in silent swallows

Leading-edge serrations are specialized feather structures, mitigating acoustic noise during foraging flight in owls, and have been extensively studied and applied to man-made noise-reducing structures. Similar structures occur in other avian species, such as swallows, although the ecological functions of the serrations in these species remain unclear. I conducted macroevolutionary analyses of hirundines (subfamily: Hirundininae), in which “leading-edge serrations” have evolved multiple times, to examine their evolution in relation to ecological context. I found that silent hirundines showed a higher probability of possessing leading-edge serrations, indicating that leading-edge serrations of swallows have an acoustic function as in owls. However, the probability of possessing leading-edge serrations had no detectable association with prey type (here, large active fliers vs. small aerial planktons) in hirundines, indicating that their acoustic functions would evolve independent of foraging adaptation in this clade. Instead, the presence of leading-edge serrations was positively associated with relative testes mass, a well-known index of sperm competition, in hirundines. These findings explain sexually dimorphic expression of leading-edge serrations, a unique characteristic in this clade. In contrast to owls, in which serrations are considered an adaptation for silent foraging, sperm competition rather than foraging adaptation would favor the evolution of leading-edge serrations in hirundines, which illustrates that totally different selection pressures favor homologous traits in two taxa.