Divergent selection on bill morphology contributes to nonrandom mating between swamp sparrow subspecies

• Birds' bills may serve as ‘magic traits’, influencing both ecological adaptation and nonrandom mating.
• We tested whether adaptive divergence in bill morphology and song production affects nonrandom mating in swamp sparrows.
• We used copulation solicitation assays to measure female preferences for song.
• Song influenced nonrandom mating between ecologically divergent populations of swamp sparrows.
• Our results support the hypothesis that bills may serve as ‘magic traits’ in ecologically divergent populations of birds.

Traits subject to ecologically based divergent selection that also affect nonrandom mating (i.e. ‘magic traits’) may hasten incipient speciation. In this study, we investigated whether nonrandom mating results from ecological divergence between swamp sparrow, Melospiza georgiana, subspecies. As a by-product of adaptive divergence in bill morphology between populations of swamp sparrows, there is a corresponding divergence in features of song shown to be salient to females, potentially providing a mechanism of reproductive isolation. Thus, female mating preferences for song could result in prezygotic isolation that prevents gene flow between subspecies. In this study we tested the hypothesis that female preferences for song provide a mechanism of reproductive isolation that limits gene flow between subspecies. Using copulation solicitation assays, we found that female coastal plain swamp sparrows showed a significant preference for consubspecific songs over heterosubspecific songs. We further tested whether mating preferences in coastal populations explain observed differences in song between subspecies. We found evidence that mating preferences in coastal females may explain the evolution of some acoustic features of song. We suggest that the bill may be an example of a ‘magic trait’ that contributes to continued divergence of swamp sparrow subspecies.