Hybrid zones provide a key natural context within which to study the barriers between incipient species. In some avian hybrid zones, there is indirect evidence of selection against hybrid offspring, yet the source of that selection is often unclear. We examined the frequency distribution of hybrids between Myrtle Warblers (Setophaga coronata coronata) and Audubon's Warblers (S. c. auduboni), using data to quantify—for the first time at a genomic scale—the composition of hybrids in this hybrid zone. We sampled birds during the breeding season and during fall migration and compared the frequencies of hybrids of different sex and age classes. Specifically, we tested for evidence of early-generation hybrids being significantly under- or over-represented in any of these classes, as would be expected if hybrids have lower or higher fitness than non-hybrids. We found that the genomic composition of birds in the hybrid zone spans the full ancestry spectrum. Across all our sampling periods, we found an excess of birds that had more Audubon's ancestry, with a stronger bias toward Audubon's ancestry in fall migrants than in breeding birds, consistent with asymmetric introgression. Notably, we did not find any differences in hybrid frequencies between juvenile and adult age classes or between males and females. Therefore, our results do not support large differences in viability between male and female hybrids or between different age classes of hybrids.
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Vol. 135 • No. 4