Genomic and plumage variation across the controversial Baltimore and Bullock's oriole hybrid zone

ABSTRACT Hybrid zones are powerful natural settings for investigating how birds diversify into distinct species. Here we present the first genomic-scale exploration of the Baltimore (Icterus galbula) and Bullock's (I. bullockii) oriole hybrid zone, which is notable for its long history of study and for its prominence in debates about avian species concepts and species limits. We used a reduced-representation sequencing approach to generate a panel of 3,067 genetic markers for 297 orioles sampled along the Platte River, a natural west-to-east transect across the hybrid zone. We then explored patterns of hybridization and introgression by comparing variation in genomic and plumage traits. We found that hybridization remains prevalent in this area, with nearly all orioles within the hybrid zone showing some degree of genomic mixing, and 41% assigned as recent-generation (F1/F2) hybrids. The center and width of the genomic and plumage gradients are concordant and coincident, supporting our finding that classically scored plumage traits are an accurate predictor of pure vs. hybrid genotypes. We find additional support for previous suggestions that the center of this hybrid zone has moved westward since it was first intensively sampled in the 1950s, but that this westward movement had slowed or ceased by the 1970s. Considered in concert, these results support previous inferences that some form of ongoing selection is counteracting the potential homogenization of these orioles via hybridization, thereby supporting their continued taxonomic separation as distinct species. LAY SUMMARY Hybridization between Baltimore and Bullock's Orioles has long captivated biologists and birders alike. Along the river valleys that cross the Great Plains, hybridization is common, leading to debates about whether these are really 2 different oriole species. We sequenced thousands of locations in the oriole genome to discover how much invisible genetic mixing is occurring in this area. Where the species' ranges overlap, many orioles are indeed hybrids, and their combination of plumage traits is a good predictor of their genetic ancestry. Yet the zone of hybridization remains narrow, suggesting that hybridization is a bit of a dead end, and adding to the evidence that these orioles are not blending together into a single species.