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

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 t...