Strong selection maintains a narrow, stable tension zone between grosbeak species in the Great Plains.

When genetically divergent and geographically isolated lineages come back into contact, their interactions allow us to observe reproductive isolating barriers in action. The avian contact zone between Pheucticus melanocephalus and P. ludovicianus in the North American Great Plains has been studied for more than 60 years, but never with the aid of genomic data. Here, we generate reduced-representation genome sequence data from a transect in South Dakota spanning pure P. melanocephalus to pure P. ludovicianus. While the presence of recent-generation hybrids and backcrosses confirms the ongoing production of fertile F1 offspring, the narrow (48-61 km) and concordant geographic transitions in nuclear ancestry, mitochondrial ancestry, and male plumage phenotypes are hallmarks of a "tension zone," where selection against hybrids is balanced by immigration of parental individuals. Additionally, the bimodal distribution of genomic ancestry in the heart of the contact zone and the highly significant association between genomic and mitochondrial ancestry both suggest a role for intrinsic genetic incompatibilities in the maintenance of this species boundary. Overall, we infer that strong selection maintains a narrow region of geographic contact between P. melanocephalus and P. ludovicianus in the Great Plains despite ongoing hybridization and introgression.