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.