Across Europe the grey-hooded and all-black forms replace each other along a line running roughly from Scotland through Denmark and central Europe to Italy. Where they meet, hybrids of intermediate appearance occur and are fertile. Under simple population genetics a zone like that should widen and blur over time.
It has not. Genome comparisons found the two forms nearly identical across most of their DNA, with differences concentrated in a small number of regions, several linked to pigmentation and feather development. The narrowness of the zone is attributed largely to assortative mating: birds tend to pair with partners resembling themselves, and plumage-based mate choice keeps the forms apart despite the absence of a genetic barrier.
The two are currently treated as separate species by most authorities, having previously been ranked as subspecies of Corvus corone. The case is a standard example of how species limits can rest on behaviour rather than genetic divergence, and the ranking is still argued over.