Two European crows that look quite different from each other are actually diverging from a single species, right before our eyes. In Western Europe, it's the inky-black carrion crow (Corvus corone). In the east, the grey hooded crow (C. cornix).
They overlap in a very narrow zone running along the River Elbe in Germany, where they sometimes mate with each other to produce fertile hybrids - a strange result, since many species hybrids are infertile.
Now, genomic analysis has revealed that C. corone and C. cornix are actually almost indistinguishable from a genetic standpoint. The only significant genetic difference between them can be found in the gene responsible for colouration.
"Defining speciation as the buildup of reproductive isolation, carrion crows and hooded crows are in the process of speciation," said evolutionary biologist Jochen Wolf of Ludwig-Maximilians-UniversitÃ¤t in Munich.
Exactly what was keeping the two populations apart was unclear - so Wolf and his colleagues performed genetic analyses of around 400 birds. These were selected from within the hybrid zone, as well as the zones on either side, where the species' territories don't overlap.
"Only two major effect genes which together encode the feather colour differ sharply on either side of the hybrid zone - the gray alleles are not found to the west of the zone and the black allele is absent in the eastern region," Wolf said.
"That's a very strong indication that there is rigorous selection on the basis of colour."
This suggests the crows are choosing mates based on colour. Birds of a feather literally, in this instance, flock together - carrion crows prefer black mates and hooded crows prefer grey ones.