Carrion crows and hooded crows share almost identical genes yet rarely interbreed
Across a line running through Europe, all-black carrion crows meet grey-bodied hooded crows. Sequencing their genomes showed the two are genetically the same except for a sliver under 0.28 per cent that switches off dark colouring on the hooded crow's body. They could interbreed freely, yet each mostly chooses a partner that looks like itself.
The carrion crow is a stocky, glossy black member of the crow family found in western Europe and the eastern Palearctic, including the mountains, forests and cities of Japan. Linnaeus described it in 1758, and it keeps his name, Corvus corone, which combines the Latin for raven with the Greek for crow. At roughly half a metre long it is noticeably smaller than a raven, and its plumage has a greener sheen than a rook's. Young birds start out brownish with blue eyes, both of which darken with age.
The split from the hooded crow probably dates to ice age cycles, which broke an ancestral population into isolated groups that later spread back and met. Researchers argue about why they stay apart. One explanation is assortative mating, a preference for similar-looking partners; another proposes koinophilia, a general aversion to mates with unusual features. Either way, appearance alone seems to hold the two populations apart.
Telling a crow from a rook comes down largely to voice. The rook calls with a high kaaa, the crow with a deeper, rougher kraa, often delivered in bursts of three or four from a rooftop or treetop while it bobs its head and hunches its shoulders. Crows are generally more solitary, though they gather in winter roosts.
They are formidable thinkers. Experiments show carrion crows can tell apart quantities up to 30, switch flexibly between rules and recognise individual faces, both human and crow; since their brains are built so differently from ours, this looks like intelligence evolved independently. They eat almost anything, from carrion and grain to eggs and ducklings, and will bully foxes or raptors off their kills. Year-old offspring often stay behind to help feed their parents' next brood.
Source: Carrion crow
