Crows are noisy scavengers, but they are also smart and savvy with a brain-to-body mass ratio equal to that of the great apes. But two types of crow have a distribution that has left scientists baffled.
The hooded crow has a black hood and wings and grey body, while the carrion crow is completely black, but the two species are so closely related they can interbreed. Their only genetic difference is a gene that gives them their different colours and yet they remain separated across Europe and overlap only in narrow zones. So what separates them?
In the last Ice Age a single species of crow in Europe was forced to seek refuge from the great ice sheets across the continent. Some of the birds found refuge in the east and evolved into hooded crows while the others went west to the Iberian peninsula and became carrion crows. The two populations remained isolated from each other.
When the ice retreated about 10,000 years ago, the two groups spread back across Europe but the carrion crow remained in the west and the hooded one in the east. They could meet in the middle of the continent but only in very narrow areas, such as the River Elbe in Germany, where the carrion crows largely stayed on the western shore and the hooded crows on the eastern side. Despite this segregation, they could still interbreed and have fertile offspring.
So birds of a feather flock together, even though their only genetic difference is the colour of their feathers, but why should that be such a barrier?
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Climate may be involved because the narrow hybrid zone where the two species can meet has shifted westwards, which for the hooded crows has been linked to a wetter and possibly milder climate. Scotland has also become a dividing line, with the carrion crows mainly settling in the east and hooded crows in the west and north. A narrow hybrid zone has developed, although that too has shifted in a northwesterly direction.
