The rain hurries me to shelter at the woods’ edge, but I’m scarcely under the branches of a mature sycamore when the canopy starts to thrash. Abrasive voices erupt from the foliage as a rabble of crows dispute. One leaps into a gap between the leaves, crouching, its ash-grey body low over a branch and fanning its black tail. The throat inflates to bray the bird’s anger. In response, the object of its fury hops on to the branch above it, all the while giving as good as it gets. Something niggles me about that one – I squint, then blink in surprise. It’s a black crow.

As a bookish youngster growing up in rural County Fermanagh, it took a while for me to grasp that the crows I encountered in real life were not, in fact, black. The hooded or grey crow is the common crow across all of Ireland. With its two-tone livery of grey torso and black extremities, it’s a handsome bird. The “hoodie” is also found in the north of Scotland. The closely related all-black carrion crow is a far more familiar sight throughout the rest of Britain, with sparse numbers along the east coast of Northern Ireland.

Hooded and carrion crows can interbreed successfully and a well‑studied “hybrid zone” is found in Scotland. Nevertheless, Carl Linnaeus (the founder of modern taxonomy) designated the two as separate species in 1758. Ever since, they have been either lumped together or split apart, depending on the current understanding of what makes a species.

Among evolutionary biologists, that’s an argument that can get as heated as the one rasping above my head. However, genetic studies suggest that hooded and carrion crows are more closely related than previously thought: their contrasting plumage is based on just 0.28% genetic difference. But because the crows give weight to the visuals in their choice of mate, that tiny genetic difference has kept hooded and carrion populations distinct, despite almost negligible difference in their ecology.

Suddenly a pair of hoodies takes off and vanishes around the campus building behind me. A second pair swoops deeper into the woods. The carrion crow fluffs its feathers. Then, gleaming with wet, it starts to preen.