In Europe, there are crows. The most famous of which is the carrion crow, which looks and behaves quite like the American crow. It is usually black, fairly omnivorous, and it often regarded as an agricultural pest.
At one point, it was believed that carrion crows existed in two distinct phases, the common all black form, which is common in Western Europe, and the phase that is marked with gray on the neck and body. This form exists in the northern parts of the British Isles, where it has been called the grey grow, the hoodie crow, or the hooded crow. This form is found in Northern Europe, Central Europe, and the Middle East.
However, ornithologists began to notice that where their ranges overlapped, it was quite unusual to see hooded crows paired off with black carrion crows, but the traditional taxonomy still thought of the hooded and black forms as being distinct phases of the same species.
Because the two forms were rarely seen mating or paired off, it was decided to call the carrion crow and hooded crow distinct species, and this is the current understanding. The hooded crow is Corvus cornix and the carrion crow is Corvus corone. But the two birds are otherwise quite similar in terms of ecology, vocalizations, and general phenotype. All that really separates them is the coloration.
Scientists that surely there was a rather deep genetic divergence between the two species, which is why there is a species barrier between the two forms of similar crow.
However, when the genomes of carrion and hooded crows were sequenced, it was revealed that they were almost identical.
Less than .28 percent of the genome varied. That variance was related to the fact that carrion crows have gray plumage on their neck and torso.
But that little variance is enough to create a species barrier between the carrion and hooded crows. Birds are highly visual, and when young crows imprint upon their parents, they imprint heavily upon what they see. If a young crow is raised by parents that are gray hooded, they will look for mates that are gray hooded. IF their parents are all black, they will look for mates that are all black.
However, it has also been suggested that these crows look for mates that appear not to have aberrant mutations, and this keeps the crows looking for mates that generally look like those belonging to their general family group and social circle.
Whatever the case, we have two very closely related species that do not hybridize. They probably became distinct during the heavily glaciation cycles of the late Pleistocene. One form evolved a gray hood and one evolved an all-black form. Maybe founder effect is the only real reason for this difference in plumage, for this difference in plumage is awfully random.
But that difference in plumage color is enough to create a species barrier, which, if it holds, will lead to greater and greater speciation between hooded and carrion crows.
This discovery about crows is quite interesting for what it tells us about dog taxonomy. Domestic dogs and wolves live together over a broad swathe of Eurasia, and for many centuries, dogs and wolves were regarded as distinct species. However, we have recently found that there is an extensive gene flow between Eurasian gray wolves and domestic dogs across Eurasia, and this gene flow is so significant that the majority of Eurasian gray wolves are estimated to have some relatively close dog ancestry.
Carrion and hooded crows have a clear species barrier that is likely only going to intensify as the two lineages continue to diverge with very limited gene flow. Dogs and gray wolves are not experiencing such a species barrier. Indeed, it looks like the gene flow between dogs and wolves is only going to increase as wolves move into human-dominated lands in Western Europe, and the Eurasian dog population continues to increase along with the human population.
So here we have two crows that are diverging, and the wild and domestic forms of Canis lupus that are continuing their gene flow. Closing down gene flow is a major part of speciation, and the crows are clearly on their way.
