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Melanism

It is genetically controlled, and many species are known to have a normal dark colour variant. Good examples of species with a melanistic phase include buzzard, woodcock, snipe, Montagus harrier, red grouse and partridge. 

A population of great tits in Surrey was reported with melanism. Oil on seabirds is sometimes mistaken for melanistic plumage.

Since melanism is a dominant characteristic, it can become more prominent in a population if there is an advantage to be darker coloured. This became known as industrial melanism. 

It was detected in house sparrows in Liverpool in the 1930s, but the famous example is that of the peppered moth, whose dark phase became dominant as it became better camouflaged against the walls and tree trunks darkened by pollution.

Melanism can be caused by dietary factors. For instance, a bird on a diet rich in hempseed with a high oil content can turn black.