Birds with unexpected colouring-blackbirds or jackdaws with white patches, blue tits with black heads and goldfinches with orange faces-may be more common than we think. ‘More than 35 species are known to exhibit unusual plumage, and most of them are common garden birds, such as blackbirds and house sparrows, plus some seasonal visitors such as bramblings and swallows, as well as goldcrests, buzzards and even moorhens,’ explains Tim Harrison of the British Trust for Ornithology (BTO), which is conducting a survey into the phenomenon.

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It’s thought that plumage abnormality may be more serious than just an oddity; it could give the wrong signals to a potential mate or make the bird more obvious to predators. Feathers with reduced pigment are often less robust and wear more rapidly, reducing flight efficiency and insulation. Despite reports of abnormal birds being loners, even ‘bullied’ by other birds, there are equal numbers of reports of abnormal individuals being more aggressive or dominant. To participate in the Abnormal Plumage Survey, visit www.bto.org/volunteer-surveys.

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