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I spotted this blackbird in the garden (see photo). It is not black but light grey, and it did not have pink eyes so I guess it is not an albino. It spread its wings and lay in the sun; in due course it flew off. I’ve never seen a blackbird with this colouring before. Can anyone tell me more about it?

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• Further to previous answers, I think this bird is sunbathing, not “anting”. The ultraviolet-B rays in sunlight facilitate a crucial step in the biosynthesis of vitamin D. In humans this occurs directly in the skin, but in birds the skin is shaded by feathers.

To resolve this, birds use an oil secreted by the uropygial gland, or preen gland, near the base of the tail. This contains a precursor steroid that is converted into vitamin D by sunlight. The bird spreads this oil on its feathers by preening, then suns itself either as shown in the photograph, or more briefly in flight, and consumes the photosynthetic product with the next preening. Perhaps this prolonged sunning behaviour is seen mostly in the spring because of a vitamin D deficiency acquired during the dark winter.

Charles Sawyer, Byron Bay, New South Wales, Australia

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This article appeared in print under the headline “Off colour”