Friday, 11 April 2014

Discoloured crow

A carrion crow waited patiently to eat some scraps on the verge until walkers passed by. I noticed the pale patches on its wing and tail feathers. This is no mutant, it is a bird that has grown its feathers while fed on poor quality food. According to van Gouw:
Partly coloured feathers are very unusual in leucism. Individual feathers that are partly coloured usually indicate a bad condition of the bird during feather growth and is not an inherit- able character (ie, is not leucism). This is often seen in, eg, Carrion Crows, especially those eating junk food in cities.
Blackbirds with white feathers on their heads or bodies are leucistic. You can see one in this post. Leucism is an inherited trait distinct from albinism, when animals are unable to produce melanin (a black pigment) at all on their bodies, resulting on all white individuals with red eyes. This doesn't affect carotenoid (yellow, red, orange) pigments, so these colours are unaffected if the normal bird displays them, e.g. an albino blackbird will have a yellow/orange bill (see an example here).

More information
van Grouw, H. (2006) Not every white bird is an albino: sense and nonsense about colour aberrations in birds. Dutch Birding, 28(2), 79-89. here.


  1. Thank you for this interesting post. The large colony of Carrion Crows in Kensington Gardens is widely affected by this bleaching. They live largely on scraps of visitors' snacks and the refuse of the cheap restaurants in nearby Queensway. Young birds in their first set of wing feathers are the worst affected, like this first-year bird photographed in autumn 2012. Two affected birds that I know as individuals have become pure black in later years, presumably because as they grow up they develop greater skill in foraging for nutritious little invertebrates.

  2. Thank you for your comment Ralph. I think you are right in saying young birds are most affected, unfortunately, the link I provided is the only thing I could find regarding carrion crows. Very interesting you noticed that individuals regrew normal feathers. I guess young might need higher quality food to grow and the adults get what they are used to feed. Here in Hull it is the crows in the largest park, East Park, that they are most affected, it is unusual to see one like this in my street.