Leucism is the lack of melanin pigment in some feathers due to the absence of melanin-producing cells; albinism is the complete lack of these pigments due to the absence of a particular enzyme (tyrosinase).
However, according to Hein van Grouw, senior curator of birds at London’s Natural History Museum, many ‘white’ birds are actually exhibiting something different – a condition known as progressive greying.
An individual starts life with normal pigmentation, then gradually loses the pigment-producing cells, so that with every moult, the amount of white feathers increases.
This isn’t a heritable condition, and the precise causes are not usually known. “In progressive greying, the white feathers are more randomly spread,” says Hein. “Whereas most forms of leucism cause a bilateral and symmetrical pattern of patches of white feathers, with the extremities most affected.”
Main image Leucistic western grey kangaroo © Getty Images
