Albinism, Leucism and Vitiligo
Th posting on the Great White Redpoll has stimulated some fascinating and educational responses. No more so than that supplied by Hein van Grouw, Bird Group Curator at the Natural History Museum, Tring. Thanks Nils van Duivendijk for contacting Hein, to Hein himself (see below) and all those whose responses are in the comments section below the first post. Worth a read.
“This Redpoll is indeed interesting but certainly not rare. There are many mutations which can cause a change in the plumage pigmentation and therefore an aberration in the colour. All kind of colour names are seemingly randomly used, now and in the past, to identify mutations in birds. Most commonly, and most often wrongly, applied is the name Albino or Partial Albino. This name is widely used for all sorts of different colour aberrations, but in only a few percent of the cases it is used correctly. Due to the mutation, an Albino is unable to produce melanin pigments at all. A mostly white bird which nevertheless shows some form of melanin pigmentation is never an Albino, by definition.
Therefore ‘Partial Albino’ does not exist and is a contradiction in terms.
This latter name is often used for what is in fact Leucism. Leucism, from the Greek Leukos = white, can be defined as the partial or total lack of pigments in feathers (and skin). The lack of pigment is due to the congenital and heritable absence of pigment cells from some or all of the skin areas where they are normally present and where they normally provide the growing feather with pigment. Depending on the sort of leucism the amount of white feathers can vary from only a few white feathers (= partial leucistic) to totally white individuals. The totally white individuals always have colourless skin as well. Partial leucistic birds can have normal-coloured bill and feet depending on where the colourless patches occur on the specimen.
However leucistic birds always have normal coloured eyes.
There are however other congenital causes for pigmentless feathers. Whereas leucism is visible after hatching, vitiligo (also called progressive greying), for example, is a progressive condition that arises after a certain age. Vitiligo is defined as the heritable progressive loss of pigment cells with age. From a certain age, when the progressive loss starts, the bird will get more white feathers after every moult. This is not uncommon in birds and it is known in Jackdaws and Blackbirds for example. And it seems it is not uncommon in Redpolls either as I have seen several identical specimens over the years.”
Hein van Grouw, Curator, Bird Group, Dept. of Zoology, The Natural History Museum, Tring.