Albinism, partial albinism and all the other -isms!

Malcolm Ogilvie

A frequent subject of letters to bird magazines and a common topic for discussion on the internet newsgroups devoted to birds is sightings of birds with unusual plumage, especially those showing strange white markings or being much paler than usual, for example, Blackbirds and crows with patches of white feathers, all-white birds, and birds which have a normal plumage pattern, but all the colours are much paler than usual.

Below - partial albino Blackbird, looking superficially like a Ring Ouzel, but shows the Blackbirds short primaries and lacks silvery wings.
All-white birds are called albinos and they are due to a complete loss of pigment in the feathers. In its most extreme form, pure albinos, even the soft parts, the eyes, legs and feet, lack pigment and so appear pinkish. These are rare in the wild, occurring slightly more often in captivity. They must, one assumes, stand out like the proverbial sore thumb to a predator.

While all-white birds are rare, birds with anything from a few to many white feathers in their plumage are commoner and are called partial albinos. Among the whitest birds I have seen, though they retained the normal black bill, legs and feet, were a handful of Barnacle Geese in the Spitsbergen-Solway population, which were all white except for a few black feathers on their wings and back. Much more usual is a Blackbird or crow with a few white feathers, on the body, wings or tail. Birds with large amounts of white are the result of a genetic flaw in both male and female. The reason they remain rare is that both parents must carry the genes responsible for the white plumage. Birds with just a few white feathers may arise in the same way, but equally may have suffered an injury or even a disease which has damaged the feather follicles so that the feathers grow without any pigment. Most reports of birds with a few white feathers are of Blackbirds, Starlings and crows, but then these not only are these common birds, but their normal colour is black or very dark so that white feathers will show up well. It is quite usual for the amount of white to grow as the bird gets older.

The third colour aberration consists of a general overall paleness. It is as if the bird has been bleached all over so that although the main patterns on the plumage are visible, everything is faded. There has been a House Sparrow like this in my garden since last autumn. It is a male and is now breeding in the byre next door with a normal female. It is a pale tan colour, though the cap and bib are darker brown. A number of visiting birdwatchers have done "double-takes" on seeing this bird and I must say I scrutinised it carefully when I first saw it in case it was some (very unusual) vagrant. There has also been a Barnacle Goose here the last two winters which is similarly "washed out", with the pale grey areas almost white and the black areas grey.

What to call such pale birds is a matter of some disagreement in the literature. The most commonly used term is "leucistic", which is certainly shorter and easier to say than the two alternatives of "chlorochroistic" and "schizochroistic". The latter is really an all-embracing term for a variety of plumage abnormalities arising from the loss of from one to several pigments.

Like albinos and partial albinos, the occurrence of pale birds is the result of genetic abnormalities in both male and female. Except in very small, inbred, populations they will always remain rare, though nonetheless of interest, and sometimes puzzlement, to birdwatchers.

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