partial albinism and all the other -isms!
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
- partial albino Blackbird, looking superficially like a Ring Ouzel,
but shows the Blackbirds short primaries and lacks silvery wings.
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.
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.
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.
Ogilvie is a natural history writer and editor, formerly a research
scientist with the Wildfowl and Wetlands Trust, and resident
on the island of Islay since 1986. Until 1997, a member of the
'British Birds' editorial board and also one of the editorial
team which produced 'Birds of the Western Palearctic'.