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Wetmore, Alexander, 1886-, et al. / Warm-blooded vertebrates

Chapter III: color and its arrangement,   pp. 25-38 PDF (5.6 MB)

Page 29

stir up, that they are much more frequently reported than
would be expected and specimens of them are often sent
to museums. The albinism may be confined to one or two
*feathers in the plumage, or may affect the entire bird, in
which case the eyes may appear pink. Briefly, albinism
results when for some reason pigment is not formed as the
feather develops, and the white appearance is due to the
structural refraction of light explained above. In a way
albinism is a diseased condition.
The reverse of albinism is melanism, in which an excess
of dark pigment makes the feathers appear sooty or black.
Melanism occurs commonly in certain hawks, which have
light and dark phases. The light phase is supposedly the
normal one, as birds of this kind are more abundant, while
only occasional individuals are brown or nearly black.
The ferruginous rough-legged hawk (see Plate I3) of the
Western States supplies us with a familiar example of this
type of coloration, the melanistic specimens being fre-
quently deep chocolate-brown. Many melanistic in-
dividuals appear so strikingly different from the normal
phase that naturalists have sometimes erroneously con-
sidered them distinct species.
What might be considered a combination of albinism
and melanism, in this case working in a normal manner, oc-
curs in the little blue heron, in which species the young are
white while the heavily pigmented adults are slaty-blue.
Xanthochroism, a rare condition of superabundant
yellow color, occurs occasionally in parrots. Normally
green parrots which show yellow feathers are usually, if
not always, victims of a form of albinism in which the
underlying pigment in the feather is lacking but in which
the external sheath remains yellow and determines the
feather's color. The common grass parrakeet or budg-
erigar of Australia, which is now reared abundantly in
captivity, normally has most of the plumage bright green.
Occasional individuals, however, are albinistic in the
pigment layer, thus appearing yellow; while more rarely

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