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Wetmore, Alexander, 1886-, et al. / Warm-blooded vertebrates
(1931)

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


Page 30

BIRDS
the albinism occurs in the outer sheath and the underlying
pigment remains normal, so that the birds appear blue.
Erythrism, or an excess of red, is still rarer in birds.
It occurs regularly in the screech owl of the eastern United
States, which has distinct gray and red phases. Curiously
enough the red phase is restricted to the subspecies of the
common screech owl found in the states east of the Great
Plains, the various subspecies found to the west of that
region being always gray.
Many ornithologists believed until comparatively re-
cently that some birds had the power to change the color
of part of their feathers after these structures had become
fully formed. They thought that in some mysterious way
new pigment entered at the base of the quill and permeated
the web, or that an amoebalike blood cell (wholly imagi-
nary, as there was absolutely no proof of the contention)
invaded the barbs and barbules by the same route and
devoured the old pigment cells, replacing them by new
ones of a different color. Thus they accounted for such a
seasonal change as takes place in the snow bunting, whose
back is light in its winter dress and very dark in its summer
dress. Similar agencies were called upon to explain the
summer changes of plumage in male ducks, and many
other seasonal differences in color in birds. Careful
studies have shown, however, that a feather when fully
matured is a dead structure, capable of change in color
only through fading and wear. When the feather is fully
formed the pulp through which the feather derives its
nourishment during growth retreats from the quill at the
base, leaving behind a series of horny caps-little parti-
tions of membrane-which effectually cut off entrance of
any further coloring or nutritive matter and thus positively
prevent any change in color from new pigment.
The explanation of the color change in the snow bunting
appears simple enough if we examine the feathers. Only
the external portions of the feathers on the head and
rump prove to be brown, the basal portions being white.
[301


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