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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 28

the color, has the following structure: A transparent,
colorless outer sheath of keratin from ten to fifteen
microns thick (a micron is the thousandth part of a milli-
meter, or about one twenty-five thousandth of an inch)
serves seemingly as a protective covering; beneath this
occurs a layer of polygonal cells about fifteen microns in
diameter, also without color; finally, the central part of the
barb consists of many minute, closely packed cells con-
taining a dark, granular pigment deposited mainly in the
cell walls. The blue appearance of the feather results
from the breaking up of the light reflected from the dark
cells by the overlying polygonal bodies. Truly a com-
plicated arrangement.
Green in birds rarely occurs as a true pigment, being
known in this form only in the feathers of a few cuckoos
and of their African relatives, the plantain eaters. With
these exceptions, green is a structural color resulting from
a modification of the arrangement that produces blue. In
the green feathers of parrots, for example, the feather
structure is essentially like that described above for the
blue of the blue jay except that the outer horny sheath is
transparent yellow instead of being colorless. The blue
produced by the basal pigment and the overlying polyg-
onal cells is modified to green by the overlying sheath of
yellow on the same principle as that by which the artist
mixes blue and yellow on his palette to obtain green.
Scrape the web of a large green parrot feather carefully
with a very sharp knife and the resulting powder will
prove to be distinctly yellow, while the web of the part of
the feather that remains after the outer sheath is cut away
will appear blue.
Birds exhibit variation from their usual coloring most
commonly in the form of albinism-the condition in which
feathers, ordinarily colored, appear partly or wholly white.
Albinos may occur in any species of bird but are suf-
ficiently rare always to attract attention. So much interest
do white blackbirds, white crows, albino robins, and so on
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