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Colour as perceived in the various parts of Birds,
is produced by pigment or by structure or by a combination of the two.
Three classes of colors can therefore be distinguished.
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Chemical or absorption colours
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Objective structural colours
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Subjective structural, prismatic, or metallic colours.
The first class, so-called chemical or absorption
colours are always due to colouring matter, which may exist in the
form of a solution diffused in the coloured parts, or in the form of pigmented
corpuscles, distributed in and between the cells of the various organs.
Such colors do not vary or change under any position of the light or eye;
and even under transmitted light a red, yellow, brown, or black feather
will always appear the same. Black, red, and brown always belong hereto,
orange and yellow mostly, but rarely green, and never blue.
The principal colour pigments are :-
Zoomelanin, the black animal colouring
matter, distributed in amorphous little corpuscles, insoluble in Water,
Alcohol, Acids, or Ether, but dissolved and destroyed when boiled in Caustic
Potash and then treated with Chlor; it consists of about 53.5% of carbon,
4.6 of Hydrogen, 8.2 of Nitrogen, and 33.7 of Oxygen.
Zoonerythrin, red, hitherto found in the
red feathers of Cotinga, Phoenicopterus, Ibis, Cockatoo,
Cardinalis, and others, and in the "rose" round the eyes of the Tetraonidae.
It is soluble in Ether, Alcohol, and Chloroform, but not in Acids or in
Potash; the variable amount of fat or oil in the feathers of the Flamingo
causes them to be more or less intensely coloured.
Zooxanthin, yellow, can be extracted by
boiling in absolute Alcohol, and is a diffused pigment which tinges the
shafts, rami, and radii of the feathers, and is possibly the same in the
yellow feet and bills of Birds-of-Prey and Anseres.
Like Zoonerythrin it is a coloured fatty oil.
Turacin is a most peculiar pigment, discovered
by Church in 1867 (Phil. Trans. 1869, pages 627-636) in the red
feathers of the Musophagidae, and seems to be restricted to these birds.
It consists of the same elements as Zoomelanin with the addition of from
5 to 8 % of copper. It can easily be extracted by weak alkaline solutions,
such as Ammonia, and with the addition of Acetic Acid, it can be filtered
off as a metallic red or blue powder. The presence of metallic copper is
indicated by the green flame of the red feathers when burnt. These
birds lose the red color when washed by the rain, but regain it when dry.
When bathing they colour the water red, and the red feathers, when wet
are distinctly shot with blue.
Turacoverdin is the only instance
of a green pigment, and is only found in the Musophagidae; it contains
comparatively much iron, but no copper.
Brown is the result of a mixture of red
and black coloring matter
White is never due to pigment; in every
white object its color is due to there being an innumerable number of interstices
between its molecules, or the air-cells in its substance. The whole substance
of a white feather, the "ceratine," is colourless, but its texture forms
a fine network which diffracts and reflects the light.
The gloss of feathers, independent of the colour
itself, is the result of their horny surface being smooth and polished,
when rough they appear more or less dull.
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