Color of Birds
source: http://www.stanford.edu/group/stanfordbirds/text/essays/Color_of_Birds.html
Birds are, hands down, the most colorful terrestrial
vertebrates—only insects and coral reef fishes rival them among animals.
Birds, like butterflies and moths, have two basic sources of color. The more
common is pigments, which are chemical compounds located in the feathers or
skin. Pigments absorb some wavelengths of light and reflect others; it is the
reflected light that reaches our eyes. The color we perceive is a function of
the wavelength of the light stimulating the receptors of our retinas. In the
visual part of the electromagnetic spectrum, we see the shortest wavelengths as
"violet" and the longest as "red." Thus a cardinal has
pigment in its feathers that absorbs all the wavelengths except the ones that,
when they enter our visual system, register as red. When no light is reflected
we see black; when all wavelengths are reflected we see white.
Blue and iridescent colors in birds are never produced by
pigments, however. They are "structural colors." The blues are
produced by minute particles in the feather that are smaller in diameter than
the wavelength of red light. These particles are able to influence only shorter
wavelengths, which appear blue, and are "scattered"—reflected in all
directions. Thus structural blue colors remain the same when they are viewed at
different angles in reflected light. If, however, they are viewed by transmitted
light (that is, with the feather between the light source and the observer), the
blue disappears.
Iridescent colors are produced by differential reflection
of wavelengths from highly modified barbules of the feathers that are rotated so
that a flat surface faces the incoming light. The detailed structure of the
barbule reflects some wavelengths and absorbs others, and the reflected
wavelength changes with the angle of reflection. The structural color is
registered by the eye in response to the reflected wavelengths and changes with
the angle formed by the light, the reflecting surface, and the eye.
Just as bird songs did not evolve to please the human ear,
bird colors did not evolve to delight our eyes. The most spectacular colors
typically function to impress members of the same species. The classic example
is the tail of the peacock, but the brilliant colors of the breeding male
Scarlet Tanager or male King Eider illustrate the same phenomenon.
Nondemonstrative colors frequently help a bird avoid predation. The camouflage
of a King Eider female on its nest is an example of such cryptic coloration.
Many inconspicuous birds exhibit what is known as "countershading";
they are darkest along the back, and gradually become lighter until the belly is
pure white. Countershading tends to eliminate a sharply defined shadow, since
the bird absorbs the most light above, where the light is brightest, and
reflects the most light below, where the light is dimmest. The vast majority of
shorebirds are countershaded, although as in the Snowy Plover the division
between darker back and lighter belly may be rather sharp.
"Disruptive"
coloration—the use of striking patterns to break up the outline of the
bird—is another technique for avoiding detection. Killdeer and Semipalmated
Plovers, for example, are very difficult to see in some circumstances. The
extreme in cryptic coloration, of course, is found among those birds that simply
take on the color of the background against which they live. Ptarmigans in their
pure white winter plumage are the classic example.
Birds often use colors to identify themselves to other
members of their flock and thus to hold it together. Examples are the color
patterns revealed in flight by shorebirds such as Ruddy Turnstones and Willets.
Colors, such as those inside the mouths of gaping chicks, may also function to
stimulate parental feeding. Other colors may direct the feeding movements of the
young, as does the red spot on the bill of the Herring Gull, which encourages
the young to solicit food and to stick its head into the adult's mouth.
Disruptive coloration in the Killdeer. The alternating
bands of white and black on the head and neck break up the outline of the bird
and make it more difficult to see against a variegated background than a bird
that is uniformly light or dark.
Some colors are apparently produced incidentally by
pigments deposited for other reasons. For instance, feathers of the wingtips are
subjected to more wear than those nearer the base of the wing. And feathers
containing pigments are more resistant to wear than those without. That is
thought to be the reason that the wingtips of many mostly white birds, such as
many gulls, terns, pelicans, and gannets, are dark.