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Bird Coloration: Pigments and Structural Colors
Jul 22 2011 5:00:00 pm EST
The remarkable diversity in color and iridescence of birds is achieved by pigments and structural colors.
Pigments are colored substances that reflect certain wavelengths of light, while absorbing others. It is the reflected light that is identified as the pigment’s color. Pigments are located on the skin or feathers of the bird and are independent of the structure of the feather. Bird pigments come in three different groups: melanins, carotenoids, and porphyrines.
Melanins are the primary determinate in human skin color. They also serve to color the skin and feathers of birds, as well as providing strength and durability to feathers. Melanins produce blacks, reddish browns, and pale yellows.
Interestingly, a feather without any pigmentation is the weakest of all. White birds will often have melanin fortified black feathers on their wings where they are subject to the greatest stress.
Carotenoids are naturally occurring organic pigments in the chloroplasts and chromoplasts of plants and some other photosynthetic organisms like algae, some types of fungus, some bacteria and at least one species of aphid. Carotenoids are most strikingly visible as the yellow, brown, and orange colors of autumn foliage.
Birds acquire carotenoids by eating plants or eating organisms that eat plants. Carotenoids are responsible for the bright yellow of goldfinches and Yellow Warblers, and the yellow-orange of Blackburnian Warblers. When carotenoids and melanins interact, they can produce colors like the olive-green of the Scarlet Tanager.
Porphyrins are synthesized from amino acids. Heme, is a well known porphyrin, responsible for the red pigmentation in blood. In birds, porphyrins produce pinks, reds, browns, and greens. Porphyrins are found in some owls, pigeons and gallinaceous species, and are responsible for the brilliant greens and reds of turacos.
Some colors are determined by the structure of the feather. These are called structural colors. Blue colors in feathers are almost always structural colors. The barbs of feathers contain nanoscopic air pockets, on the scale of a wavelength of light. When light hits the feather, shorter wavelengths of light are preferentially scattered, resulting in a consistent vivid blue color. However, when backlighting a blue feather, the scattering effect is interrupted, and the underlying color is seen.
A similar phenomenon is observed in the gorget (throat feathers) of many hummingbird species. The iridescent colors of the throat feathers are a result of numerous microscopic platelets. These thin platelets are located on each barbule of the upper third of the throat feathers. The platelets exhibit interference coloration, like the surface of a soap bubble. The color changes according to the angle of observation, the thickness of the platelets, and the concentration of air pockets in the matrix.
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The Color of Birds. Stanford Birds.
Greenewalt CH; Brandt W; Friel DD. 1960. The Iridescent Colors of Hummingbird Feathers Proceedings of the American Philosophical Society. 104: 249-253.
Thompson, D’Arcy Wentworth. On Growth and Form. 1917.