Albinism in birds

From Wikipedia, the free encyclopedia

An albino African penguin.
An albino African penguin.

Albinism in birds is rare, occurring to any extent in perhaps one in 1800 individuals. A bird that is albino (from the Latin albus, "white") has white feathers in place of coloured ones on some portion of its body. A bird that is naturally white, such as a swan, goose, or egret, is not an albino, nor is a bird that has seasonally alternating white plumage.

Four degrees of albinism have been described. The most common form is termed partial albinism, in which local areas of the bird's body, such as certain feathers, are lacking the pigment melanin. The white areas may be symmetrical, with both sides of the bird showing a similar pattern. In imperfect albinism, the pigment is partially inhibited in the skin, eyes, or feathers, but is not absent from any of them. Incomplete albinism is the complete absence of pigment from the skin, eyes, or feathers, but not all three.

A completely albino bird is the most rare, lacking any pigment in its skin, eyes, and feathers. The eyes in this case are pink or red, because blood shows through in the absence of pigment in the irises. The beak, legs, and feet are very pale or white. Completely albino adults are very rarely spotted in the wild. They are likely easier targets for predators because their colour distinguishes them from their environment. Falconers have observed that their trained birds are likely to attack a white pigeon in a flock because it is conspicuous. A complete albino often has weak eyesight and brittle wing and tail feathers, which may reduce its ability to fly. In flocks, albinos are often harassed by their own species. Such observations have been made among red-winged blackbirds, barn swallows, and African penguins. In a nesting colony of the latter, three unusual juveniles—one black-headed, one white-headed, and one full albino—were shunned and abused by companions.

Albinism has been reported in all orders and in 54 families of North American birds. The American robin and house sparrow led bird species in the incidence of albinism. Albinistic white appears to replace brown pigments more often than red or yellow ones; records suggest a greater incidence in crows, ravens, and hawks than in goldfinches or orioles.

Abnormally white feathers are not always due to albinism. Injury or disease may change their color, including dietary deficiencies or circulatory problems during feather development. Shock may also cause discolouration; a British ornithologist in 1844 described a captive blackbird that molted and became pure white after being frightened by a cat. Aging may also turn a bird's feathers white.

[edit] See also

  • Albinism, the lack of melanin pigmentation
  • Leucism, a condition similar to albinism in animals, characterized by reduced pigmentation in general
  • Melanism (or melanosis), unusually dark melanin pigmentation
  • Xanthochromism, unusually yellow pigmentation

[edit] References

  • "Albinism." Terres, John. K. The Audubon Society Encyclopedia of North American Birds, New York: Alfred A. Knopf, 1980. ISBN 0-394-46651-9.