predation. 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. Several kinds of albinism in chickens has been described: A complete albinism controlled by an autosomal recessive gene and two different kinds of partial albinism. One of the partial albinisms is sex-linked and the other is autosomal recessive. A fourth kind of albinism severely reduce pigmentation in the eyes, but only dilutes the pigment in the plumage. 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. Aging may also turn a bird's feathers white. Hen feathering in cocks Hen feathering in cocks is a genetically conditioned character in domestic fowl (Gallus gallus domesticus). Males with this condition develop a female-type plumage, although otherwise look and respond like virile males. In some breeds, one can see males that have a plumage completely similar in all aspects to that of females. The trait is controlled by a simple autosomic dominant gene, whose expression is limited to the male sex. The condition is due to an enhanced activity of the aromatase complex of enzymes responsible for estrogen synthesis. So estrogen formation in the skin is as much as several hundred-fold higher than that of normal chickens. Pigmentation conditions 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 Ino budgerigar mutation, the occurrence of this mutation