. Abnormal plumages include a variety of conditions.
Albinism, total loss of colour, is rare, but partial loss of colours is more common. Some species are colour
polymorphic, having two or more colour variants. A few species have special types of polymorphism, as in the male
ruff which has an assortment of different colours around the head and neck in the breeding season only. There are hereditary as well as non-hereditary variations in plumage that are rare and termed abnormal or aberrant plumages. Melanism refers to an excess of black or dark colours. Erythromelanism or
erythrism is the result of excessive reddish-brown erythromelanin deposition in feathers that normally lack melanin. Melanin of different forms combine with xanthophylls to produce colour mixtures and when this combination is imbalanced it produces colour shifts that are termed schizochroisms, including
xanthochromism (an overabundance of yellow), and axanthism (a lack of yellow), which are commonly bred in
cagebirds such as
budgerigars). A reduction in eumelanin leads to non-eumelanin schizochroism with an overall fawn plumage while a lack of phaeomelanin results in grey-coloured non-phaeomelanin schizochroism. Carotenism refers to the abnormal distribution of
carotenoid pigments. The term "dilution" is used for situations where the colour is of a lower intensity overall; it is caused by decreased deposition of pigment in the developing feather, and can thus not occur in
structural coloration (i.e., "dilute blue" does not exist); pale structural colours are instead achieved by shifting the peak wavelength at which light is refracted. Dilution regularly occurs in normal plumage (grey, buff, pink and cream colours are usually produced by this process), but may in addition occur as an aberration (e.g., all normally black plumage becoming grey).
Albinism Albinism in birds is rare, occurring to any extent in perhaps one in 1800 individuals. It involves loss of colour in all parts including the iris of the eyes, bills, skin, legs, and feet. It is usually the result of a genetic mutation causing the absence of
tyrosinase, an enzyme essential for melanin synthesis.
Leucism (which includes what used to be termed as "partial albinism") refers to loss of pigments in some or all parts of feathers. 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. 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. 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 an inherited genetically conditioned plumage character in
domestic fowl (domesticated
Gallus gallus) controlled by a single gene. 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, with estrogen formation in the skin is as much as several hundred-fold higher than that of normal chickens.
Abnormal 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 in captive-bred
budgerigars •
Axanthism, lack of yellow pigmentation ==See also==