Plumage

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.

Although mainly found in the Anatidae, a few other species, including related red junglefowl, most fairywrens[a] and some sunbirds also have an eclipse plumage.

In the superb and splendid fairywrens, very old males (over about four years) may moult from one nuptial plumage to another[9] whereas in the red-backed and white-winged fairywrens, males do not acquire nuptial plumage until four years of age[10] – well after they become sexually mature and indeed longer than the vast majority of individuals live.

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 – lack of yellow – which are commonly bred in cagebirds such as budgerigars).

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 colors are instead achieved by shifting the peak wavelength at which light is refracted.

[13] In some birds – many true owls (Strigidae), some nightjars (Caprimulgidae) and a few cuckoos (Cuculus and relatives) being widely known examples – there is colour polymorphism.

This means that two or more colour variants are numerous within their populations during all or at least most seasons and plumages; in the above-mentioned examples a brown (phaeomelanin) and grey (eumelanin) morph exist, termed "hepatic form" particularly in the cuckoos.

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.

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.

A complete albino often has weak eyesight and brittle wing and tail feathers, which may reduce its ability to fly.

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.

Injury or disease may change their color, including dietary deficiencies or circulatory problems during feather development.