Of this nature are zoomelanin (black), zoonerythrin (red), zooxanthin (yellow), turacin (red–only known in the Musophagidae), and perhaps turacoverdin (green, from the same family). Brown is produced by a combination of red and black; white is the appearance due to innumerable air-spaces.
Such are many yellows, oranges, greens and blues.
Albinism is due to the absence of pigment; melanism, xanthochroism and erythrism are terms implying an abnormal proportion of black, yellow, or red in the plumage. They may be caused by food.
In some cases at least Rails and Water-hens do the same.
In certain of the Tetraonidae the claws are shed in spring; in some Alcidae (Auks) the horny bill-sheath and the outgrowths over the eyes are lost after the breeding season; the American White Pelican moults a horny projection on the culmen after nesting, while the beak of Redpolls is much elongated in summer.