A bird’s feathers (its plumage) are made from keratin, the same substance found in human fingernails. Though durable, feathers become worn and must be replaced regularly. The process of feather replacement is called molting, and almost all birds molt at least some feathers at least once a year. Very large birds, such as eagles and pelicans, can have complex, irregular molts that last for months. Most birds, however, have regular, predictable molts that occur usually over a few weeks during a period when they are not nesting or migrating.
Some birds hatch with a downy coat of feathers, but most—especially passerines (perching birds)—hatch naked and acquire their first coat of contour feathers, called juvenal plumage, in their first month or so. In most species, fledglings are young birds with newly acquired juvenal plumage that are just leaving the nest; they often still have a few traces of natal down. Juveniles are young birds that have fledged and are still in their juvenal plumage. Some species, such as most warblers and sparrows, hold juvenal plumage for a relatively short time, molting into the next plumage after just a few weeks. Others, such as gulls and shorebirds, retain juvenal plumage for several months, although the appearance of the plumage changes considerably during this time through wear.
Plumages that follow juvenal plumage are attained by either a complete molt (of all feathers) or a partial molt (often just of body feathers and coverts). Some species, such as the ptarmigan, undergo several partial molts annually, producing a variety of plumages over the course of a year. Because molt can differ significantly even in closely related species, it can be difficult to apply consistent terminology to plumages and the molts that produce them. This guide uses the "life-year" system, which is closest to how human age is marked. A bird in its first-fall plumage or its first-winter plumage is still in its first year of life: less than 12 months old. These plumages differ from juvenal plumages but in many cases retain some juvenal feathers.
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| Large birds, such as this Bald Eagle, may not attain their definitive adult plumage for five or more years; most passerines and shorebirds acquire adult plumage by their second year and most gulls by their fourth. |
Most passerines attain adult plumage (sometimes called definitive plumage) around or just after their first full summer, at about one year of age; larger nonpasserines often take longer, usually reaching maturity of their plumage by the second or third year (some large birds do not attain adult plumage for nearly a decade). As birds near adult plumage, they usually resemble adults more and more closely. (The term "subadult" refers to a bird whose plumage is nearly, but not quite, that of an adult; the term "immature" applies more broadly to any bird that has not acquired its adult plumage.)
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Adults of a great many species of birds acquire breeding plumage (also called alternate plumage) just before the breeding season, and most hold this plumage for about half the year. Breeding plumage is usually more striking than non-breeding plumage (or basic plumage), especially in males. Most birds molt into their breeding plumage, but some species—such as European Starling—attain their breeding appearance by a gradual wearing of feather edges from their autumn plumage.
Species in which males and females show different plumages for at least part of the year are said to be sexually dimorphic. In many such species, males in breeding plumage are more colorful or more strikingly patterned than females. Male ducks bear a bold or colorful plumage for most of the year and molt into a very plain plumage, called an eclipse plumage, for only a few weeks in summer. Males in eclipse plumage sometimes resemble females (these briefly held plumages usually are not illustrated in this guide).
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| Albino birds, such as this American Robin, lack pigmentation in eyes and legs as well as plumage. The feathers appear white, the eyes and legs pink. | Leucistic birds, such as the Common Tern at lower left, have normal pigmentation in the eyes and often some feather; if mostly white, they usually show a hint of the normal plumage pattern. |
Some widespread bird species show a tremendous variety of colors and markings. If these variations occur in relatively regular and predictable patterns, as they do in Song and Savannah Sparrows, they may lead to
the designation of subspecies. However, variation in some species’ plumages does not show such patterns. Snow Goose and Rough-legged Hawk, for example, have different plumage morphs, or forms, which are considered not subspecies but simply color variants. Species that show different color morphs that do not relate to age or sex are said to be polymorphic in plumage.
Individual birds can also show aberrant (atypical) plumages; these include leucism (with unusually pale or whitish plumage, either overall or in certain feathers), melanism (with unusually dark plumage), and albinism (with no pigment; such birds are starkly white and have pink eyes and legs). Even more rarely, a bird will display a strong suffusion of yellow (xanthism) or red (erythrism) in the plumage.
