Despite feathers being tough and the bird's constant care (see bathing and preening), they do discolour, wear out and become damaged. Owing to the fully grown feather being a lifeless structure, unlike our finger nails and hair which grow continuously, the worn feathers must be replaced through a natural process called moulting. The old, worn feathers are loosened from their follicles and eventually pushed out by the new feathers growing below - a little like our milk teeth being replaced by permanent teeth.
In many species, particularly small, perching birds, the first moult takes place in their first autumn and replaces the juvenile plumage with a 1st-winter plumage. This is often a partial moult with head, body and wing covert feathers only being replaced; for example, the juvenile Goldfinch will acquire its red, black and white head markings.
For some species, the 1st-winter plumage will be indistinguishable from the adult plumage, but for others there will be differences. For example, the immature Blackbird can be recognised from adults by the remaining brown juvenile wing and tail feathers. For some other species the differences remain for much longer as there can be several intermediate or immature plumages between the juvenile and adult plumages. For example, sea birds, such as gulls, can take up to 4 years to reach adulthood.
Many species also have non-breeding (winter) and breeding (summer) plumages. These different plumages are often most noticeable in male birds, which have brighter colours (such as Greenfinch) or ornaments (such as Lapwing) to use in displays to attract females.
Almost every time there is a change in a bird's plumage the bird must go through the moulting process. There are occasions when the plumage changes without a moult and this arises from the feathers wearing. For example, the tips of the spotted winter plumage of the Starling wear to reveal fully the iridescent plumage, and the tips of the throat feathers of the male House Sparrow wear to reveal the black throat badge.
Moulting is costly, in terms of energy, for birds and so usually takes place when the bird is less stressed, for example, late in the summer after breeding is complete, the weather is still warm and there is still plenty of food to be found. Further, birds do not lose all their feathers at once or they would be cold and unable to fly. The moulting takes place over a period. Larger species take longer to moult than smaller ones, for example, a Tit, such as a Blue Tit, may moult all its feathers over about 6 weeks, a Herring Gull may take 6 months, but a Buzzard may take several years for a complete change of flight feathers. Also, the keratin needed to make feathers is less abundant in vegetation than in insects, so seed-eating birds, like Chaffinches, usually take a couple of weeks longer to moult than insect-eating birds, like Robins and Dunnocks.
Some species that migrate after the breeding season have a complete moult before leaving for their wintering grounds (e.g. Chiffchaff), others moult when they arrive (e.g. Garden Warbler), and still others start their moult before migrating and then complete it on arrival.
Many species that moult in the late summer also have a partial "pre-breeding" moult in the early spring to replace some body feathers and wing coverts (but not flight feathers), for example: Pied Wagtail, Spotted Flycatcher and Whitethroat. The pre-breeding plumage is usually more colourful or bolder than the winter plumage.
The large flight feathers of the wings and tail are moulted in a strategic sequence that depends on the species. Many species moult their primary wing feathers in a strict sequence, this sequence varies from species to species, but may be from the innermost to the outermost, the outermost to the innermost, or the middlemost and then inwards and outwards.
Water birds, such as ducks, swans and auks, however, shed all the wing feathers at once and remain flightless for several weeks. During this time, the male ducks, such as Mallards, acquire a drab "eclipse" plumage, which usually resembles that of the female and offers the bird greater protection through better camouflage. The breeding plumage is then acquired late in the autumn, ready for when they mate in the winter.
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