Feathers are one of the marvels of nature, they are light, strong and flexible. A bird has several different types of feathers, each is adapted for a specific purpose, whether it is flight, insulation, display or whiskers.
Feathers are made from a tough, fibrous protein called keratin. This is similar to the keratin that our finger nails and hair, and reptilian scales are made from. Indeed, birds probably evolved from reptilian ancestors, see Evolution.
The flight feathers, such as the primary and secondary wing feathers, are called remiges. These feathers comprise an airfoil-like vane with a central shaft, which is hollow and filled with air for lightness.
The quill is the end of the shaft that is attached to the bird and it fits into a follicle in the flesh just like the roots of our hair.
The feather is asymmetrical with the shaft closer to the front (anterior) edge of the feather. When a bird raises its wings, the feathers open up and allow air to pass through. On the down stroke, the feathers close up and present a solid surface to the air and this generates the lift needed for the bird to remain airborne.
The vane is not solid but is made up of thousands of hair-like filaments that zip together.
Each hair-like filament is called a barb and under a microscope the barb can be seen to have lots of smaller barbs called barbules - rather like a tree has branches, and the branches have smaller branches. The barbules have hooklets that fasten over the barbules of adjacent barbules.
In this way, if a bird collides with a twig and disrupts the smooth, aerodynamic structure of its feathers it can quickly repair the damage by reconnecting the barbs while preening.
However, not all of the bird's feathers have this mesh-like construction.
The bristle feathers are found around the eyes, mouth and nostrils of birds. These bristles provide protection (like eye lashes) and a sense of touch like a cat's whiskers. The next time a Blackbird is in your garden, take a closer look with a pair of binoculars at the bristles near the base of its bill.
Downy feathers are adapted for insulation, to keep the bird warm, and are a haphazard tangle of barbs. These feathers, on an Eider, form one of the best insulating materials in existence - Eider down.
Powder down feathers have barbs that turn to dust, like talcum powder, and are usually found in birds like pigeons that do not have preen glands. The powder helps the birds to groom. This powder can be seen when a pigeon has flown into a window and leaves behind a ghostly impression of itself.
Feathers are coloured for several reasons:
Once a feather is fully grown it becomes a dead part, unlike our finger nails and hair which grow continuously. Before the feather becomes too worn, discoloured and damaged, the bird usually replaces them in an annual moult.
Pure albino birds lack pigmentation and because feathers are made from keratin, which is naturally whitish in colour, their plumage is white. The absence of pigmentation also affects eye, leg and bill colour - the eye and legs appear pink owing to the blood vessels showing through, and the bill will be whitish. As well as pure albinos there are partial albinos which simply have a few white patches on their plumage or have white plumage but retain their proper eye or leg colour.
Albinism is usually a genetic condition that causes the absence of pigment in plumage and eyes but may also be caused through malnutrition, parasites or injuries. A common belief was that too much white bread was the cause of albinism, but this is not the case. As well as white feathers, albinos have red eyes.
Leucism is a similar condition to albinism except that the normal plumage appears very pale as a result of weak pigmentation; their eyes are coloured normally.
Some birds, such as Blackbirds, may have a single feather or patches of feathers without pigment, but their eyes are coloured normally. This is popularly referred to as partial albinism but should be properly described as leucism.
The opposite condition to leucism is melanism. A melanistic bird has excessive pigmentation, giving them a darker appearance.
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.