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Complex in form and function, feathers protect birds, are used in courtship and, most striking of all, allow them to fly
Carried on a sprightly breeze, a bird soars high over the British countryside. Its flight is powered by one of nature’s lightest and most delicate creations, feathers. These provide birds with camouflage, let them perform courtship displays and, perhaps most wonderful of all, leave the earth to fly fast, silently or even under water. A bird’s feathers are known in their entirety as its plumage. The number of feathers within this plumage varies significantly depending on the size of the particular species. It ranges from approximately 1,500 on a small passerine or perching bird such as a robin, to 25,000 on one of Britain’s largest birds, the swan. Soft downy plumage, close to a bird’s body, traps warm air against the skin, helping keep the bird warm in winter. In summer, the feathers can be fluffed up in a breeze, allowing circulating air to reach the skin, keeping it cool. At the other extreme are the stiffer primary and secondary flight feathers that create an aerofoil shape on the wings. These are there to create speed and lift, letting the bird attain flight.
Types and purposes
Feathers are made of keratin, a fibrous protein found in the hair and nails of humans and animals. Each feather has a follicle, which is a socket-like pit in the bird’s skin. In each follicle is a group of cells. These produce feathers, normally once a year, throughout the lifespan of a bird. New feathers form completely within just a few days. While they are growing a bird is said to be in pin. This term comes from the newly growing feathers being housed in a keratin sheath. This serves as protection for the feather as it matures. At this stage the developing feathers look like stiff stalks and are consequently called pin feathers. Each feather has a rachis, the shaft that runs through the middle of the feather.
On either side are barbs, individual flexible branches. These extend to varying lengths from the rachis and connect together to form a web. There are numerous feather types, all serving different purposes. On adult and immature birds, it is the contour feathers that are visible. These include the body plumage, primary and secondary flight feathers, tail feathers and coverts. The latter cover other feathers, creating a smooth outline. It is the outermost parts of the contour feathers that give the bird its individual colours. On the body plumage, the outer barbs are linked together to form a flat surface, known as the vane. Heavily overlapping, these contour feathers protect the body from exposure to the elements. They also offer protection from injuries including bites and stings. At the base of a contour feather, closest to the bird’s body, the barbs are downy and separated. It is this part of the feather which traps the warm air. Chicks are entirely covered in soft down to keep them warm in the nest.
Flying high
Flight feathers have flat barbs, which join together via hooks on the side of one barb. These secure themselves into a trough on the side of the adjacent barb. The barbs on the outer webs of flight feathers project from the rachis at a forwards angle
towards the tip. The more acute the angle, the stiffer and so stronger the feather’s web will be. Consequently, the outermost primaries of all birds’ wings have a narrow and hard outer web, with the acutest angled barbs. When a bird flaps its wings in flight, these feathers move further and faster than those on the inner wing. This provides power through the air, while the broader inner feathers create lift. Each wing usually has 10 primary feathers. A common feature on birds that soar, such as eagles, buzzards and kites, is for the distal, or furthest, portions of one or both webs (inner and outer) to be reduced. These are said to be emarginated. When wings with this feature are spread, these feathers form notches, their distal ends resembling fingers. The reduced width of the outer part of the emarginated primary is extremely stiff and of an aerofoil section in profile. In flight this feature allows air to pass through the slots, minimizing turbulence and improving stability in the air. Secondary feathers have more rounded tips than the primaries, are broader and more curved. The exception to this are the three or four tertial feathers, positioned closest to the wing joint with the body. Tertial feathers lay over one another and are more symmetrically shaped. In many cases the tertial feathers are uniformly coloured or patterned on both sides and quite elongated. When the wings are closed, they provide a top cover to the inner flight feathers. Each wing usually has between nine and 11 secondaries. Some species, however, such as seabirds that have extremely long wings, can have up to 20. The majority of species have 12 tail feathers, matched in six equal pairs. Scientifically called rectrices, they are used for control. Tails create lift and control drag during slower flights and help birds steer during turns. They are furled, or folded, to reduce drag during faster flights. As a rule, the central pair of tail feathers are reasonably straight and symmetrical, with a convex vane. As the
tail feathers progress outwards, the outer webs become narrower with broader tips and flatter vanes. When closed together, the outer tail feathers lie beneath the convex vanes of the central pair to become a compact unit. Once fanned out, the broadened tips of the outer feathers form a widespread structure. The bodily part of each wing is covered in different types of covert feathers which are quite stiff. They cover the skin and bone of that part of the body. There are also uppertail and undertail coverts, which are heavily convex in form. These smooth the bird’s body shape into the tail feathers.
Special feathers
Some birds have evolved feathers dedicated to helping with their different lifestyles and feeding habits. Woodpeckers, for example, climb up vertical tree trunks to feed on grubs in the bark and to drill out holes for their nest. Their tail feathers have very strong rachis that are pressed firmly against the trunk to provide support. The central pair, in particular, have far thicker and stronger rachis than would normally be found on a feather of this size. The tips of this central pair are often broken off. The inverted V-shaped tip that is left demonstrates the pressure exerted. Gannets, which dive into water to catch fish, have very strong, stiff feathers. These have a thick rachis and tapered webs, which form a pointed vane. The gannet also has fine, short body contour feathers, creating a streamlined shape. These characteristics combine to allow the gannet to dive into water without its feathers becoming waterlogged and so heavy it would have trouble surfacing. Softer, wider flight feathers and fluffy body plumage would absorb a lot of water. As night hunters, owls need to move through the air without sound, to avoid alerting their prey to the danger above. To
enable this, they have a velvety down on the upper surfaces of very soft webs. Their outer primaries are edged with a comb of fine extensions which cuts through the air before the main web reaches it. This mechanism creates soundless flight.
Colours and strength
White feathers are produced as a result of the feather containing pure keratin only. Colouring agents are thought to give feathers more strength. With no colour-producing substances in them, pure white ones are usually more worn than coloured examples generally are. Black and shades of brown on feathers are the result of the introduction of melanin. This is a common dark pigment found in the hair, skin and eyes of humans and animals. The actual colour produced on the feather depends on the density of melanin, and varies from a light buffish brown to darkest black. Blue feathers are produced by a colourless, translucent layer of keratin over a black pigment. Similarly, green feathers are a result of carotenoid, a colourful organic pigment, in the translucent layer over the black pigment. Red and yellow feathers are also the result of the presence of carotenoid. This pigment is found in many plants and forms of algae, so its presence in feathers may be as a result of a bird’s diet.
Fit for purpose
Whether it is to help a bird support itself on a vertical tree trunk, to fly silently, dive into the sea or even to provide camouflage on the ground, there is a feather that has evolved for that purpose. They come in all shapes and sizes and a whole array of colours and patterns. Birds’ feathers are as fascinating in their diversity as the birds themselves.