What Are Feathers?
A Stunning Variety Of Plumes
Feathers are what make birds unique in the animal world. They are used for camouflage, for advertising, for insulation, and for waterproofing. And of course, without them a bird would not be able to fly.
Feathers come in different shapes and sizes. The largest belong to the crested argus pheasant whose tail feathers can be over 5 and a half feet long and over five inches wide. The smallest feathers are to be found on birds’ eyelids.
The colours of feathers can be breathtaking- the iridescent blue of the kingfisher, the garish pink of the flamingo- and serve a wide range of purposes.
The Parts Of A Feather
The feather probably developed from the scales of birds’ reptile ancestors. Feathers are made from keratin, the same horny material that forms a reptile’s scales. They are highly complex structures which combine both strength and lightness.
Feathers are really products of the bird’s skin. The skin is made up of two parts- a thick, lower layer containing the blood vessels, muscles and nerves, and a top layer of cells which forms the epidermis. When the outer cells of the epidermis die they become filled with keratin- a feather is simply a mass of keratin sticking together.
The typical feather takes the form of a shaft and a vane, or web. The central shaft, or quill, is made up of two parts- a hollow, cylindrical section at the base called the calamus and a solid, angular shaft called the rachis. The rachis supports the vane on either side. A row of small branches makes up the vane- these small branches are called barbs and they are set at an angle which is inclined towards the tip of the feather. Branching from each barb are two sets of barbules which support barbicels. The two sets of barbules are known as hooked and spoon-shaped barbules. The hooks fit into the grooves of the spoon-shaped barbules, which thus gives the feather its strength and maintains its shape.
Primaries And Secondaries
There are several different types of feathers. The main ones are the contour feathers, those on the head and body that give the bird its outline. Downy feathers have no rachis; this helps to give them their loose, fluffy texture- they usually lie under the contour feathers and provide insulation. Semi-plumes do have a rachis, but they do not have a firm vane. They are usually shorter than the contour feathers and they, too, provide insulation. Filo-plumes are usually thin, hair-like feathers which are specialised and may be sensory. Bristles occur around the beak, nostrils and eyes of many birds. The long bristles around the beak in effect enlarge the mouth and so help the bird to capture flying insects. Those about the nose may act as a sift, purifying the air that the bird breathes.
Primary feathers are flight feathers which are attached to the hand (manus) of the wing and secondaries are flight feathers which are attached to the forearm (ulna). These are collectively known as remiges. Each wing feather quill is covered at its base on the upper side of the wing by a greater covert. From the greater coverts to the leading edge of the spread wing are successive rows of smaller and smaller feathers- middle or median coverts, lesser coverts and marginal coverts. Coverts on the underside of the wing are arranged in a similar way but are less fully developed.
The large feathers of the tail are called rectrices. They are usually of even number. The rectrices are overlain by greater and lesser upper tail coverts above and a few under tail coverts.
Birds Of Paradise: The Most Colourful Birds In The World
A Feather Without Pigmentation
The variety of colour in feathers is produced by pigments such as melanins, carotenoids and porphyrins. Males are often more brightly coloured than females, especially in the breeding season. This is for use in courtship displays and to show aggression to other males. Females are often much duller because their role is usually to incubate and care for the young, therefore it is essential for them to be well camouflaged. This sort of colouring is known as ‘cryptic.’ The opposite form to cryptic colouring is ‘phaneric’- conspicuous colouring. Uniformly white birds such as swans and egrets, or uniformly black birds, such as crows, are outstandingly conspicuous. Occasionally, such birds will fluff up their feathers to give the appearance of greater size and ferocity to potential intruders or predators.
Iridescent colours, such as the colourful gloss on a starling or lapwing, are created in two ways: either by the structure of the feather’s surface breaking up the light, or by reflections from grains of pigment inside the feather.
Prim And Proper
It’s very important that a bird’s feathers are looked after well- its very survival could depend on their condition. A bird does this by preening, ‘zipping’ the barbs of the feathers together with its beak.
The bird also uses its beak to remove dirt or parasites, and conditions its plumage by using organic oil which it gets from a gland above the root of its tail. A bird may use its feet to clean parts that it cannot otherwise reach such as its head and bill.
Feathers are often warmed in the sun to ‘iron out’ any twists (especially obvious in big birds of prey) and bathing in shallow water is common. The bird splashes about, flapping its wings and moving its bill from side to side, and its head and breast are often dipped into the water. Dust bathing is another addictive habit among birds. Fine particles of dry earth or sand are forced through the bird’s plumage and it’s thought that this helps it to deal with the many parasites that occur in its feathers.
From Colourful To Drab
Feathers have a limited life- they wear out and need to be replaced. So they are periodically shed and renewed. This is known as moulting. All birds moult at least once a year. Usually the moult occurs after breeding and before migration when brand new feathers are an advantage. Some birds such as ducks, geese and swans, lose all their flight feathers at once, which render them flightless for some time. Many feathers are used for display purposes- these obviously have to be renewed for the maximum effect in the following breeding season.