Birds Feather Structure, Function, Colour and Moult
Feathers, which distinguish birds from other animals, develop from a knob or papilla located within a feather socket or follicle.
The material of which the feather is composed is basically the same as that in the horn on their bills and the scales on their feet. Feathers are of two main types; firstly, there are the flight and contour feathers which give the bird its shape and provide outer insulation. Secondly, there are the down feathers which are found beneath the contour feathers and provide extra-insulation All other feathers are intermediate between these two. Herons and other waterbirds, as well as some birds of prey have special powder-down feathers which grow continuously and gradually disintegrate into a fine powder which is used in feather maintenance and cleanliness.
The standard feather has a shaft of which the lowest part, called the calamus, is embedded in the feather follicle. The shaft is hollow, has no veins and is translucent. The lower end tapers to a point through which, when the feather is growing, nutrients enter. The upper part of the shaft, called the rachis, supports the feather vanes. These vanes are more or less flexible and consistof hundreds of filaments called barbs. Every barb carries several hundred tiny barbules equipped with minute hooks, known as barbicels. These catch on the barbules from the next barb above them creating a most efficient fastening system, which is similar to the velcro system used on anoraks. If the weather tears open the barbs all the bird has to do is draw the feathers through its beak and the filaments knit together again.
Function of feathers
Feathers have many different and important functions: they are used for conserving warmth, for flying, and for protecting the bird against knocks. The colour of the feathers is important for both camouflage and courtship displays. During the winter the contour feathers are fluffed out in order to trap a thick pocket of warm air between the feathers and the skin; conversely on hot days in summer they are ruffled to allow hot air to escape more easily. This adaption for conserving heat is necessary because a bird must maintain a body temperature of 41 °C since the metabolism which produces the vast amount of energy required for flight, goes on much more rapidly the higher the body temperature. Although birds do not perspire they can control their body temperature within quite narrow limits.
The number of feathers varies considerably from one species to another; for instance, the density of feathering on waterfowl is greater than in other species, presumably to protect them from their coldwater environment. A count of feathers from a Whistling Swan revealed a total of no less than 25,216 individual feathers, including the down feathers. Feathers do not grow in a random fashion from all over a bird’s body but in most species they are confined to definite tracts.
Two of the functions of feather colour appear to be contradictory: one is self-advertisement and the other is self-concealment. In many species a balance is struck between these needs, the male being brightly coloured, especially in the breeding season, and the female being camouflaged by drab coloration. In the Dotterel and the Red-necked Phalarope the normal coloration and roles are reversed and the male, which is more drabbly coloured, incubates the eggs and looks after the young. In many species their striking colours can be used for both functions depending on the posture the bird adopts. For instance, the black patches of the Wheatear can be used as self-advertisement when the bird is displaying but also helps to disguise it when it is motionless amidst the rocks and stones. Furthermore, self-advertisement has the dual purpose of threatening or attracting females and thus we find that one colour in a bird can carry several messages.
The colours in the feathers are due either to the chemical substances deposited in the feathers or to their physical properties. Reds, yellows, blacks, browns and greens are usually due to the chemical properties but some greens are produced by structural conditions. Iridescent colours which are one form of structural colouring are produced from the barbules that may be flattened for part of their length or twisted and whose successive surfaces interfere with the lightwaves. Some colours, especially black, which is based on the pigment melanin, tend to be more resistant to wear. On beaches you can often find worn gull feathers in which the flecks of white have worn away whereas the black areas are still intact.
To be at peak efficiency throughout a bird’s life feathers have to be shed and renewed once or twice a year, depending on the species of bird. In almost all species a few feathers are renewed at a time, usually in pairs on either side of the body. The moulting of the wing feathers seriously impairs the ability of a bird to fly and in some cases prevents it from flying altogether. Drakes lose all their flight feathers at once and are therefore flightless for some weeks in late summer, this is when they are said to be in ‘eclipse’. However, their marshy habitat at this season of the year usually provides them with adequate food and shelter, and their sober ‘eclipse’ plumage affords them extra camouflage.
Normally birds do not moult during the breeding season, on migration or during times of food shortage as they need all the energy they can obtain to generate new feathers and to replace heat loss during the moult. However, there are all sorts of odd exceptions. Seabirds moult during the breeding season which is probably because it is safer to moult while they are on land than when they are out at sea. Migratory birds usually moult after the breeding season when there is plenty of food available and before their long flight to their winter quarters. Other species migrate to relatively safe areas to moult, the Shelduck, for example, uses Bridgewater Bay in Somerset and the Eider Duck collects off the coast of Aberdeenshire in July. The Common Scoter gathers in flocks of 150,000 birds in one spot off the Danish coast.
Some birds moult more than once a year. The Ptarmigan moults three times: one complete autumn moult produces its brown plumage, two partial moults produce the white winter plumage and then a third moult gives it the grey spring plumage. The Purple Sandpiper has what is known as an ‘arrested’ moult which begins in the breeding grounds and then continues in the winter quarters, after migration. This arrested moult may result from the fact that large birds take a long time to moult and cannot afford to wait too long as the summer food supply soon becomes exhausted. This is a particular danger if the birds happen to be insectivorous.
The time taken for moulting and changing feathers varies enormously. Some long distance migrants can lose and replace their feathers within thirty-five days prior to the migration. Others such as the Redpoll can take as much as fifty days. The Curlew Sandpiper, and shore waders which moult when they reach their winter quarters can take six or seven months to moult; eagles can take more than a year with several arrests.
The bright new feathers of a bird’s spring plumage are not always the result of a spring moult. Sometimes the brightly coloured feathers grow in the autumn but are tipped with a duller colour. In spring the tip is lost by abrasion and the brighter colours are revealed. One obvious example is provided by the black breast of the House Sparrow which is hidden by the grey feather tips in winter.