The Billy Lids

Birds are the only animals today that grow feathers. Most birds have different kinds of feathers. The most visible are the overlapping contour feathers, which give birds their smooth, aerodynamic shape. Contour feathers include the wing and tail feathers, which are vital to flight. A hummingbird may have fewer than 1,000 such feathers, and a swan more than 25,000.

Feathers are a marvel of design. The central shaft, called the rachis, is flexible and remarkably strong. Extending out from it are rows of interlocking barbs that form the smooth vane of the feather. The barbs attach to one another by means of several hundred tiny barbules, which hook onto neighbouring barbules, forming a kind of zipper. When barbules unzip, the bird simple zips them back together by preening itself.

Wing flight feathers in particular are asymmetrical - the vane is narrower on the leading edge than on the trailing edge. This classic airfoil design enables each flight feather to act like a tiny wing in itself. Also, if you look closely at a major flight feather, you will see a groove running along the underside of the rachis. This simple design element strengthens the shaft, allowing it to bend and twist without buckling.

Besides their other functions, feathers protect birds from heat, cold, and ultraviolet light. Sea ducks, for example seem to thrive despite bitterly cold ocean winds. Under their nearly impenetrable coat of contour feathers lies a dense layer of soft, fluffy feathers called down, which may be up to 1.7 cm thick and cover most of the duck's body. Natural down is so efficient an insulator that no synthetic material yet devised equals it.

Feathers eventually wear out, so birds replace them by moulting - shedding old feathers and growing new ones. Most birds moult their wing and tail feathers in a predictable, balanced order so that they always retain their ability to fly.