Feathers - A Marvel Of Design

With a downward thrust of its wings, a seagull launches itself skyward. Once aloft, it wheels and turns, rising effortlessly on the wind. Making only tiny adjustments to the angle of its wings and tail, the bird hangs nearly motionless in the air. What enables it to perform those functions with such grace and perfection? To a great extent, its feathers.

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 neighboring barbules, forming a kind of zipper. When barbules unzip, the bird simply zips them back together by preening itself. You can do the same by drawing a frayed feather gently between your fingers.

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.

Distributed among the contour feathers of many birds are long, thin feathers called filoplumes, as well as powder feathers. It is thought that sensors at the roots of the filoplumes alert the bird to any disturbance of its outer feathers and may even help it to judge its air speed. The barbs of powder feathers—the only feathers that grow continuously and are never molted—break down into a fine powder that is thought to help waterproof the bird’s plumage.

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. How? Under their nearly impenetrable coat of contour feathers lies a dense layer of soft, fluffy feathers called down, which may be up to two thirds of an inch [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 molting—shedding old feathers and growing new ones. Most birds molt their wing and tail feathers in a predictable, balanced order so that they always retain their ability to fly.

NOTE: The vivid and often iridescent colors of feathers fascinate humans. But feathers may look even more interesting to other birds. Some birds have four types of color-detecting cones in their eyes, while humans have only three. This extra visual equipment enables birds to perceive ultraviolet light, which is invisible to humans. Male and female birds of some species look alike to humans, but the male’s feathers reflect ultraviolet light differently from the female’s. The birds can see the difference, which may help them to identify potential mates.