[Diagram on page 23]

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Barb

Barbule

Rachis

[Picture on page 24]

Contour feathers

[Picture on page 24]

Filoplume

[Picture on page 25]

Powder feather

[Picture on page 25]

Down feather

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.