Feathers are the ultimate characteristic of birds. No other (living) animals have them. The qualifier is required because feathers have now been found in fossil imprints of some dinosaurs and related reptiles.
Like hair on mammals and scales on reptiles, feathers are part of the integument (skin). All are largely composed of keratin, which is also the main ingredient of human nails, animal claws, and the scales on the legs and feet of birds.
Feathers are remarkable structures, both very strong and very light. They’re subject to long flights and are bent and twisted, yet they are rarely damaged. Abrasion causes their tips and edges to wear, but this is natural and remedied periodically through molt. Melanins are common pigments that can make feathers black. Dark feathers containing melanins are more resistant to damage than other feathers. This is why the outer wing feathers of many birds with white wings, such as gulls, have black tips.
Birds have six different feather types that vary in shape, structure, and function. The most familiar is the contour feather. It consists of a central shaft and countless barbs that protrude from either side, forming vanes. The portion of the shaft that supports the barbs is called the rachis, while the bare portion that embeds in the skin is called the calamus.
Vanes of contour feathers must be rigid and flexible at the same time. Serious engineering solved the problem. A magnifying glass reveals that each barb has smaller barbs, called barbules, projecting from either side, toward the adjacent barbs. The barbules on one side of the barb are straight, while those on the other are hooked. Cleverly, the barbules of adjacent barbs overlap, so the hooked barbules attach to the straight barbules of the other, making the vane rigid. (See diagram above.) If barbs separate and the vane splits, the bird can repair it by preening. Running the barbs through its bill reconnects the hooks like a zipper.
Strong, rigid vanes are especially important for flight. The trailing, inner wing feathers, the secondaries, provide lift, while the trailing, outer wing feathers, the primaries, provide thrust. Most species have large tail feathers. They function like a rudder when flying and like brakes when landing.
Smaller contour feathers cover the body and leading edges of the wings. On the wings, the feathers help form the airfoil shape that is necessary for flight. On the body, they contribute color, which is important in courtship and for camouflage, and they form a sleek outer covering, providing an aerodynamic tear-drop shape that assists flight. From songbirds to swans, the neck is narrow and the breast muscles are massive. Where the body parts meet, contour feathers create a gradual slope.
Semiplumes are a second type of feather. As in contour feathers, barbs come off the sides of the rachis, but the barbules lack hooks, resulting in vanes that are soft and fluffy. Most semiplumes are concealed under contour feathers and help with insulation, but some, such as the enlarged, lacy feathers on the back of egrets, are used for courtship.
A third type, down feathers, are anchored in the skin by a calamus but lack a rachis. Barbs with smooth barbules radiate from the tip of the calamus, forming a short, loose, fluffy feather. Both down feathers and semiplumes lie under the body’s contour feathers, forming a mass of feathers that trap air, forming an excellent layer of insulation.
The other three feather types are quite specialized. Two, filoplumes and bristles, are hairlike. Filoplumes consist of a calamus and rachis but have only a few small barbs, near the tip. The feathers are found around contour feathers, especially on the wings. Filoplumes are associated with sensory receptors in the skin, and are thought to provide information about wind, air pressure, and feather movements that birds use to maintain efficient flight.
Bristles are short and stiff and lack barbs along most of the rachis but the base. They’re thought to be sensory in nature and are found around eyes and near the base of the beak. You will see them on insectivorous birds, such as flycatchers, which feed on the wing, and they are best developed in goatsuckers, such as Common Nighthawk, which appears to use them to funnel insects into the mouth while flying.
The final type is the powder down, a specialty feather that resembles semiplumes but differs in that the barbs grow continuously. Tips of the barbs crumble into a powder when bitten. Birds apply the powder to other feathers while preening, which serves to waterproof them. Powder downs are typically found in patches — two on the chest and two in the pelvic region — although they can be dispersed throughout the bird’s underside. Herons, bitterns, parrots, and tinamous possess powder downs.
Feathers are highly engineered structures that perform essential functions. Lightweight and strong, they allow birds to be amazing creatures. — Eldon Greij
A version of this article appeared in the February 2016 issue of BirdWatching.
Three recent articles by Eldon Greij
ON THE RUN
Why ostriches, rheas, emus, and cassowaries don’t fly
WHAT BIRDS CAN SMELL
Birds use scents to find food, mates, even their way home.
GRACE UNDER WATER
Adaptations that let birds dive and swim below the waves.
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