Feathers provide birds with warmth, protection in the rain, and coolness in heat. They are made of a tough, flexible material called keratin. Like hair, feathers are made of protein, and a bird can move its feathers using muscles. Each feather has a hollow, central shaft with vanes—the two halves of the feather—on each side. Vanes are made up of barbs—thousands of thin branches that angle toward the feather’s tip. On both sides of each barb there are even smaller branches called barbules.
Some birds don’t use their wings to fly. One group of flightless birds is called ratites. This family consists of the heaviest living bird, the ostrich, as well as rheas and kiwis. Their wings are relatively small in comparison to their bodies. Since they don’t fly, their feathers don’t have to be smooth; their outer feathers remain fluffy. In general, birds that don’t fly develop thicker bodies and strong legs and feet adapted for running. Another group of flightless birds are penguins. Penguins have wings that don’t fold. Their wing bones are flat, forming a broad paddle, which helps them swim.
Contour feathers cover the body, especially the wings. Their various parts are held together by barbules that can interlock like Velcro to create a smooth, solid surface that makes flying, gliding, and even swimming easier. Down feathers lie under contour feathers to help keep birds warm. Their fluffiness keeps in pockets of warm air for insulation.
For feathers to work properly, they must be smoothed out and arranged with their barbules hooked together. Almost all birds have an oil gland under their tail that they can reach back and squeeze with their beaks. Using their feet and beak, they rub this oil over their feathers to make them shiny and waterproof. Birds who fix their feathers this way are preening. During preening, a bird will nibble with its beak along the sides of one feather at a time. Of course, birds that lack this gland—such as frigate birds—get soaked when it rains.
Like beaks and feet, wings are adapted to the bird’s environment. There are four wing styles for flying birds. A half-moon-shaped wing is found on birds used to a rapid take-off, such as woodpeckers, and other birds that are used to small spaces and the need to flee quickly. These wings also have some spacing between the feathers, making them lighter and easier to move. The downside is that these wings aren’t made for fast or long-term flights.
Swifts, swallows, and other birds of prey have long, narrow, and pointed wings with no spaces between the main feathers. These “solid” wings are angled backward, similar to an airplane wing, allowing high-speed flight.
The other two types are similar in style, but different in function. The “gliding wing” is typical of sea birds, such as gulls. These wings are long, narrow, and flat, with no spaces between feathers. The “soaring wing” structure is typical of eagles, storks, and vultures. These are different from the gliding wings in that there are wide spaces between the feathers; they are shorter to deal with changes in the air currents; and they’re wider to enable the bird to carry prey.
Just as other animals shed their fur or skin, birds shed their feathers a few at a time and replace them in a process called molting. It usually takes place during a time that is best for the bird—usually after breeding and before migration. For this reason, many birds molt only once a year. If a new feather is damaged, it cannot be replaced until the next molting. Penguins lose all their feathers within two weeks, though new feathers are already growing by the time the old ones all fall out. Eagles, on the other hand, molt slowly. Because they need feathers for flying, clusters of feathers may stay with an eagle for two or three years.
Feather color is very important for attracting mates and displaying to other birds. Peacocks are an excellent example of a bird that uses color to its advantage. During mating season, when a male peacock sees a female—which looks like an ordinary brownish bird—he
Ill lift his green-and-blue tail straight up and fan it out, making a rattling sound. Of course, colors can also act as a disguise for birds in some settings. The Australian frogmouth hides by sitting on a branch and lifting its head up, so that its brown color blends in with the branches.
Feather colors are produced by two substances called pigments—melanin and carotenoids. Melanin, made in a bird’s body, is responsible for blacks, browns, and some yellows, while carotenoids produce bright red, oranges, and other yellows. Carotenoids are in the foods the bird eats. Pink flamingos would lose their shading if they could not eat carotenoid-filled foods such as plankton, shrimp, or—as handlers at the Philadelphia Zoo have found—carrots.
Other shimmering colorism such as the shiny green of a hummingbird or the peacock’s tail, are produced in part by the shape of the feather. Slight differences in the shape of the barbs break up the light as it hits the feather, producing an array of colors, much like a prism. The colors on the bird will vary depending in the angle from which it is viewed.