All birds are covered with feathers, collectively called plumage, which are specialized structures of the epidermis, or outer layer of skin. The main component of feathers is keratin, a flexible protein that also forms the hair and fingernails of mammals. Feathers provide the strong yet lightweight surface area needed for powered, aerodynamic flight. They also serve as insulation, trapping pockets of air to help birds conserve their body heat. The varied patterns, colors, textures, and shapes of feathers help birds to signal their age, sex, social status, and species identity to one another. Some birds have plumage that blends in with their surroundings to provide camouflage, helping these birds escape notice by their predators. Birds use their beaks to preen their feathers, often making use of oil from a gland at the base of their tails. Preening removes dirt and parasites and keeps feathers waterproof and supple.

Because feathers are nonliving structures that cannot repair themselves when worn or broken, they must be renewed periodically. Most adult birds molt-lose and replace their feathers-at least once a year. Bird wings are highly modified forelimbs with a skeletal structure resembling that of arms. Wings may be long or short, round or pointed. The shape of a bird's wings influences its style of flight, which may consist of gliding, soaring, or flapping. Wings are powered by flight muscles, which are the largest muscles in birds that fly. Flight muscles are located in the chest and are attached to the wings by large tendons. The breastbone, a large bone shaped like the keel of a boat, supports the flight muscles. Nearly all birds have a tail, which helps them control the direction in which they fly and also plays a role in landing.

small bird cages

The paired flight feathers of the tail, called retrices, extend from the margins of a bird's tail. Smaller feathers called coverts lie on top of the retrices. Tails may be square, rounded, pointed, or forked, depending on the lengths of the retrices and the way they terminate. The shapes of bird tails vary more than the shapes of wings, possibly because tail shape is less critical to flight than wing shape. Many male birds, such as pheasants, have ornamental tails that they use to attract mating partners.

Birds have two legs; the lower part of each leg is called the tarsus. Most birds have four toes on each foot, and in many birds, including all songbirds, the first toe, called a hallux, points backwards. Bird toes are adapted in various species for grasping perches, climbing, swimming, capturing prey, and carrying and manipulating food.

Instead of heavy jaws with teeth, modern birds have toothless, lightweight jaws, called beaks or bills. Unlike humans or other mammals, birds can move their upper jaws independently of the rest of their heads. This helps them to open their mouths extremely wide. Beaks occur in a wide range of shapes and sizes, depending on the type of food a bird eats.

The eyes of birds are large and provide excellent vision. They are protected by three eyelids: an upper lid resembling that of humans, a lower lid that closes when a bird sleeps, and a third lid, called a nictitating membrane, that sweeps across the eye sideways, starting from the side near the beak. This lid is a thin, translucent fold of skin that moistens and cleans the eye and protects it from wind and bright light.

The ears of birds are completely internal, with openings placed just behind and below the eyes. In most birds, textured feathers called auriculars form a protective screen that prevents objects from entering the ear. Birds rely on their ears for hearing and also for balance, which is especially critical during flight. Two groups of birds, cave swiftlets and oilbirds, find their way in dark places by echolocation-making clicks or rattle calls and interpreting the returning echoes to obtain clues about their environment.

The throats of nearly all birds contain a syrinx (plural, syringes), an organ that is comparable to the voice box of mammals. The syrinx has two membranes that produce sound when they vibrate. Birds classified as songbirds have particularly well-developed syringes. Some songbirds, such as the wood thrush, can control each membrane independently; in this way they can sing two songs at the same time.

Birds have well-developed brains, which provide acute sensory perception, keen balance and coordination, and instinctive behavior, along with a surprising degree of intelligence. Parts of the bird brain that are especially developed are the optic lobes, where nerve impulses from the eyes are processed, and the cerebellum, which coordinates muscle actions. The cerebral cortex, the part of the brain responsible for thought in humans, is primitive in birds. However, birds have a hyperstriatum-a forebrain component that mammals lack. This part of the brain helps songbirds to learn their songs, and scientists believe that it may also be the source of bird intelligence