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Bird, animal with feathers and wings. Birds are the only animals with feathers, although some other animals, such as insects and bats, also have wings. Nearly all birds can fly, and even flightless birds, such as ostriches and penguins, evolved from flying ancestors.

Birds are members of a group of animals called vertebrates, which possess a spinal column or backbone. Other vertebrates are fish, amphibians, reptiles, and mammals. Many characteristics and behaviors of birds are distinct from all other animals, but there are some similarities. Like mammals, birds have four-chambered hearts and are warm-blooded—having a relatively constant body temperature that enables them to live in a wide variety of environments. Like reptiles, birds develop from embryos in eggs outside of the mother’s body

This diagram illustrates some of the important anatomical structures common to birds. Many of these structures are adapted for greater efficiency in flying. The digital and wrist bones of the wing are fused to form ridged support for the flight feathers and the large sternum helps support muscles used in flying. The bones of many adult birds are hollow rather than filled with marrow, making them lighter and enabling them to disperse heat in flight.

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. The wings are highly modified forelimbs with a skeletal structure resembling that of arms. 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.

Nearly all birds have a tail, which helps them control the direction in which they fly and also plays a role in landing. 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.

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.

Birds have toothless, lightweight jaws, called beaks or bills. 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. 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 have well-developed brains, which provide acute sensory perception, keen balance and coordination, and instinctive behavior, along with a surprising degree of intelligence.

Scientific classification: All birds belong to the class Aves, which is subdivided into 27 orders.

See Types of Birds; Pictures of Birds