A cross-section through a bird, showing its skeletonBirds are warm-blooded, vertebrate animals with four limbs, the front two of which are adapted into wings. They are the only animals that have feathers. Birds have light, hollow bones and a toothless beak. Most can fly, but a few cannot. Birds lay hard-shelled eggs, and most species protect their young until they leave the nest. Some species migrate to warmer climates during the winter. Most birds feed on insects or plants but some larger birds are meat-eaters.
Birds have a covering of feathers over their bodies and heads. The feathers close to the skin, called down, are soft and fluffy for warmth. They trap air to keep the bird warm. Most chicks hatch with a covering of soft down.
Feather of a male peafowlThe outermost feathers on a bird's body are called the contour feathers. In most species, they give the bird a smooth, streamlined shape for flight, but in some flightless birds they are just for show.
The long, stiff feathers on the wings and tail of a bird are called the flight feathers. They are used to gain height, steer and control speed when flying. Flight feathers are divided into large, outer primary feathers and smaller secondary feathers, on the inside of the wing. Small contour feathers that overlap flight feathers on the tail and wing of a bird to give a streamlined finish are called covert feathers.
Swifts are among the fastest birds of all in level, flapping flight. The common swift can fly at 112 km/h (69 mph).
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