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.
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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