There are almost 9000 species of birds in the world today. Birds are warm-blooded, vertebrate animals with four limbs, two of which are adapted into wings. They have a toothless beak and a covering of feathers over their bodies and heads (apart from the bald-headed vultures). The feathers close to the skin are soft and fluffy for warmth, while long, stiff outer feathers help birds to gain height, steer and control speed when flying. Birds have light, hollow bones to reduce their weight in the air.
Birds lay hard-shelled eggs, usually several at a time. After mating, the male and female often work together to build a nest ready to receive the eggs. Nests can be cup-like structures made of mud, grass and twigs, holes in trees, or even burrows underground. One parent usually sits on the eggs to keep them warm, while the other collects food. After hatching, most birds feed and protect their young until they are old enough to leave the nest.
Because of their ability to fly at speed for long distances, some kinds of birds are able to migrate to warmer climates as winter approaches. They return to their breeding areas in spring, as food becomes plentiful.
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