Birds live by feeding off of both plants and animals from air, water or land. There are about 8,600 species of birds worldwide and approximately 100 billion birds overall.
Birds are vertebrates with bodies covered by feathers. Their front limbs are modified into feathered wings, which allow them to fly. They are equipped with powerful breast muscles that enable their wings to move. Most birds can fly, and their tail helps direct their flight. Their light, rigid skeleton and quick digestion help them to fly. Their jaws are covered by a horny beak, and they mostly have hind limbs with four toes, which are used for walking on land. Some birds can fly, walk and swim, although most of them are best adapted for one or two of these abilities. Water birds in particular have webbed-swimming feet.
Birds survive in a variety of habitats, including mountains, forests, tundras, deserts and near bodies of water. Many birds stay in their habitat for a long period, while others migrate to warmer countries when the weather changes. Birds have diverse diets. As they have no teeth, food moves into their two chambered stomach. They need to eat large quantities of food to produce energy necessary for flight and temperature regulation, particularly in winter and during migration.