Birds are warm-blooded vertebrate animals that have wings, feathers, a beak , sans teeth, strong, hollow bones, a skeleton in which many bones are fused together or are absent, powerful flight muscles, and an extremely efficient, one-way breathing system. Most birds can fly. Birds have a very strong heart and an efficient way of breathing - these are necessary for birds to fly. Birds also use a lot of energy while flying and need to eat a lot of food to power their flight. Not all flying animals are birds; and not all birds can fly. The ability to fly has developed independently many times throughout the history of the Earth. Bats (flying mammals), pterosaurs (flying reptiles from the time of the dinosaurs), and flying insects are not birds.

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