Birds are warm-blooded vertebrate animals that have wings, feathers, a beak, no 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.
Bird locomotion is quite varied; most can fly, some can run very well, some swim, and some do combinations of these. Some birds cannot fly.
Most birds can fly. Flying birds' wings are shaped to provide lift, allowing them to fly. These light-weight animals have adapted to their environment by flying, which makes them efficient hunters, lets them escape from hungry predators (like cats), and takes them away from harsh weather (migration).
The peregrine falcon is one of the fastest birds, and has been clocked at 90 miles per hour in a dive (and some people say that they can dive at over 200 mph).

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