Most birds move by flying through the air using their wings. The shape of a bird's wing is specially designed to force air to move faster over the top of the wing than the bottom. This causes pressure to build below the wing which lifts the bird into the air.
Birds use different techniques to gain enough momentum to force air to move rapidly over their wings. Some, like the peregrine falcon and the puffin, jump from cliffs or other high objects and use their fall to drive air over their wings and provide themselves with enough lift to fly. Others, like the hummingbird, can beat their wings so rapidly that they drive air over their wings quickly enough to achieve the required lift.
Bird's bodies are also specially designed to accommodate flight. Their bones are hollow and therefore extremely light, their lungs only fill up with a limited amount of air at one time and they do not have teeth or a bladder, all of which make birds extremely light. Birds also take advantage of the different air currents in order to provide themselves with lift. Rapidly rising plumes of hot air called thermals, act like an elevator and lift birds high into the air, while updrafts, rapidly rising air that is forced upward by an obstruction, can also be used in the same manner.