Birds have many different ways of taking off. Some, like loons, run into the wind, and the rush of air beneath their wings lifts them up. Others, like puffins and Peregrine Falcons, jump off cliffs and other high perches. Chimney Swifts simply let go of their chimney or other vertical perch, and fall into the air. Hummingbird wingbeats are so powerful that they can go straight up from a perched position without jumping. Songbirds, cranes, and many other species leap up on strong legs while flapping their wings, and there they go.
We humans could try leaping and flapping our arms, or running into a stiff wind, but we wouldn't get very far off the ground! The reason birds can is because of the special shape of their wings. The bones of bird wing are in front, covered with a smooth layer of feathers that taper toward the back. The back of the wing is just a single layer of flight feathers. People who study aerodynamics say a wing has this shape to serve as an airfoil. When air comes straight toward an airfoil (from facing directly into the wind or running fast into the air) the special shape causes the air to flow faster on top of the wing than under it. The faster air above lowers the pressure (sort of sucking the bird up) while the slower air below raises the pressure (pushing the bird up). These forces raising the bird are called lift, which makes the bird go up.