A flying hummingbird.
Birds learn to fly like a human child learns to walk: a combination of instinct and practice. However, not all birds are instinctive fliers. Flightless birds, such as penguins and ostrich, no longer have the instinct to mimic their flying cousins. Newborns of flying bird species such as pigeons or hummingbirds have an innate sense that this is a natural act. Contrast that with a human baby who instinctively understands that standing is a natural goal to achieve.
Parent birds must slowly teach baby birds to be independent and learn to fly to find food.
Most birds cannot fly until their muscular structure has had time to develop. Meanwhile, the nest becomes your whole world. The little birds are not in charge of collecting food or protecting the nest, so they often develop a psychological dependency that must be overcome. Parent birds begin to teach the chicks the importance of flight by staying a short distance from the nest during feeding. For the chicks to survive, they must move away from the nest. This often means a few hard falls to the ground followed by a long journey back to the safety of the nest.
Birds like pigeons have an innate sense of flight as a natural act.
All this practice time, oddly enough, teaches the novice the mechanics of flight. Falls to the ground become more controlled as the young bird spreads its wings, and short hops back to the nest become longer flights. Bird parents continue to encourage their young to leave the nest for longer periods. Some species actually adopt a strict love policy, leaving the young alone to develop their own instincts.
Some birds, like ostriches, cannot fly and have no instinct.
After a few weeks of practice and imitation, the young birds learn more advanced flight techniques: how to use the wind for lift, how to detect rising thermals, and how to make controlled landings. Eventually, all of these elements become instinctive and the young birds can form families of their own. The teaching process begins again when these birds teach their own chicks to fly.
For birds, flying is an extremely strenuous exercise. Some bird experts compare it to the human race multiplied by ten. Fortunately, many birds have air sacs that act as auxiliary lungs, and each breath a bird takes is much more concentrated than the equivalent human breath. Birds also have very well developed pectoral muscles for constant wing movement and an exceptionally strong heart for endurance. Hollow bones reduce drag and the natural curve of the wings creates significant lift. Most birds literally swim in the air, using the weight of the air below them to keep themselves airborne.