Birds learn how to fly much like a human toddler learns to walk: a combination of instinct and practice. Not all birds are instinctive flyers, however. Flightless birds, such as penguins and ostriches, no longer have the instinct to imitate their airborne cousins. Newborns of bird species that do fly, such as pigeons or hummingbirds, have an innate sense that it is a natural act. Compare this to a human baby who instinctively understands that standing upright is a natural goal to achieve.
Most birds cannot fly until their muscle structure has had time to develop. In the meantime, the nest becomes their entire world. Baby birds are not responsible for food gathering or protection of the nest, so they generally develop a psychological dependence that must be overcome. Parent birds begin to teach their fledglings the importance of flying by remaining a short distance away from the nest during feeding. If the young birds are to survive, they must step away from the nest. Frequently, this means a few hard falls to the ground followed a long trip back to the safety of the nest.
All of this practice time, awkward as it may seem, does teach the fledgling about the mechanics of flight. Falls to the ground become more controlled as the young bird stretches out its wings, and short hops back to the nest become longer flights. Bird parents continue to encourage their brood to leave the nest for longer periods of time. Some species actually adopt a tough love policy, leaving the fledglings alone to develop their own instincts.
After a few weeks of practice and imitation, young birds learn more advanced flying techniques: how to use the wind for lift, how to spot rising thermals and how to make controlled landings. Eventually, all of these elements become instinctive and young birds can start families of their own. The teaching process begins anew as these birds teach their own young how to fly.
For birds, flying is an incredibly taxing exercise. Some bird experts compare it to human jogging times ten. Fortunately, many birds have air sacs that act as auxiliary lungs, and each breath a bird takes is much more concentrated than an equivalent human breath. Birds also have very well-developed pectoral muscles for constant wing motion and an exceptionally strong heart for endurance. Hollow bones reduce drag and the natural curve of the wings creates significant lift. Most birds are literally swimming through the air, using the weight of the air beneath them to keep them aloft.