When a bird lifts off, it uses mass amounts of energy. This energy continues throughout the time the bird is in the air, but they are able to expend so much energy because of their natural diet. The food birds eat contains high energy proteins and carbohydrates. Birds are also equipped with hollow, lightweight bones that make it easier for them to stay in the air. Their feathers are light and positioned in a way that allows them to catch wind for greater lift. One additional perk birds have is their lung capacity, which allows for great reserves of oxygen that keep them from tiring during flight.
Gliding and Lift
Birds glide in the air in much the same way planes do. As they propel themselves through the air they also stretch their wings out and allow the moving air to flow over them. As they do this, their wings are pushing down on the air around them. Newton's third law of motion, which states that for every action, there is an equal and opposite reaction, comes into play here, meaning that as the bird puts pressure on the air, there is also pressure being put on the bird by the air. In this case it is pressure on the underside of the wing that lifts the bird in the air. This is the principle physics behind lift.
Hinged Wings and Propulsion
Birds have specialized wings that allow them to fly. First, they are slightly curved on the top, due both to bone structure and feathers. Air travels faster over the curve on top of the bird's wing than it does along the bottom. This makes the pressure under the wing higher, which puts more lift on the bird. In order to propel itself through the air, it takes advantage of its hinged wing. When the bird brings its wings down, they are straight and flat and at a slight angle that will propel the bird through the air. When the bird lifts its wings back up, though, it folds them in so they exert less pressure on the air. That keeps them from slowing down. All of these aspects combine to...