Birds have certain characteristics that help them stay light weight. They have hollow bones, light feathers, and they don't have to carry around their babies inside them. They also eat foods hight in calories to get rid of it with no digestion problems. They also don't have teeth or a nose because they would be too heavy and stick out too far forward. To chew their food birds have a gizzard near the center of their gravity. A gizzard is a muscular part of a bird's stomach that is thick walled and grinds the bird's food.
A bird uses it's muscles to flap it's wings up and down. When a bird brings up it's wings it separates it's feathers to resist resistance. When it brings it's wings down it creates resistance to bring it up. The birds also pull their wings in on the backstroke to maintain control. When a bird flaps it's wings it's being pulled up at the same time by pressure.
Birds are made in different ways, some are broad and long winged and some are thin short winged. A long curved wing is made for speed and agility. The bigger wing area creates a longer path of air. Some times birds have to use Earth's gravity which may be jumping off of something like a tree. Other birds may run on the ground to take off. Getting a start will create thrust which is needed to keep a bird flying.
When birds fly they split the air in to pieces. The air above the wing is faster air and the air below is slower. There is less pressure in the above the wing and more pressure below creating lift. Lift keeps the bird up in the air. When birds flap their wings it creates thrust. Thrust keeps the bird moving forward. Birds are light weight because their bones are hollow making it easier to stay up in the air. The shape of a bird's wing also helps it fly by the curve in the wing.