Birds have streamlined bodies, which make it easier for them to move through air. Their bones are hollow, which make them lightweight. They have wings, which help them to fly. They have powerful chest muscles, which help them flap their wings during flight.
Birds’ wings are shaped to form an airfoil. When a bird moves forward through the air, the shape and curve of the wing cause the air to flow faster above the wing than below it. The faster air above lowers the pressure (drawing the bird upward) while the slower air below raises the pressure (pushing the bird upward). This force holding the bird up is called lift, and it requires that the bird be moving forward or facing into a fairly stiff wind.