The birds have hollow bones, making it easier for them to be lighter. They also have contour feathers that help them steer, and the shape of their wing is essential. The top part of the wing is curved, making the air going over it slower than the air going under the wing(the bottom of the wing is flat). When the air underneath makes it to the end of the wing before the upper air, it causes a draft, making the bird lift.
Birds wings are curved from the top and flat from the bottom they use bernoulies principal.