What allows birds to move easily through the air

ha. I'm doing this questionare about birds for school...

birds are able to fly because of a variety of specialized adaptations. They have high metabolisms to supply their body with energy. They have lightweight bones. They have feathers, some of which are "flight feathers" that are long, strong, and able to produce lift and act as control surfaces. They also have a bone called the furcula, more commonly known as the "wishbone," in their chest, which is very important for being able to produce the strength and skeletal support needed to flap their wings.
The mechanics of flight are not unlike an airplane; the factors of lift, weight, thrust, and drag all interact to allow for controlled flight. By flapping their wings, birds create thrust and lift. They are able to steer by changing the shape and orientation of their wings and tail. They change the shape of their wing between the upstroke and downstroke as they flap to minimize drag on the upstroke and maximize the thrust generated downard on the downstroke, providing both lift and thrust. <><><> When all the feathers in a bird's wings are set in a certain way, so that the air cannot flow through them, the wing takes up a special shape which makes the air flow much faster over the top surface of the wing than it flows below the under-surface of the wing when the bird is gliding through the air. The difference in air-speed between the top and bottom surfaces of the wings gives the wings a "lift" force which can easily counteract the force of gravity.

A squirrel drinks Red Bell and gets wings!! But if they only have half of a Red Bull they become flying squirrels.

The shape of their wings and their feathers. (At least I think... Haha I SHOULD know this because I love birds so much, but I'm not an expert)

And haha yea what she ^^^^ said... <.<