Answer:
Because they have a large surface area to capture moving air when going at a certain speed.
Explanation:
When your trying to fly a kite and the wind isn't that much, you have to run so that the kite can come in contact with the fast moving air which pushes the kite to go up. Just like a eagle because when a eagle is trying to fly, it has to first flap its wings to push air downwards so that it can create lift and at a certain altitude, the eagle doesn't have to flap it's wings though it tilts its body downwards a little so that gravity can do the work, so the eagle is basically gliding downward.
