I wouldn't say it doesn't depend on the physics. Of course it does. As for my original question, i realize the flying mechanism of an eagle is completely different from that of a Quetzalcoatlus (thank you @sejje). Modern day birds use their feathers to trap air (or somthing like that), no? Where as the Quetzalcoatlus would simply use the surface-area of their leathery wings to glide, like human-made gliders.