Weight and wingsize! Flying birds have special wings for flying. It also depends on the shape of the wings. Some birds have lost the ability to fly because it is simply not a trait necessary for survival. The ostrich is a well-known flightless bird that instead is able to run very fast. Penguins also cannot fly but are adept swimmers; their modified wings are very good for swimming but useless for flight. There is also a flightless cormorant that lives in the Galapagos Islands; again, it is a good swimmer and because it does not face predation on its island that would eradicate non-flyers, has lost the ability to fly.