Why don all birds fly?
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.
