On Earth there are many birds that have lost the ability to fly. For example, penguins live where survival needs are not wings, and fins. Because in order to catch fish, which they eat, you must have a good swim.
Bird kiwi living in New Zealand, leads nocturnal. It uses its long beak finds food on earth, so it's not necessary to fly in the sky and watch for prey.
In the Indonesian jungle lives a cassowary. This bird so bold and strong that she don't have to fly, fleeing from the enemy. She violently resists claws that not every predator will cope with it.