Birds usually have many enemies, and it is possible for them to escape from these enemies by flying away.
However, on islands that are cut off from mainland, birds are comparatively safe. Their enemies are few in number, for it is not possible for predators to reach an island, except by flying or swimming.
As a result, many species of birds on remote islands lose their ability to fly. The Tasmanian native hen, the cassowary, the takahe, and the flightless cormorant are some such birds.
Some species of birds evolve wings on the mainland, but after centuries on an island, become flightless. Some flightless birds have become extinct. The dodo, which was unique to Mauritius, is one such example. Since it could not fly, it was easy to catch, and as its meat was very tasty, the Dutch sailors on the island killed and ate so many, that the species became extinct!
