Not all birds can fly. Though many people think flying is what defines birds as a group, in actuality several bird species are flightless birds.
Some birds are simply too large to fly. This includes land birds like Ostriches, Emus and Cassowaries. They do not need flight to escape predators. The Ostrich can run away, reaching speeds of 45 miles an hour. Cassowaries can defend themselves with powerful claws.
Several birds choose to swim instead of fly. Penguins, for instance, have wings evolved into narrow paddles they use to push themselves through the water. They are incredible swimmers and divers- large wings would interfere with their underwater abilities.
Many flightless birds evolved on small islands, where there are no mammal predators to fly away from. These birds include the Flightless Cormorant on the Galapagos Island, the now-extinct Dodo bird of Mauritius, many species of rails (small marsh birds), and New Zealand’s famous Kiwi birds.
These island-dwelling flightless birds are at particular risk of extinction. As humans moved onto their islands, they brought with them many mammal predators, such as housecats, mongooses and rats. Unable to escape from these new predators, several flightless birds went gone extinct.
Other flightless birds are still in danger. For instance, non-native predators reduced the Kakapo (New Zealand’s bizarre flightless parrot) to less than a hundred individual birds. Many people are now working hard to clear its home islands of non-native mammals, so that the flightless bird can recover.