An animal wing is an anatomical structure that allows the animal to fly or glide. In most instances, flightless animals don't have wings. Birds are the exception. All birds possess appendages apparently designed for flight, but in some cases, the wings are unable to keep the bird airborne. Perhaps the best-known flightless birds are ostriches and penguins. These birds belong to orders in which most or all of the species cannot fly. Less well known are birds, such as the flightless cormorant, that are the only flightless species in their order.
Ratites are birds with flat sternums that lack a keel, the point where the breast muscles attach. This flat sternum prevents ratites from developing breast muscles strong enough to lift themselves off the ground. Their wings are also disproportionately small relative to their body mass. Their legs, however, are large and powerful, making them good runners. Ostriches, emus, cassowaries, rheas and kiwis are the five types of flightless ratites currently living. Other types are now extinct. The tinamou, a quail-like bird, is often included in the ratite order, even though it has a keel and can fly a little.
There are 17 species of penguins, all of which are native to the southern hemisphere and none of which can fly. Their dense bodies and small wings, often called flippers, prohibit them from obtaining the necessary lift. However, their breast and wing muscles are powerful. Penguins propel themselves through the water with the same motion other birds use for flight. They are excellent swimmers, and some penguins spend over half their lifetime beneath the waves.
Cormorants are a type of large waterfowl related to pelicans. Galapagos cormorants, also known as flightless cormorants, are the largest species of their order and the only ones that cannot fly. Both their wings and the keels on their sternums are proportionally too small and lacking in strength. Flightless cormorants are native to the Galapagos Islands in Ecuador and are an extremely endangered species.
Another flightless bird that is also the largest of its species, is the kakapo parrot. These nocturnal parrots have an owl-like appearance. In their native country of New Zealand, their name means "night parrot." They cannot fly because their sternums are lacking keels, so they live on the ground and exist on a diet of plants. Kakapo parrots are also critically endangered.
Other Flightless Birds
Other orders of birds, such as ducks, rails and wrens, contain a species or two that is flightless, usually the biggest or heaviest species. Flightless birds have a natural disadvantage, especially if they are not aquatic. They can fall prey to almost anything. The last living Stephen's Island wren was killed by a cat in 1894, and numerous other flightless species have met with similar fates or exist only because of preservation measures taken on their behalf.