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Flightless Birds

Ostriches, Rheas, Emus, Cassowaries, Kiwis

Ratites are flightless birds that lack the keel (high ridge) on the breastbone to which the flight muscles of flying birds are attached. Instead, the entire breastbone looks rather like a turtle's shell. It has also been described as a raft, which gives this group of flightless birds its name, Ratitae (Ratis means raft in Latin). Ratites have heavy, solid bones, while flying birds have lightweight, hollow ones. Several ratites, such as ostriches, rheas, emus, and cassowaries, are the largest living birds. The kiwis of New Zealand, however, are about the size of chickens.

These flightless birds are the oldest living birds. All older species of ratites are now extinct. However, several ratite species became extinct only recently. Genyornis of Australia survived long enough to be hunted by aborigines about 30,000 years ago. The largest bird ever found, the elephant bird or roc (Aepyornis), lived in Madagascar. A huge New Zealand moa (Dinornis), which became extinct only about 200 years ago, may have been as tall, but did not weigh as much as the roc. The moa had no wings at all.

Although ratites are the most ancient of the living birds, they are no more closely related to the reptiles from which they evolved than other birds are. In fact, they are probably descended from flying birds. Their ancestors lost the ability to fly because they did not need to fly to obtain food or escape from predators. They probably had no important enemies in their habitats.

The structure of their feathers also prevents ratites from flying. On a flying bird's feathers, the barbs, those branches that grow at an angle from the shaft (or quill), are fastened together by hooked structures called barbules. This design makes a smooth, flat, light surface that can push against the air during flight. The feathers of ratites, however, lack barbules. The strands that grow from the quill separate softly, allowing the air through. This quality of softness is what makes the feathers of many ratites particularly desirable. Ostrich plumes, for example, have long been used as decoration on helmets and hats.

The living flightless birds are classified into four orders and five families. The single species of ostrich is in the order Struthioniformes, family Struthionidae. The two species of rhea are in the order Rheiformes, family Rheidae. Emus and cassowaries are classified in the same order, Casuariiformes; emus belong to the family Dromaiidae, while cassowaries comprise the family Casuariidae. Kiwis belong to the order Apterygiformes, family Apterygidae. Some ornithologists consider the tinamous of Central and South America to be ratites because they seldom fly. However, tinamous are capable of flight (although they prefer to run from predators and other danger) and have keeled breastbones. Penguins are also flightless birds, but they are not regarded as ratites. Their powerful breast muscles are used for swimming instead of flying.

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