Rheas are large, flightless birds with grey-brown plumage, long legs and long necks, similar to an ostrich. Large males of R. americana can reach 170 cm (67 in) tall at the head, 100 cm (39 in) at the back and can weigh up to 40 kg (88 lb), The lesser rhea is somewhat smaller as they are only 90 cm (35 in) tall at the back. Their wings are large for a flightless bird (250 cm (8.2 ft)) and are spread while running, to act like sails. Unlike most birds, rheas have only three toes. Their tarsus has 18 to 22 horizontal plates on the front of it. They also store urine separately in an expansion of the cloaca.

The common ostrich or simply ostrich, is a species of large flightless bird native to certain large areas of Africa. It is distinctive in its appearance, with a long neck and legs, and can run for a long time at a speed of 55 km/h (34 mph) with short bursts up to about 70 km/h (43 mph), the fastest land speed of any bird. The common ostrich is the largest living species of bird and lays the largest eggs of any living bird. The bird has just two toes on each foot (most birds have four), with the nail on the larger, inner toe resembling a hoof. The outer toe has no nail.

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