The ostrich is the largest living bird on the planet, reaching heights of 8 feet and weighing between 300 and 345 lbs. They are the only living member of the Struthionidae family of birds. Its closest living relatives, classified under the order Struthioniformes, are kiwis, cassowaries, emus and the moas. All birds in this order are flightless ground nesters.
The species most resembling Africa's ostriches are the South American rheas, of which there are two species: the greater rheas and Darwin's rheas. Standing at around 5 feet in height, the greater rhea resembles most closely a small female ostrich. These rheas are found mostly in the pampas grasslands of Brazil and Argentina. Like the ostrich, the rheas nest on the ground with the male or a dominant female incubating eggs from several females within the male's harem. Both species also are omnivorous. Their diet consists of fruits, insects, lizards and small birds.
Native to Papua New Guinea and areas of Australia are three species of Cassowary. Although they are members of the same order as the ostrich, they do not look much alike. Cassowarys are colorful birds, the largest species is the double-wattled that can get up to 5 feet in height. Unlike the ostrich, they are found in forested areas, not open plains. They are highly territorial and do not live in flocks but instead in mated pairs. Like ostriches, the males do much of the egg incubation.
The emu is the second-largest bird species in the world, reaching up to 6 feet in height. Found commonly throughout Australia, only one species of emu still exists. They have a similar shape to ostriches but are easily recognized as different because instead of normal feathers, emus appear to have a more loose, fur-like coat. Like the ostrich, they are found on open grasslands and do not venture into thick woodlands, which are more the domain of the cassowary.
The kiwi bird is by far the smallest relative of the ostrich, standing just about 19 inches in height. With hairlike feathers and a long, thin beak, the kiwi looks very little like its much larger cousin. Native to New Zealand, six species of kiwi exist, which are the Okarita brown, Stewart Island brown, North Island brown, Haast brown, greater spotted and little spotted. Like the cassowary and emu, kiwi birds make breeding pairs with the males doing most, if not all of the incubation.