The rheas are large ratites, native to South America, related to the ostrich and emu.
There are two extant species: the greater or American rhea (Rhea americana) and the lesser or Darwin's rhea (Rhea pennata).Both are currently rated as near-threatened in their native ranges; a feral population of the greater rhea in Germany appears to be growing.
Rheas are from South America only and are limited within the continent to Argentina, Bolivia, Brazil, Chile, Paraguay, Peru, and Uruguay. They are grassland birds and both species prefer open land. The greater rheas live in open grasslands, pampas, and chaco woodlands. They prefer to breed near water and prefer lowlands, seldom going above 1,500 metres (4,900 ft). On the other hand, the lesser rhea will inhabit most shrubland, grassland, even desert salt puna up to 4,500 metres.
A small population of rheas has emerged in northeastern Germany, after several couples escaped from an exotic meat farm near Lübeck in the late 1990s. Contrary to expectations, the large birds have adapted well to the conditions in the German countryside.
Rheas tend to be silent birds with the exception being when they are chicks or when the male is seeking a mate. During breeding season, the male will attempt to attract females by calling. This call is a loud booming noise. While calling like this, they will lift the front of their body, ruffle their plumage, all while keeping their neck stiff. They will then extend and raise their wings, and run short distances, alternating with their wings. He may then single out a female and walk alongside or in front of her with a lowered head and spread wings. If the female notices him, then he will wave his neck back and forth in a figure-eight. Finally, a female may offer herself and copulation will commence.
During the non-breeding season they may form flocks of between 20 and 25 birds,although the lesser rhea forms smaller flocks than this. When in danger they flee in a zig-zag course, using first one wing then the other, similar to a rudder. During breeding season the flocks break up.
For the most part, rheas are vegetarian and prefer broad-leafed plants but they also eat fruits, seeds and roots, as well as insects such as grasshoppers and small reptiles and rodents.Young rheas generally eat only insects for the first few days. Outside of the breeding season they gather in flocks and feed with deer and cattle.
Rheas are polygamous, with males courting between two and twelve females. After mating, the male builds a nest, in which each female lays her eggs in turn. The nest consists of a simple scrape in the ground, lined with grass and leaves.The male incubates from ten to sixty eggs. The male will use a decoy system and place some eggs outside the nest and sacrifice these to predators, so that they won't attempt to get inside the nest. The male may use another subordinate male to incubate his eggs, while he finds another harem to start a second nest.The chicks hatch within 36 hours of each other. The females, meanwhile, may move on and mate with other males. While caring for the young, the males will charge at any perceived threat that approach the chicks including female rheas and humans. The young reach full adult size in about six months but do not breed until they reach two years of age.
The numbers of both the lesser and greater rhea are decreasing as their habitats are shrinking. Both are considered near threatened by the IUCN and have been for the last 15 years. The IUCN also states that they are both approaching vulnerable status.