One of two species in the genus Rhea, in the family Rheidae, it inhabits a variety of open areas, such as grasslands, savanna or grassy wetlands.

They are closely related to other ratites such as emus, ostriches, cassowaries, and kiwi, along with the extinct forms: moa and elephant birds.

There are five subspecies of the greater rhea; their ranges meet around the Tropic of Capricorn:[9] Main subspecific differences are the extent of the black coloring of the throat and the height.

[15][16] Large males can weigh up to 40 kg (88 lb), stand nearly 1.83 m (6.0 ft) tall and measure over 150 cm (59 in) long, although this is uncommon.

The wings of the American rhea are rather long; the birds use them during running to maintain balance during tight turns, and also during courtship displays.

Greater rheas have a fluffy, tattered-looking plumage, that is gray or brown, with high individual variation, The head, neck, rump, and thighs are feathered.

It is absent from the humid tropical forests of the Mata Atlântica and planalto uplands along the coast of Brazil[22] and extends south to 40° latitude.

These birds survived the winter and succeeded in breeding in a habitat sufficiently similar to their native South American range.

[28] The greater rhea is a silent bird except during mating season, when they make low booming noises, and as chicks, when they give a mournful whistle.

Favorite food plants include native and introduced species from all sorts of dicot families, such as Amaranthaceae, Asteraceae, Bignoniaceae,[30] Brassicaceae, Fabaceae,[30] Lamiaceae,[30] Myrtaceae[30] or Solanaceae.

However, the leaves of particular grass species like Brachiaria brizantha can be eaten in large quantities, and broad-leaved plants (e.g. the sarsaparilla Smilax brasiliensis) have also been recorded as foodplants.

[9][30] Like many birds which feed on tough plant matter, the greater rhea swallows pebbles which help grind down the food for easy digestion.

It will eat any large invertebrate it can catch; its food includes locusts and grasshoppers, true bugs, cockroaches, and other pest insects.

It is not clear whether this applies to the species in general but for example in pampas habitat, beetle consumption is probably lower simply due to availability while Orthoptera might be more important.

Sometimes, greater rheas will gather at carrion to feed on flies; they are also known to eat dead or dying fish in the dry season, but as vertebrate prey in general not in large quantities.

Then they start courting females by calling and raising the front of their body up while keeping their neck straight and ruffling their plumage.

Classical conditioning of greater rhea juveniles against predator models can prevent this to some degree, but the personality type of the birds – whether they are bold or shy – influences the success of such training.

In 2006, a protocol was established for training greater rheas to avoid would-be predators, and for identifying the most cautious animals for release.

[35] The greater rhea is considered a Near Threatened species according to the IUCN, and they have a decreasing range of about 6,540,000 square kilometres (2,530,000 sq mi).

[5] The species is believed to be declining due to increased hunting[1] and the conversion of central South American grasslands to farmland and ranchland.

[9] Farmers sometimes consider the greater rhea pests, because they will eat broad-leaved crop plants, such as cabbage, chard and bok choy.

In its new home, the greater rhea is considered generally beneficial as its browsing helps maintain the habitat diversity of the sparsely populated grasslands bordering the Schaalsee biosphere reserve.