American Rhea, R. Americana
Darwin's Rhea, R. pennata
Rheas are polygamous: the male courts between two and twelve females. After mating, he builds a nest, in which each female lays her eggs. The male incubates from ten to sixty eggs; 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 anyone--including humans and female rheas--who approaches the chicks.
Rheas are omnivorous: they prefer broad-leafed plants, but also eat seeds, roots, fruit, insects, and small vertebrates.
Rhea Americanus (the gray or common rhea) is not only the largest species of rhea, but the largest South American bird, with adults averaging 23 kilograms (51 lbs).
Farmers sometimes consider them pests, because they will eat almost any crop plant. Because of this, farmers sometimes kill the birds. This, along with egg gathering and habitat loss, has led to a sharp population decline; the species is listed as being of special concern.
American rheas live in grassland, savanna, scrub forest, chaparral, and even desert, but prefer areas with at least some tall vegetation. During breeding season (which ranges from August to January, depending on location), they stay near water.
Rhea pennatus (Darwin's rhea, syn. R. darwini) is 90 to 100 centimeters (3' to 3'4") tall, and has larger wings than other ratites, enabling it to run particularly well. They can reach speeds of 60 km/hour, enabling them to outrun predators. The strong claws at the end of each wing are effective weapons.
The males of this species become aggressive once they are incubating eggs. The females thus lay the later eggs near the nest, rather than in it. Most of the eggs are moved into the nest by the male, but some remain outside, where they rot and attract flies. The male, and later the chicks, eat these flies.
Outside the breeding season, Darwin's rheas are quite sociable: they live in groups of from 5 to 30 birds, of both sexes and a variety of ages.
The "lesser rhea", Pterocnemia pennata, formerly R. macrorhyncha, appears to be a synonym for R. darwini.