Rhea (bird), common name for either of two South American ostrichlike birds. They are smaller than African ostriches, however, and have three toes instead of two. The head and neck are completely feathered. The tail is undeveloped, but long feathers droop over and cover the posterior region of the body. The coloration varies from pale gray to brown. Like the ostrich, the males are polygamous and hatch the eggs; several females use the same nest and a single male may hatch 50 eggs. Rheas run very rapidly on long legs and travel in family groups of about 6 birds, except in the mating season, when flocks of about 25 congregate. The greater rhea inhabits grassy plains from Bolivia and Brazil to central Argentina. The lesser rhea is found in the high puna region of the southern Andes and in grasslands south of the range of the greater rhea.

Scientific classification: Rheas make up the order Rheiformes. They are sometimes placed in the order Struthioniformes. The greater rhea is classified as Rhea americana, the lesser rhea as Pterocnemia pennata.