The rheas ( REE-əz), also known as ñandus ( NAN-dooz) or South American ostriches, are large ratites (flightless birds without a keel on their sternum bone) in the order Rheiformes, native to South America, distantly related to the ostrich and emu. Most taxonomic authorities recognize two extant species: the greater or American rhea (Rhea americana), and the lesser or Darwin's rhea (Rhea pennata). The International Union for Conservation of Nature (IUCN) classifies the puna rhea as another species instead of a subspecies of the lesser rhea. The IUCN currently rates the greater and puna rheas as near-threatened in their native ranges, while Darwin's rhea is of least concern. In addition, a feral population of the greater rhea in Germany appears to be growing, though control efforts are underway, and seem to be succeeding in controlling the birds' population growth.