The rheas (/ˈriː.əz/REE-əz), also known as nandus /ˈnænduːz/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 IUCN classifies the puna rhea as a 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.