The rheas are large ratites (flightless birds without a keel on their sternum bone) in the order Rheiformes.
They are native to South America and related to the ostrich and emu.
There were formerly two, but now three recognized extant species: the greater or American rhea (Rhea americana), the lesser or Darwin's rhea (Rhea pennata) and the puna rhea (Rhea tarapacensis).
The greater and puna rheas are currently rated as near-threatened in their native ranges, while Darwin's rhea is of least concern; Reference: Wikipedia