Rheidae  is a family of flightless ratite birds which first appeared in the Paleocene.It is today represented by the sole living genus Rhea, but also contains several extinct genera.
Rheas are the largest birds in South America and are common residents of open grasslands or croplands throughout the southern part of the continent. Though endowed with excellent running ability, rheas usually saunter away long before a visitor draws too close. If left in peace, they will slowly graze through an area, searching for anything edible, animal or vegetable, at a pace less than a human’s slow walk. Males incubate the eggs laid in their nests by multiple females, and after hatching, the chicks forage in the grass, accompanied by their father until gradually reaching their independence about seven months later.

Family Rheidae,  Bonaparte 1849