The Greater Rhea is among the largest living birds. Its long, powerful legs carry a football-shaped body with a long protruding neck that can dip to the ground to feed and rise above tall grass to scan for danger. The plumage is gray, with black around the head, neck, and shoulders, lighter gray feathering the legs, and white on the underparts. The rhea, like all ratites, cannot fly, but retains large wings with which it supports bursts of running and performs impressive displays for mates and competitors. Its distinctive profile can be identified from a distance, and in its native habitat could only be mistaken for its sister species, the Lesser Rhea (Rhea pennata).
The Lesser Rhea (Rhea pennata) is the only other species in this family, and the only other ratite in the New World. As its name implies, the Lesser Rhea is slightly smaller, and its plumage is brown with a pattern of white flecks. In contrast the plumage of the Greater Rhea is unspotted and primarily is gray, although the base of the neck often is blackish. The two species replace each other geographically, with little or no overlap.
The following description is based on Blake (1977):
Very large, with long neck and tarsi. Tarsus entirely bare, with transverse scutes. Males larger and darker than female. Plumage of upperparts generally gray or grayish brown. Crown, nape, base of neck, and upper back usually dark brown or black. Underparts whitish.
Height: 1.5 to 1.7 m
Mass: 20-25 kg (Blake 1977)
Linear measurements (from Blake 1977):
Rhea americana americana
Culmen length, males: 95 mm (n=1)
Culmen length, females: mean 88 mm (range 80-94 mm, n=3)
Tarsus length, males: 370 mm (n=1)
Tarsus length, females: mean 358 mm (range 335-370 mm, n=3)
Rhea americana araneipes:
Culmen length, males: 100.3 mm (range 96-104 mm, n=3)
Culmen length, females: mean 92.5 mm (92, 93 mm, n=2)
Tarsus length, males: mean 352.7 mm (range 351-354 mm, n=3)
Tarsus length, females: mean 363.5 mm (range 344-370 mm, n=4)