Rhea (bird) males of "R. americana" can reach tall at the head, at the back and can weigh up to , The lesser rhea is somewhat smaller as they are only tall at the back. Their wings are large for a flightless bird () and are spread while running, to act like sails. Unlike most birds, rheas have only three toes. Their tarsus has 18 to 22 horizontal plates on the front of it. They also store urine separately in an expansion of the cloaca. Rheas are from South America only and are limited within the continent to Argentina, Bolivia, Brazil, Chile,
Bird flight streamlined to help overcome air-resistance. Also, the bird skeleton is hollow to reduce weight, and many unnecessary bones have been lost (such as the bony tail of the early bird "Archaeopteryx"), along with the toothed jaw of early birds, which has been replaced with a lightweight beak. The skeleton's breastbone has also adapted into a large keel, suitable for the attachment of large, powerful flight muscles. The vanes of each feather have hooklets called barbules that zip the vanes of individual feathers together, giving the feathers the strength needed to hold the airfoil (these are often lost in flightless birds).