The Strthioniformes is a large order of flightless birds. Consisting of the Struthionidae (Ostrich), Rheidae (Rhea), Casuariidae (Emu, Cassowary) and the Apterygidae (Kiwi). Unlike other flightless birds, the Strthioniformes have no keel on their sternum—hence the name from the Latin ratis (for raft). Without a keel to anchor the wing muscles, they could not fly even if they developed suitable wings.
Author: Josh V.
Photo Credit: http://en.wikipedia.org/wiki/Ratite