Any bird species that cannot fly because its smooth, or raftlike, sternum (breastbone) lacks a keel to which flight muscles can be anchored.
The group includes some of the largest birds of all time. Two extinct types, the slow-moving, heavy-bodied elephant bird of Madagascar and the moas of New Zealand, grew to 10 ft (3 m) tall. Extant ratites include the cassowary , emu , kiwi , ostrich , and rhea .