Rhea (bird) Rhea (bird) The rheas () are large ratites (flightless birds without a keel on their sternum bone) in the order Rheiformes, native to South America, distantly related to the ostrich and emu. Most taxonomic authorities recognize two extant species: the greater or American rhea ("Rhea americana") and the lesser or Darwin's rhea ("Rhea pennata"). The IUCN lists the puna rhea ("Rhea tarapacensis") as a separate species. The IUCN currently rates the greater and puna rheas as near-threatened in their native ranges, while Darwin's rhea is of least concern. In addition, a feral population of the greater rhea in Germany appears
Flightless bird the terror birds (and their relatives the bathornithids), eogruids, geranoidids, gastornithiforms, and dromornithids (all extinct) all evolved similar body shapes – long legs, long necks and big heads – but none of them were closely related. Furthermore, they also share traits of being giant, flightless birds with vestigial wings, long legs, and long necks with some of the ratites, although they are not related. Divergences and losses of flight within ratite lineage occurred right after the K-Pg extinction event wiped out all non-avian dinosaurs and large vertebrates 66 million years ago. The immediate evacuation of niches following the mass extinction