Classification Paleognathae, the primitive clade defined by a unique conformation of the palate, comprise the Tinamous (whose wings are attached to a keeled sternum and can fly) and the Ratites (Ostriches, Rheas, Emus, Cassowaries and Kiwis), which cannot fly as a result of their flat sternum. Until recently, Tinamous and Ratites were considered two sister clades originating from the Paleognath ancestor. A surprizing result of recent phylogenomic studies has been that ostriches are an independant branch within the paleognathae tree. This branch is sister to another monophyletic branch comprising all the other paleognaths, volant or flightless.

The ostrich relies on its strong legs (uniquely two-toed) to escape its enemies, chiefly humans and the larger carnivores. A frightened ostrich can achieve a speed of 45 miles per hour. If cornered, it can deliver dangerous kicks. Breeding males emit lionlike roars and hisses as they fight for a harem of three to five hens. A communal nest scraped in the ground contains more than a dozen shiny, whitish eggs. The male sits on the eggs by night; the females take turns during the day. The chicks hatch in about 40 days and when a month old can keep up with running adults. Egyptian vulture is the main predator of ostrich eggs, which it breaks by using stones.

Smith JV et al (2012): Ratite Nonmonophyly

http://en.wikipedia.org/wiki/Ostrich

ostrich. (2011). Encyclopædia Britannica Ultimate Reference Suite.  Chicago: Encyclopædia Britannica

http://www.oiseaux.net/birds/struthioniformes.html