Cassowary in the context of Paleognaths

The common ostrich (Struthio camelus), or simply ostrich, is a species of flightless bird native to certain areas of Africa. It is one of two extant species of ostriches, the only living members of the genus Struthio in the ratite group of birds. The other is the Somali ostrich (Struthio molybdophanes), which has been recognized as a distinct species by BirdLife International since 2014, having been previously considered a distinctive subspecies of ostrich.

The common ostrich belongs to the order Struthioniformes. Struthioniformes previously contained all the ratites, such as the kiwis, emus, rheas, and cassowaries. However, recent genetic analysis has found that the group is not monophyletic, as it is paraphyletic with respect to the tinamous, so the ostriches are now classified as the only members of the order. Phylogenetic studies have shown that it is the sister group to all other members of Palaeognathae, and thus the flighted tinamous are the sister group to the extinct moa. It is distinctive in its appearance, with a long neck and legs, and can run for a long time at a speed of 55 km/h (34 mph) with short bursts up to about 70 km/h (43 mph), the fastest land speed of any bipedal animal. The common ostrich is the largest living species of bird and thus the largest living dinosaur. It lays the largest eggs of any living bird (the extinct giant elephant bird (Aepyornis maximus) of Madagascar and the south island giant moa (Dinornis robustus) of New Zealand laid larger eggs).

Flightless birds are birds that cannot fly, as they have, through evolution, lost the ability. There are over 60 extant species, including the well-known ratites (ostriches (Struthio), emus (Dromaius), cassowaries (Casuarius), rheas, and kiwis (Apteryx)) and penguins (Sphenisciformes). The smallest flightless bird is the Inaccessible Island rail (length 12.5 cm, weight 34.7 g). The largest (both heaviest and tallest) flightless bird, which is also the largest living bird in general, is the common ostrich (2.7 m, 156 kg).

Some domesticated birds, such as the domestic chicken, have lost the ability to fly for extended periods, although their ancestral species, the red junglefowl and others, respectively, are capable of extended flight. A few particularly bred birds, such as the Broad Breasted White turkey, have become totally flightless as a result of selective breeding; the birds were bred to grow massive breast meat that weighs too much for the bird's wings to support in flight.

Kiwi are flightless birds endemic to New Zealand of the order Apterygiformes. The five extant species fall into the family Apterygidae and genus Apteryx. Approximately the size of a domestic chicken, kiwi are the smallest ratites (which also include ostriches, emus, rheas, cassowaries and the extinct elephant birds and moa).

DNA sequence comparisons have yielded the conclusion that kiwi are much more closely related to the extinct Malagasy elephant birds than to the moa with which they shared New Zealand. There are five recognised species, four of which are currently listed as vulnerable, and one of which is near threatened. All species have been negatively affected by historic deforestation, but their remaining habitat is well protected in large forest reserves and national parks. At present, the greatest threat to their survival is predation by invasive mammalian predators.

Palaeognathae (/ˌpæliˈɒɡnəθi/; from Ancient Greek παλαιός (palaiós) 'old' and γνάθος (gnáthos) 'jaw') is an infraclass of birds, called paleognaths or palaeognaths, within the class Aves of the clade Archosauria. It is one of the two infraclasses of birds, the other being Neognathae, both of which form Neornithes. Palaeognathae contains five extant orders consisting of four flightless lineages (plus two that are extinct), termed ratites, and one flying lineage, the Neotropic tinamous. There are 47 species of tinamous, five of kiwis (Apteryx), three of cassowaries (Casuarius), one of emus (Dromaius) (another became extinct in historic times), two of rheas (Rhea) and two of ostriches (Struthio). Recent research has indicated that paleognaths are monophyletic but the traditional taxonomic split between flightless and flighted forms is incorrect; tinamous are within the ratite radiation, meaning flightlessness arose independently multiple times via parallel evolution.

There are three extinct groups that are undisputed members of Palaeognathae: the Lithornithiformes, the Dinornithiformes (moas) and the Aepyornithiformes (elephant birds), the latter two of which became extinct in the last 1250 years. There are other extinct birds which have been allied with the Palaeognathae by at least one author, but their affinities are a matter of dispute.

The Casuariiformes /kæsjuːˈæri.ɪfɔːrmiːz/ is an order of large flightless birds that has four surviving members: the three species of cassowary, and the only remaining species of emu. They are divided into either a single family, Casuariidae, or occasionally two, with the emu splitting off into its own family, Dromaiidae. The IOC World Bird List and Cornell Lab of Ornithology's Birds of the World both do not recognize Dromaiidae, placing the emu in the family Casuariidae.

All four living members are native to Australia-New Guinea, but some possible extinct taxa occurred in other landmasses.

The northern cassowary (Casuarius unappendiculatus), also known as the one-wattled cassowary, single-wattled cassowary, or golden-necked cassowary, is a large, stocky flightless bird of northern New Guinea. It is one of the three living species of cassowary, alongside the dwarf cassowary and the southern cassowary. It is a member of the superorder Palaeognathae.

The dwarf cassowary (Casuarius bennetti), also known as Bennett's cassowary, little cassowary, mountain cassowary or muruk, is the smallest of the three extant species of cassowaries.

Casuarius lydekkeri is an extinct species of cassowary.