Question: The African ostrich is the world's tallest living bird; where does the second tallest, the emu, come from?

full_context: Document 1: Arabian ostrich: The Arabian ostrich or Syrian ostrich ("Struthio camelus syriacus") is an extinct subspecies of the ostrich that lived on the Arabian Peninsula and in the Near East until the mid-20th century.
Document 2: Vulturine guineafowl: The vulturine guineafowl ("Acryllium vulturinum") is the largest extant species of guineafowl. Systematically, it is only distantly related to other guineafowl genera. Its closest living relative, the white breasted guineafowl, "Agelastes meleagrides" inhabit primary forests in Central Africa. It is a member of the bird family Numididae, and is the only member of the genus "Acryllium". It is a resident breeder in northeast Africa, from southern Ethiopia through Kenya and just into northern Tanzania.
Document 3: Giraffe: The giraffe ("Giraffa") is a genus of African even-toed ungulate mammals, the tallest living terrestrial animals and the largest ruminants. The genus currently consists of one species, "Giraffa camelopardalis", the type species. Seven other species are extinct, prehistoric species known from fossils. Taxonomic classifications of one to eight extant giraffe species have been described, based upon research into the mitochondrial and nuclear DNA, as well as morphological measurements of "Giraffa," but the IUCN currently recognizes only one species with nine subspecies.
Document 4: Guineafowl: Guineafowl ( ; sometimes called "pet speckled hen", or "original fowl" or "guineahen") are birds of the family Numididae in the order Galliformes. They are endemic to the continent of Africa and rank among the oldest of the gallinaceous birds. They are phylogenetically intermediate between peafowl and the Odontophoridae. One Eocene fossil lineage, Telecrex, is represented by the black guineafowl, an extant species native to the primary forests of Central Africa. Telecrex, which inhabited Mongolia, may have given rise to the oldest of the true Tetraophasianids like "Ithaginis" and "Crossoptilon", which evolved into high-altitude montane adapted species with the rise of the Tibetan Plateau. While modern guineafowl species are endemic to Africa, the helmeted guineafowl has been introduced widely elsewhere.
Document 5: Masai ostrich: The Masai ostrich ("Struthio camelus massaicus"), also known as the pink-necked ostrich or East African ostrich, is a large subspecies of the common ostrich native to East Africa. Today it is farmed for eggs, meat and feathers.
Document 6: Flightless bird: Flightless birds are birds that through evolution lost the ability to fly. There are over 60 extant species including the well known ratites (ostrich, emu, cassowary, rhea and kiwi) and penguins. 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, is the ostrich (2.7 m, 156 kg). Ostriches are farmed for their decorative feathers, meat and their skins, which are used to make leather.
Document 7: Royal Australasian Ornithologists Union: The Royal Australasian Ornithologists Union, now part of BirdLife Australia, was founded in 1901 to promote the study and conservation of the native bird species of Australia and adjacent regions. This makes it Australia's oldest national birding association. It was also Australia's largest non-government, non-profit, bird conservation organisation. In 1996 it adopted the trading name of Birds Australia for most public purposes, while retaining its original name for legal purposes and as the publisher of its journal the "Emu". In 2012 it merged with Bird Observation & Conservation Australia to form BirdLife Australia.
Document 8: South African ostrich: The South African ostrich ("Struthio camelus australis"), also known as the black-necked ostrich, Cape ostrich or southern ostrich is a subspecies of the common ostrich endemic to Southern Africa. It is widely farmed for its meat, eggs and feathers.
Document 9: North African ostrich: The North African ostrich or red-necked ostrich ("Struthio camelus camelus"), also known as the Barbary ostrich, is the nominate subspecies of the common ostrich from West and North Africa. It is the largest subspecies, making it the largest living bird.
Document 10: Pachyornis: Pachyornis is an extinct genus of ratites from New Zealand which belonged to the moa family. Like all ratites it was a member of the order Struthioniformes. The Struthioniformes are flightless birds with a sternum without a keel. They also have a distinctive palate. This genus contains three species, and are part of the Anomalopteryginae or lesser moa subfamily. "Pachyornis" moa were the stoutest and most heavy-legged genus of the family. The most notable species being "Pachyornis elephantopus" - the Heavy-Footed Moa. They were generally similar to the Eastern Moa or the Broad-billed moa of the genus "Euryapteryx", but differed in having a pointed bill and being more heavyset in general. At least one species ("P. australis") is assumed to have had a crest of long feathers on its head. The species became rapidly extinct following human colonization of New Zealand, with the possible exception of "P. australis", which may have already been extinct by then.
Document 11: Emu: The emu ("Dromaius novaehollandiae") is the second-largest living bird by height, after its ratite relative, the ostrich. It is endemic to Australia where it is the largest native bird and the only extant member of the genus "Dromaius". The emu's range covers most of mainland Australia, but the Tasmanian emu and King Island emu subspecies became extinct after the European settlement of Australia in 1788. The bird is sufficiently common for it to be rated as a least-concern species by the International Union for Conservation of Nature.
Document 12: Bird: Birds (Aves) are a group of endothermic vertebrates, characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweight skeleton. Birds live worldwide and range in size from the 5 cm bee hummingbird to the 2.75 m ostrich. They rank as the class of tetrapods with the most living species, at approximately ten thousand, with more than half of these being passerines, sometimes known as perching birds. Birds are the closest living relatives of crocodilians. Birds are descendants of extinct dinosaurs with feathers, making them the only surviving dinosaurs according to cladistics.
Document 13: List of largest birds: The largest living bird, a member of the Struthioniformes, is the ostrich ("Struthio camelus"), from the plains of Africa and Arabia. A large male ostrich can reach a height of 2.8 m and weigh over 156 kg . A mass of 200 kg has been cited for the ostrich but no wild ostriches of this massive weight have been verified. Eggs laid by the ostrich can weigh 1.4 kg and are the largest eggs in the world today.
Document 14: Common ostrich: The ostrich or common ostrich ("Struthio camelus") is either one or two species of large flightless birds native to Africa, the only living member(s) of the genus "Struthio", which is in the ratite family. In 2014, the Somali ostrich ("Struthio molybdophanes") was recognized as a distinct species.
Document 15: North Island brown kiwi: The North Island brown kiwi ("Apteryx mantelli"; "Apteryx australis" or "Apteryx bulleri" as before 2000, still used in some sources), is a species of kiwi that is widespread in the northern two-thirds of the North Island of New Zealand and, with about 35,000 remaining, is the most common kiwi. This bird holds the world record for laying the largest eggs relative to its body size.
ground_truth: Australia