Most of the largest birds in the world are flightless which allows them to have denser bones and heavier bodies.
Ostriches (Struthio camelus) are the largest living birds in the world, with an average mass of 104 kg and average length of 210 cm. They lay the largest eggs and hold the record for the fastest land speed of any bird, reaching speeds of 70 kmph. It is also thought that ostriches have the largest eyes of any land vertebrate at 50 mm in diameter.
Ostriches are native to Africa but are farmed all over the world, particularly for their feathers, skin which is used in leather products, and meat. In 2014, the Somali ostrich (Struthio molybdophanes) was recognized as a distinct species.
The southern cassowary (Casuarius casuarius), also known as the double-wattled cassowary, Australian cassowary or two-wattled cassowary have an average weight of 45 kg and are around 155 cm long. Southern cassowaries are found in north-eastern Australia, Indonesia and New Guinea.
Northern cassowaries (Casuarius unappendiculatus) also known as the single (one)-wattled cassowary or golden-necked cassowary, are slightly smaller with an average weight of 44 kg and average length of 149 cm. They are endemic to northern New Guinea.
Endemic to Australia, emus (Dromaius novaehollandiae) are the second tallest living birds in the world that can reach up to 190 cm tall. They weigh an average of 33 kg. Female emus are usually slightly larger than the males and have significantly wider rumps.
Emus have evolved to run fast and despite being flightless they have vestigial wings, which they flap when they run to stabilise themselves. They only have 3 toes and a reduced number of bones and foot muscles. Emus are the only birds with gastrocnemius muscles found in the back of the lower legs.
Emperor penguins (Aptenodytes forsteri) are endemic to Antarctica and are the tallest and heaviest of all living penguins weighing an average of 31.5 kg and measuring 114 cm in height. During breeding season male Emperor penguins lose up to 15 kg as they do not eat anything while they protect their eggs from extreme cold for more than 2 months. Like all penguins, Emperor penguins are flightless but they have adapted to the harsh conditions of their environment which is the coldest for any breeding bird. Feathers provide up to 90% of their insulation and they are able to thermoregulate their core body temperature without altering their metabolisms.
King penguins (Aptenodytes patagonicus) weigh 13.6 kg on average and are 92 cm in length. They breed on Subantarctic islands, as well as Tierra del Fuego, the Falkland Islands, and other temperate islands of the region.
Greater rheas (Rhea americana) are flightless birds native to South America. They are also known as grey, common, or American rheas. Greater rheas have an average mass of 23 kg and an average length of 134 cm. A small population of greater rheas has established itself in Germany. One male and five females escaped from a farm in August 2000. They survived the winter and began breeding. It is estimated that there is now a population of about 250 birds which are considered domestic and therefore protected from hunting.
Lesser rheas (Rhea pennata), also known as Darwin’s rhea, are found in Altiplano and Patagonia in South America. It weighs on average 13.5 kg and has an average length of 96 cm.
The wild turkey (Meleagris gallopavo) is the largest species of bird that is able to fly. They weigh on average 13.5 kg and have an average length of 124 cm. Despite their size, they are quiet agile fliers staying close to the ground and flying for no more than 400 m at a time.
Wild turkeys are the same species as the domesticated turkey. They are native to North America but got their name from the domestic turkey which was misidentified as an unrelated species imported into Europe by Turkish merchants. Domestic turkeys are unable to fly as they are too fat. Because they don’t use their breast muscles, the breast meat is white, unlike that of wild turkeys which is dark and tastes more like game.
Mute swans (Cygnus olor) are the heaviest of all waterfowl weighing an average of 11.87 kg. They average 100-130 cm in length. Trumpeter swans weigh slightly less at 11.6 kg but are longer at 138-165 cm.
A Polish mute swan cob (Cygnus immutabilis), a morph of the mute swan, and a pure white version with pink legs instead of the usual black colour, has been verified as the largest bird to take flight weighing in at 23 kg.
Great bustards (Otis tarda) weigh on average 10.6 kg and are 115 cm in length. They breed in open grassland in southern and central Europe and Asia with 60% of the population resident in Spain and Portugal. Great bustards were driven to extinction in the UK by trophy hunters in the 1830s but have recently been introduced and are breeding on Salisbury Plain.
The Kori bustard (Ardeotis kori) is Africa’s largest flying bird and a contender for the heaviest flying living bird weighing an average of 11.4 kg and measuring 150 cm in length.
The wandering albatross (Diomedea exulans) is also known as the snowy albatross, the white-winged albatross and the goonie. It weighs an average 11.9 kg and is up to 135 cm in length. It has the longest wingspan of any living bird ranging from 2.5 m to 3.5 m and unverified reports put the largest wingspans at 4.22 m and 5.3 m.
Wandering albatrosses spend most of their life in flight and can remain in the air without flapping for several hours at a time. Albatrosses have some of the longest ranges of all birds and some individual wandering albatrosses have been known to circumnavigate the Southern Ocean three times in one year.
The Dalmatian pelican (Pelecanus crispus) is the largest member of the pelican family and perhaps the largest freshwater bird in the world rivalled in length and weight by swans. They weigh on average 11.5 kg and are 183 cm in length. It has a wingspan similar in size to the great albatrosses and is one of only 4 species of birds having verified wingspans of over 350 cm, the others being the wandering albatross, the southern royal albatross and the great white pelican.
Dalmatian pelicans are found in lakes, rivers and estuaries in south east Europe, Russia, India and China. During the 20th century the population underwent a drastic decline in numbers most likely due to habitat loss caused by the drainage of wetlands.