You won't find any of these in a tree in your garden...
When you think of the world's biggest species, you probably think of mammals – giraffes, elephants, blue whales. Birds are comparatively small, at least in the UK. Here, you're more likely to see a robin or a sparrow than something that weighs tens of kilograms.
But bird species can be surprisingly large, with the very largest even taller than a human.
Even these species have nothing on the extinct elephant bird. The species Vorombe titan, which lived in Madagascar over 1,000 years ago, may have weighed up to 800kg.
Maximum weight: 17.6kg
Living in forests on hills and mountains up to 3km high, Casuarius bennetti is the smallest of the three species of cassowary. Over a metre long, this bird feeds on fungi, fruit, and even insects or small animals.
Maximum weight: 22.5kg
Distribution:Central and Northern Europe; Central and Eastern Asia
Despite its name, the mute swan (Cygnus olor) is a vocal bird, though its voice is quieter than other species and it only uses it occasionally. They are usually found in lowland freshwater marshes, lagoons and rivers.
Maximum weight: 25kg
Distribution:Argentina, Bolivia, Chile and Peru
Rhea pennata is also known as Darwin's rhea, after the naturalist studied the species on his second voyage on theBeagle. Though Darwin struggled in his search for the bird, he was successful after the artist Conrad Martens, who joined him on the voyage, shot and prepared one for dinner.
Maximum weight: Over 25kg
Distribution: Argentina, Bolivia, Brazil, Paraguay, and Uruguay
This grassland-dwelling species, Rhea americana, is omnivorous. As well as plants, it likes to eat grasshoppers, lizards, frogs, small birds and some snakes.
Maximum weight: 46kg
This iconic species can stand as tall as 115cm and weigh up to 46kg. Aptenodytes forsteri will trek up to 120km over ice to reach a breeding colony.
Maximum weight: 55kg
Of the four surviving members of the family Casuariidae, three are cassowaries, and the fourth is the emu. Adult emus can stand at 190cm tall, and though they weigh on average between 30 and 45kg, the largest reach 55kg.
Like other species of cassowary, Casuarius unappendiculatus is a flightless bird. After being laid by a female, the eggs of this species are incubated by a male who raises them for nine months.
Maximum weight: 58kg
Distribution:New Guinea and Queensland, Australia
The southern cassowary (Casuarius casuarius) is only slightly larger than its cousin, the northern cassowary. They live in rainforests, swamp forests and savannah woodland, and can grow up to 170cm in length.
Maximum weight: 105kg
Distribution:Horn of Africa
Once believed to be a subspecies of the common ostrich, Struthio molybdophanes was confirmed as a separate species in 2014.
Maximum weight: 156kg
Distribution:Savannahs of Africa
Males of the species Struthio cameluscan reach a whopping 2.75 tall. These flightless birds are incredible runners, and can reach speeds of 70km/h (43mph) in a sprint.