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Flightless birds are birds that cannot fly. There are about 40 species in existence today, the best known being the ratites (ostrich, emu, cassowary, rhea and kiwi) and the penguins. Some birds evolved flightlessness in response to the release from predation, for example on oceanic islands, although this is likely not the case for the ratites as evolutionary origins suggest a continental biogeographical providence.
Two key differences between flying and flightless birds are the smaller wing bones of flightless birds and the absent (or greatly reduced) keel on their breastbone. (The keel anchors muscles needed for wing movement.) Flightless birds also have more feathers than flying birds.
New Zealand has more species of flightless birds (including the kiwis, several species of penguins, and the takahe) than any other country. One reason is that until the arrival of humans roughly a thousand years ago, there were no large land predators in New Zealand; the main predators of flightless birds were larger birds.
Some flightless varieties of island birds are closely related to flying varieties, implying flight is a significant biological cost.
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), although some extinct species grew to larger sizes.
Flightless birds are the easiest to take care of in captivity because they do not have to be caged. Ostriches were once farmed for their decorative feathers. Today they are raised for meat and for their skins, which are used to make leather.
There were also other families of flightless birds, such as the now extinct Phorusrhacidae, that evolved to be powerful terrestrial predators.
Many domesticated birds, such as some breeds of domestic chicken, jon byrd and domestic duck, have lost the ability to fly, though their wild ancestors, the red junglefowl and mallard, respectively, are perfectly capable of flight.