Ratites are flightless birds. They have wings but do not fly. Is it due to their size?
Ratites include emus and cassowaries in the Casuariidae family, kiwis in the Apterygidae family, ostriches in the Struthionidae family, and rheas in the Rheidae family.
Most ratites are very large birds: emus, cassowaries, ostriches, and rheas, growing up to 2.8 metres, which is 280 centimetres (9.2 feet or 110 inches) tall. The largest flightless bird in the world is the Common Ostrich (Struthio camelus).
These large flightless birds have similar body shapes: long legs, long necks, and big heads. But their families are not closely related to each other.
The penguin in the Spheniscidae family is also a large, aquatic, flightless bird, but it is not a ratite, and it has a different body shape to the other large, flightless birds.
The Kiwi is the smallest ratite in the world at 25-45 centimetres (10-18 inches) tall.
Another small flightless bird, that is not a ratite, is the Inaccessible Island Rail in the Rallidae family. The Inaccessible Island Rail (Laterallus rogersi) is the smallest flightless bird in the world, growing to about 12 centimetres (5 inches) tall.
Other small, non-ratite, flightless birds in the Rallidae family include the Takahe swamphen (Porphyrio hochstetteri) and the Weka woodhen (Gallirallus australis).
The Kiwi and the Rallidae (rails) have similar body shapes, but their families are not related: like a domestic chicken, but with short legs, and very small neck.
Being flightless is not due to their size or body shape.
The emu is from Australia, the cassowary is from northern Australia and New Guinea, the ostrich is from countries in Africa, and the rhea is from countries in South America. The penguin is mostly from the Antarctic.
The Kiwi, Takahe, and Weka are from New Zealand and the Inaccessible Island Rail is from a remote island of Tristan da Cunha in the Atlantic Ocean.
So, flightless birds are mainly from the Southern Hemisphere, with a few from African and South American countries north of the equator.
Flightless birds evolved independently of each other, due to the lack of predators, living on remote oceanic islands, or other factors.
Being flightless is mainly due to the absence or reduced size of the keel (or carina) on their breastbone. The keel anchors the muscles that are needed for wing movement, so without the keel, the wing movement is limited.
Photographer: Martina Nicolls
Martina Nicolls: SIMILAR BUT DIFFERENT IN THE ANIMAL KINGDOM