Flightless birds evolved from flying ancestors; there are about forty species in existence today. Most flightless birds evolved in the absence of predators, on islands. A notable exception, the ostrich, which lives in the African savannas, has claws on its feet to use as a weapon against predators.

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[1] (http://www.nhm.org/birds/guide/pg019a.html). Flightless birds also have more feathers than flying birds.

New Zealand has more species of flightless birds (including the kiwi, penguin, and takahe) than any other country. One reason is that until the arrival of humans roughly 1000 years ago, there were no land mammals in New Zealand other than three species of bat; the main predators of flightless birds were larger birds[2] (http://www.kiwirecovery.org.nz/Kiwi/AboutTheBird/NewZealandsIcon/KiwiCharacteristics/Flightless.htm).

With the introduction of mammals (among them humans) to the habitats of flightless birds, many have become extinct, including the Great Auk, the Dodo, and the Moa.

The smallest flightless bird is the Inaccessible Island Rail (12.5 cm and 34.7 g). The largest - heaviest and tallest - flightless bird (and, incidentally, the largest living bird) is the ostrich (2.7 m and 156 kg)[3] (http://www.birding.com/BirdRecords1.htm).

See also: Extinct birds, Ratite