The kiwis comprise three species - the common or brown kiwi (A. australis), the little spotted kiwi (A. oweni), and the great spotted kiwi (A. haasti). All three species are restricted to New Zealand, with the great and little spotted species found only on South Island. They prefer a forested habitat with high rainfall.
Kiwis are the smallest of the ratites, standing up to 40 cm high and weighing about 2 kg. The feathers are long and flexible, with a shaggy appearance similar to fur. The bird has no external tail, and the vestigial wings are hidden by the plumage. The legs are strong, used not only for running but also burrowing and fighting.
The kiwi feeds primarily on earthworms, although it also eats insects, berries, and seeds. The bird probes soil with its long, sensitive beak and finds worms by means of its unusually strong sense of smell. (The olfactory bulb has a diameter 34% of that of the cerebral hemisphere, close to the highest ratio among birds.)
Kiwis nest in holes among tree roots. They lay one to three eggs weighing about 500 grams, an unusually high egg to adult weight ratio.
New Zealand separated from the supercontinent Gondwanaland about 80 million years ago, before mammals could populate the region. Since New Zealand has no native terrestrial mammals (other than two species of bat), birds occupied the ecological niches which would normally be filled by mammals. Kiwis descended from flying birds, as can be seen from their vestigial wings, but lost the ability to fly due to atrophy of their wings and pectoral muscles.