Kiwi are flightless birds endemic to New Zealand, in the genusApteryx and family Apterygidae. At around the size of a domestic chicken, kiwi are by far the smallest living ratites and lay the largest egg in relation to their body size of any species of bird in the world. There are five recognised species, two of which are currently vulnerable, one of which is endangered, and one of which is critically endangered. All species have been negatively affected by historic deforestation but currently large areas of their forest habitat are well protected in reserves and national parks. At present, the greatest threat to their survival is predation by invasive mammalian predators.
The kiwi is a national symbol of New Zealand, and the association is so strong that the term Kiwi is used in some parts of the world as the colloquial demonym for New Zealanders.
Their adaptation to a terrestrial life is extensive: like all the other ratites (Ostrich, Emu, Rhea and Cassowary) they have no keel on the sternum to anchor wing muscles. The vestigial wings are so small that they are invisible under the bristly, hair-like, two-branched feathers. While most adult birds have bones with hollow insides to minimise weight and make flight practicable, kiwi have marrow, like mammals and the young of other birds. With no constraints on weight due to flight requirements, Brown Kiwi females carry and lay a single egg which may weigh as much as 450 g (16 oz). Like most other ratites, they have no uropygial gland (preen gland). Their bill is long, pliable and sensitive to touch, and their eyes have a reduced pecten. Their feathers lack barbules and aftershafts, and they have large vibrissae around the gape. They have 13 flight feathers, no tail and a small pygostyle. Finally, their gizzard is weak and their caecum is long and narrow.