[5] Approximately the size of a domestic chicken, kiwi are the smallest ratites (which also include ostriches, emus, rheas, cassowaries and the extinct elephant birds and moa).

DNA sequence comparisons have yielded the conclusion that kiwi are much more closely related to the extinct Malagasy elephant birds than to the moa with which they shared New Zealand.

All species have been negatively affected by historic deforestation, but their remaining habitat is well protected in large forest reserves and national parks.

[7] Other unique adaptations of kiwi, such as short and stout legs and using their nostrils at the end of their long beak to detect prey before they see it, have helped the bird to become internationally well known.

[9] However, some linguists derive the word from Proto-Nuclear Polynesian *kiwi, which refers to Numenius tahitiensis, the bristle-thighed curlew, a migratory bird that winters in the tropical Pacific islands.

[12][failed verification] The genus name Apteryx is derived from Ancient Greek 'without wing': a- (ἀ-), 'without' or 'not'; ptéryx (πτέρυξ), 'wing'.

haastii A. owenii A. australis A. rowi A. mantelli Relationships in the genus Apteryx[17] 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.

With no constraints on weight due to flight requirements, brown kiwi females carry and lay a single egg that may weigh as much as 450 g (16 oz).

The eye has small specialisations for a nocturnal lifestyle, but kiwi rely more heavily on their other senses (auditory, olfactory, and somatosensory system).

The same experiment examined three specific specimens that showed complete blindness and found them to be in good physical standing outside of ocular abnormalities.

[26] A 2018 study revealed that the kiwi's closest relatives, the extinct elephant birds, also shared this trait despite their great size.

[27] Unlike virtually every other palaeognath, which are generally small-brained by bird standards, kiwi have proportionally large encephalisation quotients.

[28] Before the arrival of humans in the 13th century or earlier, New Zealand's only endemic mammals were three species of bat, and the ecological niches that in other parts of the world were filled by creatures as diverse as horses, wolves and mice were taken up by birds (and, to a lesser extent, reptiles, insects and gastropods).

They prefer subtropical and temperate podocarp and beech forests, but they are being forced to adapt to different habitat, such as sub-alpine scrub, tussock grassland, and the mountains.

The large egg is instead thought to be an adaptation for precocity, enabling kiwi chicks to hatch mobile and with yolk to sustain them for two and half weeks.

For example, prior to a joint 1080 poison operation undertaken by DOC and the Animal Health Board in Tongariro Forest in 2006, 32 kiwi chicks were radio-tagged.

[51] A number of other mainland conservation islands and fenced sanctuaries have significant populations of kiwi, including: North Island brown kiwi were introduced to the Cape Sanctuary in Hawke's Bay between 2008 and 2011, which in turn provided captive-raised chicks that were released back into Maungataniwha Native Forest.

Operation Nest Egg is a programme run by the BNZ Save the Kiwi Trust—a partnership between the Bank of New Zealand, the Department of Conservation and the Royal Forest and Bird Protection Society.

Kiwi eggs and chicks are removed from the wild and hatched and/or raised in captivity until big enough to fend for themselves—usually when they weigh around 1200 grams (42 ounces).

Young kiwi chicks are vulnerable to stoat predation until they reach about 1–1.2 kg (2.2–2.6 lb) in weight, at which time they can usually defend themselves.

Shaw's description was accompanied by two plates, engraved by Frederick Polydore Nodder; they were published in volume 24 of The Naturalist's Miscellany.

[77][78] In 2023, Zoo Miami apologized for mistreating a kiwi, after footage of visitors patting the nocturnal bird under bright lights caused outrage in New Zealand.

[83] A song, "Sticky Beak the Kiwi", with words by Bob Edwards and music by Neil Roberts, was recorded in 1961, sung by Julie Nelson (aged 14) and accompanied by the Satins and the Don Bell Orchestra of Whangārei.

A Christmas song, it portrays Sticky Beak as insisting on pulling Santa Claus's sleigh when distributing presents south of the equator.

It uses elements of Māori mythology, such as Tāne Mahuta, and the World War I symbol of cowardice, white feathers, in a pourquoi story explaining features of New Zealand birds.

Owen portrays the kiwi as nobly sacrificing its wings and flight in order to protect the trees from depredation by ground-dwelling creatures, and thereby winning its unique renown.