The kakapo (Strigops habroptila) is a critically endangered, giant, nocturnal bird. It is a classic example of evolution on an isolated island, and has a number of characteristic features that make this species unique. The kakapo is the only member of the subfamily Strigopinae and is the only flightless bird in the world. It is also the heaviest tropical bird known and is possibly the longest-lived; the oldest known kakapo was elderly when found in 1975 and was still alive in 2002. Adult kakapos have beautiful mossy green plumage mottled with brown and yellow, which provides excellent camouflage against the forest floor. The face is owl-like, yellowish-brown, and framed with modified whisker-like feathers. Juvenile kakapos are slightly duller in color than adults and have browner faces. The feathers of the kakapo are downy and soft; the scientific name habroptila means ‘soft feathers'. The male kakapo produces a strange 'boom' call to attract potential mates, which can be heard up to 5 kilometres away. The kakapo does not reach sexual maturity until 9 to 10 years of age, and it only breeds every 2 to 5 years.