Apteryx spp. - The Kiwi
Aside from its obvious qualities, the kiwi bird is surprisingly unique in general. Where flighted birds have hollow bones to conserve weight, kiwis have bone marrow, akin to mammals. They have no keel on the breastbone (to be expected with such nubby wings, as the keel is there to anchor wing muscles), no tail, and a weak gizzard.
Most of the unique traits of the kiwi evolved to allow the bird to fill ecological niches that were open on the New Zealand islands, owing to the lack of mammalian species. The diet, behavior, and even sense of smell are all directly tied to filling in the niche of terrestrial insectivore. Other notable oddities about the kiwi are the cat-like whiskers around their beak and the massive size of their eggs: the bird itself is the size of a chicken, but can lay eggs over six times as large - one quarter the weight of the adult female.
Wild life of the world: a descriptive survey of the geographic distribution of animals. Richard Lydekker, 1915.