The cockatiel (Nymphicus hollandicus), also known as weiro bird, or quarrion, is a bird that is a member of its own branch of the cockatoo family endemic to Australia. They are prized as household pets and companion parrots throughout the world and are relatively easy to breed. As a caged bird, cockatiels are second in popularity only to the budgerigar.

The cockatiel is the only member of the genus Nymphicus. It was previously considered a crested parrot or small cockatoo; however, more recent molecular studies have assigned it to its own subfamily, Nymphicinae. It is, therefore, now classified as the smallest of the Cacatuidae (cockatoo family). Cockatiels are native to Australia, favouring the Australian wetlands, scrublands, and bushlands.

The cockatiel is now biologically classified as a genuine member of Cacatuidae on account of sharing all of the cockatoo family's biological features, namely, the erectile crest, a gallbladder, powder down, suppressed cloudy-layer (which precludes the display of blue and green structural colours), and facial feathers covering the sides of the beak, all of which are rarely found outside the family Cacatuidae. This biological relation to other cockatoos is further supported by the existence of at least one documented case of a successful hybrid between a cockatiel and a galah, another cockatoo species.

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