Three weasel-like animals or Mustelids have been liberated in New Zealand to control rabbits; the wild ferret (Mustela putorius), the stoat (M. erminea), and the weasel (M. nivalis). The ferret attains the size and appearance of the polecat; the smaller stoat is darkish brown with white underneath and a bushy, black-tipped tail; the weasel is much smaller than the stoat and has no black tip to its tail. All three Mustelids have spread since their liberation. The stoat is the most widely spread, being found in open country and in forests. The ferret is mostly found in rabbit areas. The weasel is the rarest, being less widespread than the two other species.
Since their liberation in the late eighties and nineties of the last century, these Mustelids have been the subject of a considerable controversy. People in charge of rabbit control claim that Mustelids have helped to reduce rabbits, but conservationists say they contribute to the extinction or scarcity of some species of native birds. A recent study by W. H. Marshall indicates, however, that native birds have been probably more affected by the enormous changes of habitat since the white settlement of New Zealand than they have been by the presence of Mustelids. There is no organised control of Mustelids, except some trapping by acclimatisation societies and national park rangers.