Weasels are predators of native birds, eggs, lizards and insects. They will tackle a prey much larger than themselves and nesting birds are easy targets.
By the mid 1870s, rabbits were becoming a serious agricultural pest in New Zealand. Farmers demanded that the natural enemies of the rabbit (mustelids) be introduced. Despite the protests of bird experts, ferrets, stoats and weasels were released throughout pastoral areas and by the mid 1890s they had spread into forests west of Lake Manapouri. Far too late, after many official protests, the government changed its policy on mustelids in 1903. However, it was not until 1936 that all legal protection for mustelids was removed.
Weasels are found in low numbers in most habitat types in New Zealand but impact on lizard and invertebrate populations as well as bird life. Whittaker’s skink on the mainland has been reduced to a single remnant population by predation pressure and loss of habitat from bush clearance and conversion to pasture.
New Zealand provides only a marginal habitat for weasels and their impact on native birds is similar to stoats although on a much smaller scale. Weasels were originally introduced in larger numbers than stoats but have not thrived - probably because of the absence of abundant easy prey.