New Zealand, like many other islands, has suffered extinctions of native species and successful introductions of exotic species. It has been uncertain whether the introductions caused the extinctions or whether the extinctions permitted the introductions. On New Zealand's Hauraki Gulf islands, which are unusual in their near lack of introduced mammalian predators and complete lack of mammalian browsers, exotic bird species abundant in mainland New Zealand forest and reaching these islands are virtually absent from unmodified forest. Exotic bird species disappeared from Cuvier Island's forest after elimination of mammalian predators and browsers. Hence extinctions of native species were not due to competition from introduced species but to other factors (such as mammalian predators and habitat alteration). Only after decimation of native species and forest alteration by browsing mammals could exotic birds invade forest.