Distributions of New Zealand Birds on Real and Virtual Islands

This paper considers how habitat geometry affects New Zealand bird distributions on land-bridge islands, oceanic islands, and forest patches. The data base consists of distributions of 60 native land and freshwater bird species on 31 islands. A theoretical section examines how species incidences should vary with factors such as population density, island area, and dispersal ability, in two cases: immigration possible or impossible. New Zealand bird species are divided into water-crossers and non-crossers on the basis of six types of evidence. Overwater colonists of New Zealand from Australia tend to evolve into non-crossers through becoming flightless or else acquiring a fear of flying over water. The number of land-bridge islands occupied per species increases with abundance and is greater for water-crossers than for non-crossers, as expected theoretically. Non-crossers are virtually restricted to large land-bridge islands. The ability to occupy small islands correlates with abundance. Some absences of species from particular islands are due to mancaused extinctions, unfulfilled habitat requirements, or lack of foster hosts. However, many absences have no such explanation and simply represent extinctions that could not be (or have not yet been) reversed by immigrations. Extinctions of native forest species due to forest fragmentation on Banks Peninsula have especially befallen non-crossers, uncommon species, and species with large area requirements. In forest fragments throughout New Zealand the distributions and area requirements of species reflect their population density and dispersal ability. All these patterns make abundantly clear that the future of New Zealand bird species confined to native forest hinges on preserving large tracts of native forest.