Determinants of geographical range sizes: a test using introduced New Zealand birds

1. The 34 species of birds that have been successfully introduced to New Zealand offer a unique opportunity to study patterns of variation in geographical range sizes, and to test mechanisms that may be responsible for that variation, because the New Zealand range sizes have established within the last 160 years and because there are data on the starting conditions, including the year of first recorded release and the subsequent effort put into the introduction of each species. 2. We collated data on geographical range sizes, life history traits, dates of introduction and initial introduction effort for the birds successfully introduced to New Zealand. To test whether range size-life history correlations show a consistent pattern between regions, we collated further data on geographical range sizes and life history traits for British breeding birds. 3. The geographical range sizes of birds introduced to New Zealand did not depend on the length of time since they were introduced. Instead, large geographical ranges were exhibited by species whose preferred habitat is widespread in New Zealand, species with life history traits associated with higher rates of population growth (high fecundity, fast development and small body size), species that are partial migrants in part of their natural range, and species that were initially introduced more often and in greater numbers to New Zealand. 4. The strength and direction of geographical range size-life history correlations in introduced New Zealand and British breeding birds were very similar. There was also a strong positive correlation between the geographical range sizes of the species introduced to New Zealand from Britain and their geographical range sizes in Britain. However, the similarity of the correlations between life history traits and geographical range sizes in both regions was not a simple consequence of this; the similarity persisted when species introduced from Britain were excluded from the New Zealand data. 5. We discuss the implications of these results for understanding variation in geographical range sizes in the introduced New Zealand avifauna.