Enhanced niche opportunities: can they explain the success of New Zealand's introduced bird species?

The niche hypothesis could explain why some species introduced to new locations reach higher densities than in their native range: it posits that the new environment provides more abundant or higher quality resources or habitat, a more suitable physical environment or both. We investigate whether 11 bird species occur at higher densities in their introduced range than in their native range and whether the differences can be explained by the availability of preferred habitat or the suitability of climatic conditions in their introduced range relative to their native range. South Island, New Zealand (the introduced range); UK (the native range). We first develop a series of models that accurately predict the density of 11 bird species at 54 UK farmland sites, which are closely matched to our New Zealand sites, from habitat and climatic variables. We then use these models to predict the density of the 11 species at 54 New Zealand farmland sites and compare the predicted and observed values. Actual densities at New Zealand sites were on average (median) 22 times (range: 1-6361) higher than predicted from the UK models and similarly higher than actually observed at comparable UK sites. Habitat and climatic variables can accurately predict bird densities in the UK but grossly underestimate densities for all species except Turdus merula in New Zealand. These findings indicate that factors other than the measured habitat and climatic variables must differ between the two regions and explain the much higher densities of New Zealand birds. We suggest that introduced birds, other than T. merula, in New Zealand may still experience enhanced niche opportunities due to greater availability of higher quality resources within habitats, release from natural enemy regulation, less exposure to extreme weather events, particularly during winter, or some combination of these processes.