Correlates of Introduction Success in Exotic New Zealand Birds

Whether or not a bird species will establish a new population after invasion of uncolonized habitat depends, from theory, on its life-history attributes and initial population size. Data about initial population sizes are often unobtainable for natural and deliberate avian invasions. In New Zealand, however, contemporary documentation of introduction efforts allowed us to systematically compare unsuccessful and successful invaders without bias. We obtained data for 79 species involved in 496 introduction events and used the present-day status of each species as the dependent variable in fitting multiple logistic regression models. We found that introduction efforts for species that migrated within their endemic ranges were significantly less likely to be successful than those for nonmigratory species with similar introduction efforts. Initial population size, measured as number of releases and as the minimum number of propagules liberated in New Zealand, significantly increased the probability of translocation success. A null model showed that species released more times had a higher probability per release of successful establishment. Among 36 species for which data were available, successful invaders had significantly higher natality/mortality ratios. Successful invaders were also liberated at significantly more sites. Invasion of New Zealand by exotic birds was therefore primarily related to management, an outcome that has implications for conservation biology.