The Influence of Numbers Released on the Outcome of Attempts to Introduce Exotic Bird Species to New Zealand

Information on the approximate number of individuals released is available for 47 of the 133 exotic bird species introduced to New Zealand in the late 19th and early 20th centuries. Of these, 21 species had populations surviving in the wild in 1969-79. The long interval between introduction and assessment of outcome provides a rare opportunity to examine the factors correlated with successful establishment without the uncertainty of long-term population persistence associated with studies of short duration. 2. The probability of successful establishment was strongly influenced by the number of individuals released during the main period of introductions. Eight-three per cent of species that had more than 100 individuals released within a 10-year period became established, compared with 21% of species that had less than 100 birds released. The relationship between the probability of establishment and number of birds released was similar to that found in a previous study of introductions of exotic birds to Australia. 3. It was possible to look for a within-family influence on the success of introduction of the number of birds released in nine bird families. A positive influence was found within seven families and no effect in two families. This preponderance of families with a positive effect was statistically significant. 4. A significant effect of body weight on the probability of successful establishment was found, and negative effects of clutch size and latitude of origin. However, the statistical significance of these effects varied according to whether comparison was or was not restricted to within-family variation. After applying the Bonferroni adjustment to significance levels, to allow for the large number of variables and factors being considered, only the effect of the number of birds released was statistically significant. 5. No significant effects on the probability of successful establishment were apparent for the mean date of release, the minimum number of years in which birds were released, the hemisphere of origin (northern or southern) and the size and diversity of latitudinal distribution of the natural geographical range.