Recovery and maintenance of North Island kokako (Callaeas cinerea wilsoni) populations through pulsed pest control Original Research Article

Kokako (Callaeas cinerea wilsoni) population recovery on the North Island of New Zealand depends primarily on control of key introduced mammal pests, especially ship rats (Rattus rattus) and brushtail possums (Trichosurus vulpecula). Recovery can still occur if pest control is pulsed (x years ‘on’; y years ‘off’) because kokako sub-adults and adults are generally long-lived, although chick production is high only during ‘on’ years. Pulsing effort means that conservation resources can be extended to other sites or problems during ‘off’ years; that toxin input at any one site is reduced; and that project staff do not burn out by repeatedly working at a site. Mathematical modelling supports empirical evidence that pests need not be controlled every year in order to maintain or greatly increase kokako populations. It predicts that the total number of years during which there is pest control is the main factor determining population size. Three years of pest control in each 10 should be sufficient to at least maintain a population with 20 females when mean parameters apply, but pulsed control should still be effective with very pessimistic parameters. In the safest strategies, control should occur in minimum pulses of 2–3 years to avoid single poor years when few breeding attempts are made. Very small populations should first be increased to at least 20 females by translocation or continuous pest control. This will greatly reduce the probability of chance extinction, and increase the efficiency of subsequent pest control. The model will apply best to closed kokako populations below carrying capacity, in which pests are controlled over the entire block. Empirical data on the effects of habitat carrying capacity on kokako dispersal, and on the importance of stoats as predators of adult females are required to further strengthen the model.