Successful recovery of North Island kokako Callaeas cinerea wilsoni populations, by adaptive management

Kokako Callaeas cinerea wilsoni (Callaeidae) populations are declining in unmanaged primary forests of the North Island, New Zealand. An 8-year experiment to determine the cause of decline was undertaken by controlling introduced browsing and predatory mammal pests in two forest areas, then monitoring pest abundance, kokako chick output and adult density in the managed forests and an unmanaged non-treatment block. Treatments were switched between the unmanaged and one of the managed areas after 3-4 years. Reduction of pests, especially brushtail possums Trichosurus vulpecula and ship rats Rattus rattus, to very low levels resulted in significant increases in kokako chick output and adult density in all three study populations. This was due primarily to increased success of nesting attempts, which then increased the number of pairs attempting to breed, initially as newly recruited young females formed pairs with residual single males. The 'adaptive management' approach of using routine large-scale pest control in a co-ordinated experiment to directly test the pest-limitation hypothesis enabled researchers and managers to investigate the cause of decline and to increase populations simultaneously. Predation is a more immediate cause of current kokako declines than competition. Management to recover vulnerable kokako populations should aim to reduce possums and ship rats to very low levels (<1% trap catch for possums; <1% tracking rate for ship rats, using particular indexing techniques) at the onset of the kokako nesting season, for several consecutive years. (C) 1998 Elsevier Science Ltd. All rights reserved.