The effects of introduced predators on the saddleback (Philesturnus carunculatus), and implications for management

Abstract

Factors predisposing many endemic New Zealand birds to predation are examined, and an investigation of one of the most susceptible species, the saddleback (Philesturnus carunculatus), is presented. Like other island endemic birds, many New Zealand species are vulnerable because they lack prior exposure to predatory mammals. Birds such as the saddleback, which feed and nest on the ground, and nest in cavities, or which are flightless, have been most at risk to introduced predators. Arboreal feeders and nesters, and non cavity-users, and species which can fly, appear to have been less susceptible. The saddleback and other species with similar features, which had previously coexisted with the Maori and their rats (Rattus exulans), declined quickly when European rats, cats and mustelids were introduced. A translocation of saddlebacks to Kapiti Island where R. exulans and R. norvegicus, occur, provided an opportunity to re-examine the effects of rats. Circumstantial evidence suggested coexistence with terrestrial R. norvegicus before the arrival of arboreal R. rattus in about 1860. However, on Kapiti Island R. norvegicus preyed on saddlebacks at ground roost and nest cavities, and the release failed. This contrasts with islands with R. exulans only, where some nests are robbed, but adults, juveniles and fledglings are never killed. An experiment was conducted on Kapiti Island in which roost and nest boxes were provided as substitutes for natural cavities, and box-using birds were introduced from another island. Box-users had significantly higher survival than non box users. Although some young learned to use boxes, recruitment was still insufficient to balance adult losses. The experiment provided indirect evidence that R. norvegicus was an important predator at roost and nest cavities. Translocations were also used to investigate the relevance of familiarity to saddleback conservation, and to refine translocation and release methods. Translocated saddlebacks aggregated near conspecifics, probably because the chances of finding mates were higher, and because other birds provided information about the habitat. Familiar and unfamiliar birds settled randomly with respect to each other. Familiar birds did not have enhanced survival or breeding productivity in the first year, but benefits from familiarity accrued in subsequent years. There was no difference in survival in hard (direct) and soft (gentle) releases. Hard releases are appropriate for wild birds, where the time between capture and release should be minimized. Successful releases of wild forest birds depend more on high quality habitat, sufficient numbers and a balanced sex ratio than release method. Future management depends on predator-free islands. As a result of improved methods for ridding islands of rats there is much scope for establishing new populations on restored islands. Since saddlebacks were widespread in pre-human times, releases of this species should be an integral part of the ecological restoration of New Zealand islands.