Download

The Saddleback (Philesturnus carunculatus) is now confined to New Zealand offshore islands free of the introduced carnivorous mammals, i.e. rats, feral cats and mustelids, which are assumed to have exterminated the species on the mainland during the last century. The North Island Saddleback (Philesturnus carunculatus rufusater) coexists with the Polynesian rat (Rattus exulans) while the South Island Saddleback (P c. carunculatus) thrives only on rat-free islands. An experimental transfer to Kapiti island, where Norway (R. norvegicus) and Polynesian rats occur, provided an opportunity to test the hypothesis that North Island Saddlebacks could coexist with both rat species. I compared the survival of Saddlebacks on Kapiti Island wlth that on Cuvier and Little Barrier Islands where the only rodent is R. exulans. On Kapiti Island, Saddlebacks suffered high mortality, and despite high productivity, there was insufficient recruitment of young to balance losses of adults. Significantly more nests sited less than one metre above the ground were preyed on by rats on Kapiti Island than on Cuvier and Little Barrier Islands. Saddlebacks on Kapiti Island which roosted in high or secure cavities survived longer than those birds which roosted in low or vulnerable places. These observations are consistent with the ground-foraging and predatory behaviour of R. norvegicus. On Kapiti Island, 21 rat-killed birds were found at nests and roosts, whereas no rat-killed birds were found at nests and roosts on Cuvier and Little Barrier Islands, although the contents of some nests were preyed on by R. exulans. On Kapiti Island, R. norvegicus faeces were found with the remains of several dead birds, providing direct evidence that this rat was the predator. The poor survival and recruitment of Saddlebacks on Kapiti Island, coupled with observations of rat-killed birds and plundered nests near the ground, suggest that Saddlebacks are unable to coexist with both rat species, and that R. norvegicus is probably an important predator. The cavity nesting and roosting habits of the Saddleback make this species especially vulnerable to mammalian predators.