From Mainland Beech to Island Hardwood/Podocarp Habitat: the Challenges of Translocating Mohua (Mohoua Ochrocephala) to Nukuwaiata, an Island in the Marlborough Sounds

Abstract

Mohua (Mohoua orchrocephala) were translocated from two source populations to Nukuwaiata, a predator free island in the Marlborough Sounds. Four mohua were transferred from Mt Stokes in the Marlborough Sounds in 1999, and a further transfer of 27 mohua, sourced from the Dart Valley, Mt Aspiring National Park, occurred in 2001. As mohua are now absent from podocarp and hardwood forests on the South Island mainland establishing a population on Nukuwaiata, where these forest types are present, is important to secure the future distribution of this endangered species. Detailed research has not previously been conducted on mohua on predator free offshore islands, or in forest types other than beech (Nothofagus spp.). Survival, establishment patterns, breeding behaviour, habitat requirements and preferences of mohua were investigated within the hardwood and podocarp forest types on Nukuwaiata. The results of this research were used to recommend effective management strategies for this species. Prey availability was also determined, to provide a comparative baseline for future mohua transfers to islands with similar forest types to those on Nukuwaiata. After the two monitored breeding seasons (2001-02 and 2002-03), survival rates of mohua were greater for Mt Stokes birds (75% and 50%, respectively) than for Dart Valley mohua (51.9% and 14.8%, respectively). Mt Stokes mohua bred successfully, producing one male chick in the first breeding season and two chicks (one male) in the second season. Dart Valley mohua did not breed in the first two seasons, however in the third season the Mt Stokes female paired with a Dart Valley male and successfully produced one chick. Later that season the female paired with another Dart Valley male, which is the first reported case of polyandry in mohua. Differences in dialects were evidently not a breeding barrier between the Mt Stokes and Dart Valley mohua on Nukuwaiata. Only two territorial areas were used in the three breeding seasons and within each matai (Prumnopitys taxifolia) density was greater than elsewhere on the island. Habitat utilisation and preference differed between the two transferred groups of mohua. Mt Stokes birds behaved differently from Dart Valley birds, since breeding behaviours only occurred with Mt Stokes mohua. Mt Stokes mohua allocated their time in certain tree species differently from Dart Valley birds. Matai, kohekohe (Dysoxylum spectabile), hinau (Elaeocarpus dentatus), tawa (Beilschmiedia tawa) and rata (Metrosideros sp.) were the most commonly used tree species by Mt Stokes mohua in the first breeding season, whereas hinau, matai, kohekohe, tawa, pigeonwood (Hedycarya arborea) and rata were used more frequently in the second season. Dart Valley mohua used kanuka (Kunzea ericoides), matai, pigeonwood and kohekohe more frequently. Larger and taller trees were also used more frequently by Mt Stokes than Dart Valley mohua. Mohua did not use the habitat on Nukuwaiata at random as Mt Stokes mohua were found to prefer matai, hinau, rata, kohekohe and five-finger (Pseudopanax arboreus) during the first season and hinau, rata, pigeonwood and five-finger during the second season. Kanuka, pigeonwood, five-finger and Coprosma grandifolia were the preferred tree species of Dart Valley mohua, in relation to availability within the habitat. An assessment of prey availability on Nukuwaiata revealed differences in invertebrate diversity and abundance between the two breeding seasons and between some tree species. Invertebrate diversity and abundance differed within and outside the Mt Stokes mohua territories and were greater in matai, within the mohua territories. Average body length of invertebrates was larger outside the mohua territories. Both the Mt Stokes and the Dart Valley mohua appeared to allocate their foraging times within certain tree species in accordance with the diversity and abundance of invertebrates. Invertebrate body length however, did not appear to influence foraging time allocation within tree species used by the Mt Stokes and Dart Valley mohua. This research suggests that as the Mt Stokes mohua were the first to be translocated to Nukuwaiata, they settled in the most optimal habitat on the island, where matai and prey availability were highest. Habitats available to the Dart Valley mohua therefore appeared to be of lower quality and may have been the primary factor limiting the success of this translocation. Appropriate recommendations for the conservation management of mohua, based on this research, are presented and discussed.