Habitat use by Tasman parakeets (Cyanoramphus cookii) and Crimson rosellas (Platycercus elegans) on Norfolk Island, South Pacific : a thesis submitted in partial fulfilment of the requirements for the degree of Master of Science in Conservation Biology, Massey University, New Zealand

Abstract

Islands harbour a disproportionate amount of threatened vertebrate species and remain the focus of intense conservation research and management. Two important components of the ecological restoration of islands include revegetation and control of introduced animal species. The Tasman parakeet (Cyanoramphus cookii) endemic to Norfolk Island, is one of the rarest bird species in the South Pacific, with a long history of assisted conservation in particular during the breeding cycle. An introduced parrot, the crimson rosella (Platycercus elegans), is thought to compete strongly with Tasman parakeets for nesting and feeding resources, however, the degree of competition for feeding resources has not been quantified. No study has aimed at understanding the selection of available habitats by Tasman parakeets and crimson rosellas, or the relevance of restored vegetation patches for these species for foraging activities. In order to provide information to better manage a growing population of the Tasman parakeet within the Norfolk Island National Park, it is important to understand patterns of habitat use and key resources, as well as the degree of competition with the introduced crimson rosella. I studied the habitat use of Tasman parakeets and crimson rosellas during two seasons, autumn and spring 2017 by surveying 986 vegetation plots. I encountered a total of 80 woody plant species occurring on nine habitat types. The highest plant species richness was in forest edges (70 species), followed by remnant hardwood forest (43 species). The lowest plant species richness was on regenerating vegetation patches (26 species). Tasman parakeets and crimson rosellas used these habitats differently. In autumn and spring, Tasman parakeets preferred remnant hardwood forests (Manly Selectivity Index autumn: 0.50; Chi square 6.86, P < 0.001, n = 35; Manly Selectivity Index Spring: 0.32; Chi square 6.86, P < 0.001, n = 55). Crimson rosellas preferred forest edge in autumn (Manly Selectivity Index Autumn: 0.34; Chi square 24.51, P < 0.001, n = 69) and remnant hardwood in spring (Manly Selectivity Index Spring: 0.28; Chi square 4.56, P < 0.001, n = 130). In terms of feeding species, Tasman parakeets and Crimson rosellas exhibited a high degree of overlap in only one food type: ake ake (Dodonea viscosa) seeds and fruits. Opportunistic observations during summer, revealed a similar degree of overlap for consumption of seeds and pulp of red guava (Psidium cattleianum), although habitat preferences for summer or winter were not quantified in this study. I did not register a single instance of aggressive interactions between Tasman parakeets and crimson rosellas after nearly 300 hours of field observations. My results indicate that Tasman parakeets and crimson rosellas used the available habitats in the Norfolk Island National Park differently. In terms of foraging resources, Tasman parakeets and crimson rosellas do not appear to overlap greatly in food types or feeding species during autumn and spring. Future research should focus on the patterns of habitat use during summer and winter and the degree of competition for feeding resources during these seasons as well. While there has been evidence (prior to this research) of intense competition for nesting cavities between Tasman parakeets and crimson rosellas, my research reveals little inter-seasonal overlap in habitat use and feeding resource. From a management perspective, control of crimson rosellas in an around nest cavities of Tasman parakeets should be favoured over control on foraging areas. Nevertheless, crimson rosellas outnumber Tasman parakeets 3:1, and therefore my results need to be interpreted with caution as it is unclear whether foraging and habitat use patterns consistent with my research would be encountered in the future, as numbers of Tasman parakeets continue to increase.