Aspects of the ecology and morphology of Cyanoramphus parakeets and hybrids from Mangere Island, Chatham Islands

Abstract

Two parakeet species of the genus Cyanoramphus, which are hybridizing on Mangere Island in the Chatham group were investigated by field observation and examination of preserved specimens. A description is given of the distribution on Mangere, and habitat selection of Forbes Parakeet (C. auriceps forbesi), the Chatham Island Red-crowned Parakeet (C. novaezelandiae chathamensis) and hybrids. Degradation of the former Forbes habitat appears to have reduced the zone of allopatry and thus the ability of the Forbes population to withstand introgression of Red-crowned genes. The diets of Red-crowned and hybrid parakeet groups on Mangere were similar. Crop content analysis and feeding observations indicate that differences in feeding between the two parental species are not as great as when they occurred on separate islands with different habitat. Marked seasonal variation in diet was noted. Mangere parakeets harboured Forficuloecus feather lice, and heavy infestations of Ascaridia platyceri nematodes — the first record of this worm in wild birds. The Chatham Island Red-crowned Parakeet was found to conform to geographic patterns in morphometric,head plumage marking, and colour variation within the Red-crowned species. More extreme development of features characteristic of island life were noted in Forbes Parakeet such that there is a convergence of body size in Chatham Island species. A smaller third species possibly inhabited the Chatham Islands. The coexistence of species is discussed in relation to ecological strategies and reproductive isolation. Recolonization of disrupted habitat seems to be the primary cause of hybridization. Contributory factors may also include: convergent size, limited mate choice, and the close morphological and physiological similarity within the genus. A preliminary electrophoretic study of the group revealed no fixed allelic differences in plasma proteins between species. Theories of speciation and biogeography are discussed as they relate to parakeets. The rare orange-fronted Parakeet (C. malherbi) of the New Zealand mainland was found to be morphologically indistinguishable from the Yellow-crowned Parakeet (C. auriceps auriceps), supporting the hypothesis that the former is a colour-morph of the latter species.