Cyanoramphus forbesi (Chatham, or Forbes’ Parakeet) is restricted to Little Mangere and Mangere Islands in the Chatham Island group, New Zealand. By 1930, it was extinct on Mangere Island, but by 1973 it had recolonised and numbered 40 birds and a small number of hybrids with C. novaezelandiae chathamensis (of which there were 12 on the island) (Higgins 1999). In 1996, the population was estimated at 120 birds (Aikman et al. 2001).
The Department of Conservation has collected trend data for parakeets on Mangere Island since 1999. Although many hybrids occurred in the early years of survey (e.g. Chan et al. 2006), this appears to have now changed in favour of Chatham Parakeets, possibly due to the impact of positive assortative mating (Greene 2000). The most recent population estimate is around 600 individuals, and this population level appears to have remained stable since 2003 (Aikman and Miskelly 2004, T. Greene in litt. 2016). A small cull of non-forbesi phenotypes was carried out in 2012, and since then only a small proportion <10% of hybrids has been observed (T. Greene in litt. 2016).
Although the population now appears stable, the greatest present threat to this species is hybridisation with C. n. chathamensis which, despite culling, continues to establish itself on Mangere Island (J. Kearvell in litt. 1999). The previously high rate of hybridisation is believed to be the result of the then low population sizes of the two species (D. Houston and C. Miskelly in litt. 2008). Levels of hybridisation have remained below 10% of the total Mangere Island parakeet population for much of the past decade. Ongoing vigilance for unsustainable levels of hybridisation will be required in future years.
As the population of C. forbesi is stable or increasing, and is now considered to have exceeded 250 individuals for more than five years, the species no longer qualifies as Endangered under criterion D. It therefore proposed to downlist it to Vulnerable under criteria D1 (population less than 1,000 mature individuals) and D2 (AOO <20 km2 and number of locations <5, with a plausible future threat that could drive the species to Critically Endangered or Extinct in a very short time).
Additional information and comments on this proposal are welcomed.
Aikman, H.; Davis, A.; Miskelly, C.; O’Connor, S.; Taylor, G. 2001. Chatham Islands threatened birds: recovery and management plans. Department of Conservation, Wellington, New Zealand.
Aikman, H.; Miskelly, C. 2004. Birds of the Chatham Islands. Department of Conservation., Wellington.
Chan, C.H., Ballantyne, K.N., Aikman, H., Fastier, D., Daugherty, C.J. and Chambers, G.K. 2006. Genetic analysis of interspecific hybridisation in the world’s only Forbes’ parakeet (Cyanoramphus forbesi) natural population. Conservation Genetics 7(4): 493-506.
Higgins, P. J. 1999. Handbook of Australian, New Zealand and Antarctic birds: parrots to dollarbirds. Oxford University Press, Oxford, U.K.