South Island Wren (or New Zealand Rock Wren) (Xenicus gilviventris) is endemic to New Zealand. Once found in the North Island and South Islands prior to European settlement, it is now restricted to the South Island, where it ranges from north-west Nelson, down through Westland and the Southern Alps, to Fiordland (Heather and Robertson 2015). It was once described as locally common (Heather and Robertson 1997), but its distribution is fragmented, and the population is now estimated to number less than 5,000 mature individuals with a predicted population decline of 50-70% over the next 10 years (Robertson et al. 2013).
Perhaps due to the difficulty of studying such a widespread alpine species, there is a lack of detailed information on the population size of X. gilviventris, and it is described as “data poor” (Robertson et al. 2013). The New Zealand threat status of X. gilviventris was reclassified in 2012 from ‘nationally vulnerable’ to ‘nationally endangered’ due to significant perceived ongoing population decline (Gaze 2013; Robertson et al. 2013). Both stoats and mice are known predators of adult and juvenile South Island Wrens, and their eggs (Michelsen-Heath & Gaze 2007). A number of recent unpublished studies led by the New Zealand Department of Conservation have observed extremely high (80-100%) nest failure due to predation (DOC, unpublished data per Weston 2014, Webb 2015). High rates of predation within isolated populations may be contributing to their demographic instability (Weston 2014). It is likely that predation rates vary significantly from year to year (Gaze 2013).
Conservation efforts to date have involved localised trapping of predators by volunteer groups and translocations to predator-free offshore islands (Weston 2014). One such translocation (to Secretary Island) appears to have succeeded, whereas another to Anchor Island seems to have failed (Weston 2014). There is an additional threat to this species from climate change, especially if warming temperatures make its core habitat more suitable for ship rats – a potential nest predator which is currently absent (Gaze 2013).
Due to the predicted levels of decline for this species based on the very low apparent nesting success in many areas, and the disappearance of some populations (K. Weston in litt. 2014), it is recommended that this species is uplisted to Endangered under criteria A3ce and A4ce.
Additional information and comments on this proposal are welcomed.
Gaze, P.D. 2013. Rock wren. In Miskelly, C.M. (ed.) New Zealand Birds Online. www.nzbirdsonline.org.nz
Heather, B. D.; Robertson, H. A. 1997. The field guide to the birds of New Zealand. Oxford University Press, Oxford, U.K.
Heather, B. D.; Robertson, H. A. 2015. The field guide to the birds of New Zealand. Penguin Random House NZ, Auckland, N.Z.
Michelsen-Heath, S; Gaze, P. 2007. Changes in abundance and distribution of the rock wren (Xenicus gilviventris) in the South Island, New Zealand. Notornis 54(2): 71-78.
Robertson, H. A; Dowding, J. E; Elliott, G. P; Hitchmough, R. A; Miskelly, C. M; O’Donnell, C. F. J; Powlesland, R. G; Sagar, P. M; Scofield, R. P; Taylor, G. A. 2013. Conservation status of New Zealand birds, 2012. NZ Threat Classification Series 4. Department of Conservation. Wellington.
Webb, D. 2015. The effect of management on rock wren nesting success. Unpublished MSc Report to University of Otago Zoology Department.
Weston, K.A. 2014. Conservation Genetics of Alpine Rock Wren (Xenicus giliventris). Thesis submitted to for the Degree of Doctor of Philosophy at University of Otago, Dunedin, New Zealand.