We saw no pure Black Stilts on our visit. These appear to be hybrids. The Black Stilt is endemic to New Zealand. Formerly common it is now critically endangered with a total population of fewer than 100. The main cause of the decline is predation by introduced mammals but hybridization with Pied Stilt is also a factor. Although immature Black Stilts can be difficult to separate from immature Pied Stilts, the adult Black Stilt is entirely black. Typical hybrids resemble Pied Stilts but have a variable black band across their foreneck. The top/middle individual is on the dark side and may represent a backcross. It is close to hybrid type F described in HANZAB as "white on underparts increased to form irregular belly-patch, with scattered black streaking; and white on face expanded but retaining neat circular pattern round base of bill." The bottom individual is more typical and consistent with type D. Digiscoped with Panasonic DMC-LX5 | Nikon FS 3 | 30XWA | hand-held, no adapter.
References:
Adams, R. 2013. Pied stilt. In Miskelly, C.M. (ed.) New Zealand Birds Online. www.nzbirdsonline.org.nz
BirdLife International. 2016. Himantopus novaezelandiae. The IUCN Red List of Threatened Species 2016: e.T22693690A93418187. http://dx.doi.org/10.2305/IUCN.UK.2016-3.RLTS.T22693690A93418187.en. Downloaded on 22 March 2017.
Marchant, S.; Higgins, P.J. (eds) 1993. Handbook of Australian, New Zealand and Antarctic birds. Vol. 2, raptors to lapwings. Oxford University Press, Melbourne.
Pierce, R.J. 2013. Black stilt. In Miskelly, C.M. (ed.) New Zealand Birds Online. www.nzbirdsonline.org.nz
Pierce, R.J., Kirwan, G.M., Boesman, P. & Sharpe, C.J. (2017). Black Stilt (Himantopus novaezelandiae). In: del Hoyo, J., Elliott, A., Sargatal, J., Christie, D.A. & de Juana, E. (eds.). Handbook of the Birds of the World Alive. Lynx Edicions, Barcelona. (retrieved from http://www.hbw.com/node/53760 on 22 March 2017).
Pierce, R.J., Kirwan, G.M. & Boesman, P. (2017). Black-winged Stilt (Himantopus himantopus). In: del Hoyo, J., Elliott, A., Sargatal, J., Christie, D.A. & de Juana, E. (eds.). Handbook of the Birds of the World Alive. Lynx Edicions, Barcelona. (retrieved from http://www.hbw.com/node/53759 on 21 March 2017).