This species occurs in two color morphs, one bronze and one pied. This is a pied-morph. Some authorities (H&M4) lump this bird with the New Zealand King Shag (Phalacrocorax carunculatus) and Chatham Island Shag (Phalacrocorax onslowi) under the collective name "Rough-faced Shag." In fact the pied morph is almost impossible to distinguish from the other two in the field.
However new research (Rawlence et al. 2016) indicates that our bird is not really a Stewart Island Shag, but a previously undescribed species, the Foveaux Shag (Phalacrocorax stewarti). This study found a large genetic divergence between two populations of Stewart Island Shag and calls for recognition of Foveaux Shag as a new species separate from Phalacrocorax chalconotus which is renamed the Otago Shag. They found osteological, morphological, and behavioral differences between the two species with the Foveaux Shag actually closer to the Chatham Shag than to the Otago Shag. The latest IOC ver 7.1 accepts this split.
Canon PowerShot SX50 HS.
References:
McKinlay, B. 2013. Stewart Island shag. In Miskelly, C.M. (ed.) New Zealand Birds Online. www.nzbirdsonline.org.nz
Orta, J., Jutglar, F., Garcia, E.F.J., Kirwan, G.M. & Boesman, P. (2017). Stewart Shag (Phalacrocorax chalconotus). 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/52644 on 2 February 2017).
Rawlence, Nicolas J.; Scofield, R. Paul; Spencer, Hamish G.; Lalas, Chris; Easton, Luke J.; Tennyson, Alan J.D.; Adams, Mark; Pasquet, Eric; Fraser, Cody; Waters, Jonathan M.; Kennedy, Martyn (2016). "Genetic and morphological evidence for two species of Leucocarbo shag (Aves, Pelecaniformes, Phalacrocoracidae) from southern South Island of New Zealand". Zoological Journal of the Linnean Society. Retrieved 2 Feb 2017. [PDF]