Two species of this wader occur in New Zealand-the South Island pied oystercatcher (Haematopus ostralegus finschi), which is only subspecifically different from the European bird, and the variable oystercatcher (Haematopus unicolor), which is a species found only in New Zealand. The South Island pied oystercatcher breeds only in the South Island; the variable oystercatcher breeds in North, South, Stewart, and Chatham Islands.
In spite of its scientific name, the variable oyster-catcher (as its common name suggests) may occur in pied or black plumage or in a range of plumages intermediate between these two. It was originally named after the black phase and before it was realised that its other colour phases all belonged to the same species. The South Island pied oyster-catcher, on the other hand, has a constant pied plumage and is noticeably smaller than the variable oystercatcher. Totally black and intermediate mottled forms of the variable oystercatcher will not easily be confused with the South Island pied oystercatcher, but the pied form may be, for it is almost as white below as the South Island pied oystercatcher and also has a white rump. The boundaries between its black and white areas are, however, not so precise below, and there is a mottled band on the leading edges of its underwing surfaces. Its white rump patch is only a band across the base of the tail instead of a broad wedge extending to the middle of the back, as in the South Island pied oystercatcher. And, finally, the white bars on the backs of its wings, as seen in flight, are short and narrow instead of long and broad.
In both species the sexes are very similar in appearance, except that the females are a little bigger than the males. There are differences in behaviour between the two species: the South Island pied oystercatcher breeds inland mainly on shingly river beds; the variable oystercatcher breeds on coastal sand dunes. The South Island pied oystercatcher breeds earlier than the variable and is mainly a migrant, moving to the mudflats of Auckland and Northland in winter, whereas the variable oystercatcher is predominantly sedentary.
The totally black form of the variable oystercatcher is uncommon, except in a few localities in the South Island. It is the dominant oystercatcher about Foveaux Strait.
The long, stout, and orange-red bill, scarlet eyes, and pink legs and feet of both species clearly characterise these waders in the field, as does their shrill flight call of “kleep, kleep”.
Nests are simple scrapes in shingle or sand, and two or three stone-coloured eggs, heavily marked with patches of light or dark brown, are the usual clutch. During the breeding season a trilling, bubbling song is heard.
by Gordon Roy Williams, B.SC.(HONS.)(SYDNEY), Lecturer in Agricultural Zoology, Lincoln Agricultural College.