Tuatara: Volume 2, Issue 3, September 1949
Identification of New Zealand Mainland Shags
Identification of New Zealand Mainland Shags
As the usual form of identification key would be of value only to those who are concerned with handling museum specimens and dead birds, the accompanying sketches have been prepared as an aid to distinguishing in life seven of the eight species of shags or cormorants to be found in New Zealand, including Stewart Island. A further six not figured here occur in the Chathams and the subantarctic.
The superficial similarity of shags disguises many differences of structure and habit; while age and seasonal plumage phases may lead to further confusion. These are mentioned in the following notes. Distribution and habitat are also important guides. As a group, those species frequenting inland waters and extending their range to estuaries and bays all have black feet. The oceanic shags, which never venture into fresh water, all have pink or yellow feet.
1. Little Shag (Phalacrocorax brevirostris), variously called “White-throated,” “Little Pied,” or “Frilled.” The diagnostic features are short yellow bill and long tail. The plumage pattern is not uniform: the commonest form when adult is black with a white throat (1 c), and at the other extreme is the “little pied” plumage (1 a). Most young in their first plumage are entirely black (1 b), but the extreme “little pied” form* assumes the white breast with its first plumage. Feet black.
Distribution: Lakes, rivers and sheltered coasts throughout New Zealand.
2. Little Black Shag. (P. sulcirostris). Distinguished from the young of the last by its longer bill, which is also dark, and its shorter tail. Feet black.
Distribution: Lakes, rivers, and estuaries of Auckland isthmus south to Hawke's Bay.
3. Pied Shag. (P. varius.) Larger than the last two, the male being nearly as large as a black shag. The bill is pale, and facial skin yellow, blue, and dull purple. Immature birds have white underparts slightly mottled with dark. Feet black.
Distribution: Warmer coastal waters and estuaries of Auckland, Bay of Plenty, East Coast, Marlborough, Nelson, North Canterbury, and Stewart Island.
4. Black Shag. (P. carbo). The largest inland shag, with a pale bill, and yellowish about the face. White chin, crests, a whitish mane, and white thighs are breeding ornaments only and are not always present. Immature birds have the underparts mottled with white. Feet black.page 117 page 118
Distribution: Lakes, rivers, estuaries, and coastline throughout New Zealand.
5. Spotted Shag. (P. punctatus). A slenderly-built sea shag with pale-grey plumage except for oil-green back and thighs. Bright facial colours are developed in the breeding season; double crests, and pure-white feathers form a neck stripe and sprout from the darker areas of the plumage. The young are pale-grey below, and darker above. Feet, cream to yellow.
Distribution: Rocky coasts of both islands. The Stewart Island form, with less white, is known as the Blue Shag (P. steadi).
6. Rough-faced or King Shag. (P. carunculatus). The largest of the subantarctic shags, which are alike in having steel-blue necks and backs and oil-green wings. They have a cluster of orange pimples at the base of the bill, and blue eyelids. Young have the dark plumage dull brown. Feet pink.
Distribution: Western Cook Strait only, and not common.
7. Stewart Island Shag (P. chalconotus). Two distinct plumage forms (7a and 7b) occur, and they interbreed. The glossy dark bird is unmistakeable; and the white-breasted one is not unlike the King Shag. They differ in being smaller, having red throat pouches, and developing crests in the breeding season. Feet pink.
Distribution: Coasts of Stewart Island and Ruapuke. A larger form of the same bird inhabits Otago Peninsula. These birds are entirely marine. Superficially, the pied form could be confused with the Pied Shag (P. varius), and the dark form with the Black Shag (P. carbo).
Identification of shags is a matter of practical importance because while many are on the schedule of absolutely protected birds they are somewhat indiscriminately subject to shooting and nest raiding because they are fish-eaters. The basic falsity of the assumption that any predator effects serious reduction of its own food supply has long been revealed for what it is worth, but it still influences the views of gamekeepers, fish culturists, and some commercial fishermen.
The official attitude, maintaining a reasonable balance on available evidence, accords protection to all the purely marine cormorants; although prosecutions are infrequent except when heads are submitted for bounty payable on some river shags. With the exception of the Little Black Shag (P. sulcirostris), the estuarine and river species are not protected, and a good deal of lively controversy centres round the relationship of the Black Shag to stocks of introduced trout. Although the late E. F. Stead, who was an experienced angler, considered that the presence of shags on trout streams was beneficial (Emu, vol. 8, p. 71, 1908) the empirical views of the majority of anglers is summed up in a booklet by H. G. Williams (The Shag Menace, 1945). The evidence presented in this book has some serious defects, but its main page 119 page 120 conclusions have been tentatively accepted by D. F. Hobbs (Trout Fisheries in N.Z., 1948) who considers that a policy of control should be adopted.
A good deal more investigation is required, backed by a good knowledge of birds, fish, and stream ecology. Some attempt at presenting evidence that can be checked has been made by Falla and Stokell (T.R.S.N.Z. vol. 74, 1945); but the New Zealand standard of investigation has not yet reached the level at which similar work has been carried out in the U.S.A., Canada, and Australia.
Australian investigations, because they deal with shag species also found in New Zealand, are perhaps of more immediate interest. Dealing with conditions on an estuary—the Swan River—D. L. Serventy (The Feeding habits of Cormorants and South-western Australia. Emu, 1938, p. 293) examined a total of 441 shags of four species in one year. His results showed the effect of the Pied Shag on commercially important fishes is negligible, the Little Pied is still more harmless, the Little Black Shag likewise harmless; the Black Shag alone being shown to include an appreciable percentage of marketable fishes in its diet. In Victoria, G. Mack (Cormorants and the Gippsland Lakes Fishery. Mem. Nat. Mus, Vic. LX, 1941, p. 95) found no evidence from stomach analyses that shags were detrimental to stocks of commercial or sporting fishes in that area. In New South Wales, K. McKeown (The Food of Cormorants and other Fish-eating Birds. Emu. 1944, p. 259) has published data which, as far as they go, confirm the findings from other States.
It may be concluded, on the basis of available evidence that no native marine or estuarine fisheries in New Zealand are adversely affected by the presence of a population of Cormorants. The problem seems to be reducible to the single issue of the black shag in relation to introduced trout. The whole of the fresh water system of the country has been stocked, or there have been attempts to stock it, with introduced salmonids. To those whose idea of adequate trout population really requires an artificial surplus to satisfy it, shag depredations can never be regarded with equanimity. But it should be possible to contemplate without alarm a state of affairs, and surely a desirable one, in which the growth of trout food keeps pace with the needs of a trout population, and the trout keep pace with their predators. The predatory relationship is not necessarily a destructive one and it has yet to be shown whether the level at which black shags affect trout has an adverse or a beneficial effect on the stock.
* Specifically separated by most authors as P. melanoleucus.