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Some often overlooked birds are a wonderful example of our region’s diversity, writes Hamish Spencer.
Shags, or what the rest of the world calls cormorants, are not often the most popular of birds. Even among professional ornithologists (bird scientists), they seem neglected. For example, there are fewer formal studies of shags compared to, say, penguins, even though there are more than twice as many living shag/cormorant species (~41) as penguins (~18).
Aotearoa-New Zealand’s list of shag species is remarkably long. Some 13 species, almost 32% of the world’s total, are found here. In the South we have striking biodiversity: six species occur within the bounds of Dunedin City, the same number as in the whole of North America and a larger number than Australia.
This wonderful diversity has not always been celebrated, however. Once upon a time, it was even legal to kill our native shags. Seventy-five years ago, Dunedin resident H.G. Williams argued for the mass culling of shags in order to protect introduced trout. A council member of the Otago Acclimatisation Society, Williams published a book entitled The Shag Menace, documenting examples of shags hunting and eating trout. (Indeed, the book was re-published in 2002 by an M.E. Rollinson, of Timaru.) Unfortunately, the complete lack of evidence that predation by shags made any difference to trout numbers didn’t stop the widespread shooting of shags for decades. Even today, Fish and Game officers are permitted to kill black and little shags if they are found in or close by fish hatcheries; about 10 are shot each year.
Yet, the more we find out about shags, the more interesting they are. We are even finding new species of shags, one right on our doorstep. Scientists only recognised the Otago shag as a separate species from the Foveaux shag in 2016. Prior to that, the two species had been lumped as one species, the Stewart Island shag. Genetic research at the University of Otago showed that the Otago shag is actually more closely related to the Chatham Island shag than the nearby Foveaux shag.
Although they are difficult to tell apart outside the breeding season, there are some subtle differences among these species. For example, the size and number of the orange caruncles (bare warty skin) above the bill seems to differ when they are in their breeding dress. Chatham Island shags have the most extensive orange, easily visible in the field with binoculars. The Otago shag has a smaller orange patch and the Foveaux shag almost none. The two South Island species start to breed at different times, too: May to September for the Otago shag and September onwards for the Foveaux.
The Otago Shag once occurred all along the eastern coast of the South Island. Subfossil remains have been found as far north as Marlborough. Soon after the arrival of people, however, the population crashed to less than 1% of its previous size and it was exterminated from the northern part of its range. By contrast, in the far south, where relatively few people lived, the population size of Foveaux shag was relatively unchanged. The timing of these events, the presence of Otago shag bones in middens and the difference between the two species’ population histories all point to human impacts (e.g., predation) being a major factor in the decline of the Otago shag.
The Otago, Foveaux and Chatham shags, together with the King shag from the Marlborough Sounds, are all members of the genus Leucocarbo. Other species are found on our subantarctic islands, as well as in South America and various remote islands ringing Antarctica. This group is particularly interesting to evolutionary biologists, as almost every island group has a species that is found nowhere else. It seems that new species quickly evolve from early colonisers of these islands.
Recent research examining so-called "ancient DNA" has shown that, sadly, New Zealand has lost a species of Leucocarbo. The Kohatu shag, named in 2017, lived in the far north of New Zealand and possibly further south along the Northland coast, but disappeared just over 600 years ago. Leucocarbo shags are prone to abandon their breeding colonies if disturbed by people, but hunting may also have played a part in the demise of this species. The Kohatu shag is one of only two shag or cormorant species worldwide known to have become extinct in historical times.
- Hamish G. Spencer is Sesquicentennial Distinguished Professor in the Department of Zoology at the University of Otago.