Often seen perched on rocks or trees drying their outspread wings, shags are large birds with long hooked bills, snaky elastic necks, and powerful webbed feet for underwater hunting. Of the world’s 36 shag species, 12 are found in New Zealand. Eight of these are endemic – they are found nowhere else. The remaining four are native to other countries as well as New Zealand. Under the Wildlife Act 1953, the black shag, little shag and pied shag may be hunted or killed under certain circumstances.
The various shag species have evolved to specialise in different environments. Some are coastal, and have adapted to feed in either shallow or deeper water. Others are found inland, where they dive for fish and other prey in lakes and rivers. Several that live on offshore islands have probably evolved into new species through isolation.
Members of the shag family belong to three groups, based on the colour of their feet: black, yellow or pink. Outside New Zealand, all species (except the European shag, Phalacrocorax aristotelis) are better known as cormorants.
Shags capture and eat fish larger than would seem possible. The New Zealand naturalist Edgar Stead observed a black shag catching a 60-centimetre eel:
[T]he Shag is able to hold it without difficulty, for the edges of its bill are so sharp they can cut into the eel’s skin and so give a good grip … I have seen an eel half swallowed by a Shag give (no doubt when its head came into contact with the bird’s gastric juices) such a convulsive heave that it flipped itself clean out of the bird’s throat. 1
Shags capture their prey by chasing it underwater (pursuit diving), propelling themselves with powerful webbed feet. They can stay underwater for an extraordinary length of time. A king shag in the Marlborough Sounds was clocked diving for 190 seconds in search of prey, while a pied shag lasted 220 seconds before coming up for air. But although they might be agile at sea, shags are as awkward as penguins on land.
Compared to other seabirds such as penguins or petrels, shags have less fat to keep them warm. In addition, so that they can dive better to catch their prey, their feathers are less oily, and can become partially waterlogged. As a result they are at risk of hypothermia if they are in the water too long. Studies show that shags in cold waters catch fish at a very fast rate. At the end of a fishing expedition, the northern species characteristically hang out or flap their wings to dry and get warm again. The more southerly pink-footed shags do not, probably because the risk of hypothermia from rapid heat loss is too great.