Gulls, terns and skuas
Gulls, terns and skuas belong to the order Charadriiformes, which includes both sea and shorebirds. They have webbed feet and are partly dependent on marine or freshwater food sources. In New Zealand all of these birds are protected, except black-backed gulls. Subantarctic skuas are partially protected.
Gulls and terns are gregarious birds common to New Zealand’s inland and coastal regions. Subantarctic skuas are found on the Chatham Islands and on islands south of the mainland.
Gulls in New Zealand slang have a rough reputation. In the 1930s wharf labourers waiting for scraps of work were called seagulls. And in rugby, a seagull is a loose forward who scavenges for pickings on the edges of tight play.
Black-backed gulls or karoro (Larus dominicanus) can be found in the southern hemisphere from Antarctica to the subtropics. Elsewhere they are known as Dominican or kelp gulls. The subspecies in New Zealand is the widespread Larus dominicanus dominicanus. There are probably over two million in coastal and near-shore environments, and inland waterways. They do not generally venture far out to sea.
Of New Zealand’s gull species, black-blacked gulls are the largest, at 60 centimetres long. Males weigh over 1 kilogram, and females about 850 grams. Adults have white bodies, black wings, and yellow bills and legs. Juveniles look very different, with mottled brown plumage and black bills and legs.
The gulls threaten rare birds such as the New Zealand dotterel, and some terns and petrels by preying on eggs and chicks. They can also attack newborn lambs. They were found to spread Salmonella brandenburg, an infection causing spontaneous abortions in sheep and cattle, which the gulls got by eating dead foetuses with the disease.
Black-backed gulls also feed on fish, shellfish, offal and carcasses, and fruit. They are often seen in ploughed fields taking worms and grubs. The gulls often return to favourite feeding spots.
Training and taming
Black-backed gulls’ predatory nature has been put to good use. Māori trained them to eat the caterpillars that infested kūmara (sweet potatoes). Some birds became tame enough to follow people around, while others had their wings clipped to stop them flying away.
Breeding and life span
Black-backed gulls generally breed in large colonies, anywhere from coastal sites to mountain lakes. They make nests of plant material, and lay one to three greyish-green eggs in October–November, which both sexes incubate. Chicks fly when they are 50 days old, and the birds breed from the age of four. They usually live for 14 years, but can live twice as long.