Skuas are aggressive seabirds that rob other birds of their food, prey on eggs, chicks (such as fatty muttonbirds) and fish, and scavenge for sheep, cattle, seal and sea lion carcasses. Although skuas are occasionally found around mainland coasts, they are more often seen on the Chatham and subantarctic islands, and in Antarctica. Of the five species recorded in the New Zealand region, subantarctic skuas are the only ones that breed there. The skuas most commonly seen on New Zealand coasts are Arctic skuas – visitors from the northern hemisphere.
Known to Māori as hākoakoa, the subantarctic, southern or brown skua (Catharacta antarctica lonnbergi) breeds in New Zealand, throughout the Southern Ocean, and on islands of the Indian, Atlantic and Pacific oceans down to Antarctica.
Relatively large and stocky, skuas are dark brown with white flashes on the wings. They have a sturdy, hooked bill. Adults grow to 63 centimetres long, with males weighing 1.76 kilograms and females 1.95 kilograms.
Subantarctic skuas breed on islands off Stewart Island and the Chatham, Snares, Antipodes, Auckland and Campbell islands.
Skua females lay two eggs from September to December, and the young are fed for 4–5 months. Unlike any other seabirds, 10% of subantarctic skuas on the Chatham, Snares and Stewart islands tend their nests in cooperative groups, usually consisting of a female and two or more unrelated males. Some trios remain together for many years.
The New Zealand population is fewer than 2,000, and subantarctic skuas are partially protected (they may be killed as pests by landowners). Scientists believe that numbers are dwindling, although there has been no systematic monitoring. The reduction has been linked to plummeting numbers of penguins and elephant seals – the carcasses of which are favourite food for skuas – on subantarctic islands. Another problem is farmers and fishers shooting them.
South Polar skuas
The South Polar skua (Catharacta maccormicki) breeds on the Antarctic continent and its offshore islands. They spend winter at sea, including offshore New Zealand, and some head to the northern hemisphere. They feed on shoaling fish, and take eggs, chicks and spilt krill from penguin and petrel colonies.
These skuas are 59 centimetres long, with males weighing 1.27 kilograms and females 1.42 kilograms. The New Zealand-administered Ross Dependency has an estimated population of 15,000 South Polar skuas.
Why do skua chicks kill their siblings? Evolutionary biologists who have puzzled over this have considered that parents produce more offspring than they can raise, resulting in a fight to the death over resources. To test this, two New Zealand scientists gave supplementary food to South Polar skua parents. Still, the chicks killed their siblings. The explanation was that skuas are naturally aggressive.
The Arctic skua (Stercorarius parasiticus) breeds in the Arctic and spends the northern winter in southern hemisphere seas. Common visitors to New Zealand coastal and inshore waters from November to April, they are more likely to be seen than the New Zealand-breeding subantarctic skuas. They feed by stealing the catch of terns and gulls in mid-air. They are relatively small, at 43 centimetres long and 400 grams in weight.