Being a seabird is a specialised occupation. Of the 24 bird orders (groups of related species), only five have seabirds among their members. Seabirds in New Zealand come from four of these orders:
- Sphenisciformes – penguins
- Procellariiformes – albatrosses, shearwaters and other petrels
- Pelecaniformes – shags, gannets and their kin
- Phaethontiformes – tropicbirds
- Charadriiformes – terns, gulls and skuas.
Penguins belong to the order Sphenisciformes. Three of the world’s 17 species breed on the New Zealand mainland, and a further three on the subantarctic islands. Penguins live only in the southern hemisphere. Of all the birds, penguins are the most accomplished divers, with some species capable of reaching depths of 100 metres or more. Their small wings or flippers, stiff oily plumage, dense bones and thick fat deposits are all adaptations to diving. They catch fish, crustaceans (such as krill) and squid by underwater pursuit.
Not quite seabirds
The most familiar ‘seabirds’ are the gulls, but because they find much of their food on land and seldom venture far from shore they are not true seabirds. The black-billed gull, black-fronted tern and some shags are often found around fresh water. The wading birds such as oystercatchers, godwits and herons which feed in estuaries or along the shoreline are not regarded as seabirds. No New Zealand duck is primarily marine, with the possible exception of the extinct New Zealand merganser (Mergus australis).
Albatrosses, shearwaters and other petrels
The order Procellariiformes is arguably the most successful seabird order, with about 124 species and living in all the world’s oceans. They range in size from tiny 35-gram storm petrels to huge albatrosses weighing in at 9 kilograms, with a 3-metre wingspan. These birds find all their food at sea, and most species come to land only to breed. Petrels and shearwaters are adept divers – some shearwaters regularly dive to 60 metres. Storm petrels, prions and albatrosses obtain their food close to the water’s surface.
Shags, gannets and their kin
Pelecaniformes is the most variable seabird order. Its members include gannets and shags (cormorants), as well as pelicans, frigate birds and boobies. Species from the latter three groups of seabirds live on New Zealand’s Kermadec Islands and occasionally stray to the mainland coasts.
Three species of New Zealand shag live primarily in freshwater habitats, harbours and estuaries. The other nine species are exclusively marine, with some also making use of estuaries and harbours. All but one of the marine shags are endemic to New Zealand – they live nowhere else. The other four species occur in other countries as well as New Zealand. All but one of the endemic species have restricted ranges, being confined to particular island groups or to limited parts of the New Zealand mainland. Shags pursue their prey under water, using their feet to propel themselves.
Australasian gannets (Morus serrator) breed at 24 sites around the New Zealand coast, the best-known colonies being at Muriwai (near Auckland), Cape Kidnappers in Hawke’s Bay and on Farewell Spit. Gannets sight fish while flying overhead and capture them by plunging into the water.
Terns, gulls and skuas
Most members of the order Charadriiformes are not marine species. Gulls and some terns take a large proportion of their food ashore or from freshwater habitats. Black-billed gulls and black-fronted terns breed by braided rivers and some larger lakes, and many visit the coast in winter. However, some tern species and all skuas are largely marine, and both of these groups have New Zealand representatives. This order also includes the phalaropes that occasionally stray into New Zealand waters, and the auks which live only in the northern hemisphere.
Marine terns such as the white-fronted tern (Sterna striata) feed by dipping – hovering above the water then dropping to catch surface-shoaling fish.
Skuas are well-known predators of eggs, chicks and small birds at seabird colonies, but they do in fact take a large proportion of their food at sea. In the New Zealand region subantarctic skuas (Catharacta antarctica) breed around Stewart, Chatham and the subantarctic islands. Four other species visit New Zealand waters each year, including the small Arctic-breeding skuas which obtain much of their food by harassing terns, inducing them to regurgitate their food.