Black-backed gull and red-billed gull

The two species of gulls most commonly seen on and around our harbour and beach areas are the Southern black-backed gull (Karoro) and the red-billed gull (Tarāpunga). Nationally, both species are abundant with the red-bills occurring in much greater numbers locally.

Red-billed gull (left), black-backed gull (right). Photo credit Corinne Bowie.

In recent years the red-billed gull has suffered declines at their main breeding colonies. These losses are thought to be associated with the reduced availability of their main food krill and introduced predators. The red-billed gull can be confused with its black-billed cousin, especially the juvenile forms of these sub-species. The black-billed variety is far less common, found more often inland, and has a much finer and longer bill than the red-bill.

Red-billed gulls, also known as silver gulls, inhabit mainly coastal areas throughout New Zealand and are seldom seen inland. A small breeding population is present at Sulphur Point Lake Rotorua but the main breeding populations are in large dense colonies on the eastern coasts of both mainland islands. These are commonly sited at river mouths, rocky cliff faces and sandy or stoney beaches. They also breed singly or in small numbers on a number of off-shore islands with their nests hidden amongst low coastal vegetation.

They are monogamous with one banded pair known to have remained together for 17 years. Nests are well constructed from twigs, grasses and feathers, normally 2 eggs are laid which hatch after about 23 days incubation. Both parents share the incubation, feeding and caring duties. Chicks begin to fly at about 35 days and are fed by their parents for another 30 days.

Red-billed gulls. Photo credit Corinne Bowie.

During the breeding season most feeding is undertaken on inshore waters targeting the surface swarming small shrimp-like crustacean known as krill. They are opportunistic feeders however and will take alternative foods such as kelp flies, small fish, earthworms, rubbish tip garbage, offal from fishing boats etc. Some have even been observed predating the eggs and young chicks of the endangered NZ dotterel and shore plover.

The black-backed gull is about 3 times the size of the red-billed gull. With the mature adults displaying their distinctive black and white plumage, yellow legs, and yellow bill tipped with red, they are easily distinguished from the red-billed gulls.

Juvenile black-backed gull. Photo credit Corinne Bowie.

The speckled brown plumage of the juvenile birds does confuse some people, mistakenly identifying them as southern or south polar skuas. They live in a wide variety of habitats, ranging from coastal areas and plains to high country farms and subalpine zones. The only habitat they don’t frequent is forested areas.

The species is also common in parts of other countries in the southern hemisphere including southern Australia, southern Africa, southern America, most subantarctic islands and the Antarctic Peninsular. They are one of the most abundant and familiar large birds in New Zealand.

Black-backed gull. Photo credit Corinne Bowie

The New Zealand population has benefited significantly from human colonisation. The resultant land development, growth of agricultural and fishing industries, open landfill and other waste disposal methods, have all contributed to providing additional new food sources for these voracious birds. They have increased in number to the extent they are causing serious harm and are considered a pest.

At airports where grassed runway berms and outfields provide suitable breeding, feeding and roosting habitat they pose a serious bird-strike threat. On farmland some develop a habit of attacking cast sheep and newborn lambs. They do not have any legal protection under the Wildlife Act and are subjected to various control measures when required.

Black-backed gulls breed in colonies or scattered pairs in a variety of habitats ranging from river beds, rocky outcrops, open pastures, grassed airfields, alpine tarns and wetlands, coastal dunes and beaches. They are even known to roost and nest on the roofs of city buildings. They are both predators and scavengers and will consume almost any organic waste they find and any living or dead animal matter small enough to manage.

Some have developed a learned behaviour of opening bivalve shellfish by way of flying high above concrete and tar-sealed surfaces or rocks and repeatedly dropping them from a great height until they are broken open sufficiently for them to access the succulent contents.