Red-billed gulls
The red-billed gull or tarāpunga (Larus novaehollandiae) has a white body with grey wings tipped with black then white. Their bills and legs are red when they are adult. They are 37 centimetres long – considerably smaller than black-backed gulls. Males weigh 300 grams and females about 260 grams.
There are several subspecies, of which Larus novaehollandiae scopulinus occurs only in New Zealand. Red-billed gulls are known as silver gulls in Australia and New Caledonia.
Gulls of Mokoia
In 1823 the Ngāpuhi tribe attacked the Te Arawa people living on Mokoia Island, in Lake Rotorua. Te Arawa only knew of the imminent attack when a flock of red- and black-billed gulls flew up, screaming a warning. The gulls circled overhead while Te Arawa were defeated. Following the battle, the Te Arawa priests honoured the birds by declaring them tapu (sacred).
The gulls are mainly coastal and offshore island birds, with the exception of a few at Lake Rotorua in the North Island. Here they nest alongside black-billed gulls, and occasionally interbreed.
The largest colonies were at Kaikōura Peninsula and on outlying islands, but numbers have declined catastrophically because of a diminishing food supply. In 2014 Otago populations of the gulls were increasing.
Since the 1960s studies of red-billed gulls on Kaikōura Peninsula have documented their habits and social organisation.
The gulls breed from October to December, laying one to three brownish eggs with purple blotches. Both sexes incubate the eggs, and the young fledge at 37 days. Once the breeding season is over, the colony disperses, and Kaikōura gulls have been seen as far afield as Auckland and Invercargill. Female gulls live around nine years and males about six, but some live up to 25 years.
Black-billed gulls
Of New Zealand’s three species of gull, the black-billed gull (Larus bulleri) is the only endemic gull. Similar in size to red-billed gulls, their bills are black, and they are longer and finer in shape, with reddish-black legs and paler wings.
Black-billed gulls eat small fish, whitebait and flatfish, and take earthworms and grass grubs from pastureland. They also feed on the wing, taking cicadas, moths and aquatic insects. In winter they fly to estuaries and harbours to eat marine invertebrates and shellfish, or to parks for worms and human handouts.
Black-billed gulls mainly breed inland, beside South Island rivers. In the late 20th century their breeding habitat became more varied, and some were breeding at Kaipara and Manukau harbours. They nest in colonies, and make nest mounds of dry grass and twigs on shingle. They lay one to four pale green-grey eggs from September to December, and parents share incubation.
Chicks are left alone within a day of hatching, and parents return to feed them by regurgitating food onto the ground. Fledglings leave the nest when they are 26 days old. The birds live about 18 years.
Despite extending their range, black-billed gulls are declining in number. Populations have dropped along Canterbury and North Otago rivers, where hydroelectric schemes and irrigation have affected all bird species which nest there. Even in Southland, where 78% of the gulls nest, populations are falling. In 1974 there were 84,900 gulls breeding along the Oreti River; in 1997 there were 15,308. A New Zealand-wide count in 1996 found 48,000 nests.