Adult red-billed gull on nest (New Zealand)
The red-billed gull (Chroicocephalus scopulinus; in Māori tarapunga or akiaki), once also known as the mackerel gull, is a native of New Zealand, being found throughout NZ coastal areas and on outlying islands. Except for a colony at Lake Rotorua, the red-billed gull rarely is found inland. They are commonly seen in coastal towns, scavenging on human refuse and offal from fish and meat processing facilities.
The red-billed gull is a fairly small gull with an all-red bill, a white iris with red eye ring, and red legs and feet. The scarlet colour is duller outside the breeding season. The mantle, back and wing coverts are pale grey. The main flight feathers (wingtips) are black with white tips. The rest of the body is white. Sexes are virtually similar, but males are slightly larger with a longer and stouter bill. Juvenile gulls have a dark brown bill with only hints of red, making them difficult to distinguish from the closely related black-billed gull. The legs and iris are also brown and there are brown patches on the grey mantle and the primaries are brownish in colour rather than black. Adult plumage is attained in the second year; birds of this age class can be recognised by the brownish-black tip to the bill, and the primary feathers have a brownish tinge instead of black in older individuals.
The red-billed gull is the smallest gull commonly seen in New Zealand. Until recently it was regarded as a subspecies of the silver gull found in Australia, and the two species are very similar in appearance. However recent research suggests that they are not particularly closely related. Its vernacular name is sometimes also used for the dolphin gull, a somewhat similar-looking but unrelated species. As is the case with many gulls, the red-billed gull has traditionally been placed in the genus Larus. At Lake Rotorua in a mixed colony of red-billed gulls and black-billed gulls interbreeding has been recorded. The hybrids have been fertile and have bred with red-billed gulls.
POPULATION AND THREATS
The red-billed gull is an abundant species that has recently suffered huge declines at its three main breeding colonies (Three Kings Islands, Mokohinau Islands and Kaikoura Peninsula). At Kaikoura the decline began in 1994, and between 1983 and 2005 the population declined by 51%. In contrast, with mammalian predator control at the Otago Peninsula, the population has seen a 6-10% annual increase in the 20 years since 1992, leading to a three- or four-fold increase in the local population.
A major threat to breeding birds is predation from introduced predators such as cats, ferrets, rats and stoats. Climate-induced fluctuation in the availability of krill, the principal food of the birds during the breeding season, has a major impact on breeding success.
On mainland New Zealand, red-billed gulls breed in large, dense colonies, mainly restricted to the eastern coasts of the North and South Islands on stacks, cliffs, river mouths and sandy and rocky shores. On the outlying islands their nests are scattered or solitary concealed and located singly or in small groups (probably to help avoid skua predation). The bird tends to nest at the same locality from one season to the next, and offspring mostly return to their natal colony to breed. The birds form pair bonds which endure across seasons, but there is a certain amount of extra-pair copulation. Divorce is highest in young birds and in pairs breeding together for the first time, especially if they were unsuccessful the previous season.
An important part of the preparation for mating is courtship feeding of the female by the male. Courtship feeding increases in frequency 16 days prior to the laying of the first egg. If the female is being adequately fed by the male, the female is able to remain in the nest territory for much of the day. Like most species of gulls and terns, the red-billed gull has a sex ratio that is biased in favour of females. As a result, some pairings are female-female pairs. These often produce fertile eggs because at least one of the females allows herself to either be force-copulated with by a male paired with another female, or solicits a copulation.
Red-billed gulls have an extremely long egg-laying period that can extend from mid September to January. The sexes share approximately equally in nest-building, incubation and provisioning the chicks. Nests are well formed and may be constructed of grass, seaweed, leaves and twigs. Generally the clutch size is two eggs, but one and three egg clutches sometimes occur. Supernormal clutches up to five eggs can occur in female-female associations. Eggs are ovoid, mainly brownish or greenish-grey with light or dark brown spots or blotches. Incubation lasts 21-25 days and chicks begin to fly at 35 days and continue to be fed for another c. 30 days.
The main food at the largest colonies is krill (small shrimp-like planktonic crustacean and their larvae). During the breeding season adult gulls can sustain themselves on alternative foods such as small (shell) fish, earthworms, lizards, berries, garbage and insects (e.g. kelp flies), but they are dependent upon an abundant and regular supply of the surface-swarming krill for successful breeding. Outside of the breeding season the diet is highly variable. Some still feed at sea, others feed on small invertebrates along the shore and others will feed from human sources such as handouts in towns or cities, offal being discarded from fishing boats and garbage at rubbish dumps.
Source: Wikipedia, nzbirdsonline.org.nz