Greater crested tern

The terns, subfamily Sterninae, are small to medium-sized seabirds closely related to the gulls, skimmers and skuas. They are gull-like in appearance, but typically have a lighter build, long pointed wings (which give them a fast, buoyant flight), a deeply forked tail and short legs. Most species are grey above and white below, and have a black cap that is reduced or flecked with white in the winter. The greater crested tern was originally described as Sterna bergii by German naturalist Martin Lichtenstein in 1823, but was moved to its current genus Thalasseus, after mitochondrial DNA studies confirmed that the three main head patterns shown by terns (no black cap, black cap, black cap with a white forehead) corresponded to distinct clades. The nearest relative of the greater crested tern is the critically endangered species Chinese crested tern (T. bernsteini), with the next closest being a group of three orange-billed species, West African crested tern T. albididorsalis, lesser crested tern (T. bengalensis), and royal tern (T. maximus). Earlier studies of the genus Thalasseus had not included T. bernsteini, as genetic material was not then available, but otherwise had broadly similar results. The generic name of the greater crested tern is derived from Greek Thalassa, "sea", and the species epithet bergii commemorates Karl Heinrich Bergius, a Prussian pharmacist and botanist who collected the first specimens of this tern near Cape Town. The greater crested tern has four or five geographical subspecies (depending on authority), differing mainly in the darkness of the upperparts and the bill dimensions. These are listed below in taxonomic sequence. A similar number of other potential subspecies have been proposed, but are not considered valid. == Description ==

The greater crested tern is a large tern with a long () yellow bill, black legs, and a glossy black crest that is noticeably shaggy at its rear. The breeding adult of the nominate subspecies T. b. bergii is long, with a wing-span; weight . The forehead and the underparts are white, the back and inner wings are dusky-grey. In winter, the upperparts plumage wears to a paler grey, and the crown of the head becomes white, merging at the rear into a peppered black crest and mask. The adults of both sexes are identical in appearance, but juvenile birds are distinctive, with a head pattern like the winter adult, and upperparts strongly patterned in grey, brown, and white; the closed wings appear to have dark bars. After moulting, the young terns resemble the adult, but still have a variegated wing pattern with a dark bar on the inner flight feathers. The northern subspecies T. b. velox and T. b. thalassinus are in breeding plumage from May to September or October, whereas the relevant period for the two southern African subspecies is from December to April. For T. b. cristatus, the moult timing depends on location; birds from Australia and Oceania are in breeding plumage from September to about April, but those in Thailand, China and Sulawesi have this appearance from February to June or July. The royal tern is similar in size to this species, but has a heavier build, broader wings, a paler back and a blunter, more orange bill. The greater crested often associates with the lesser crested tern, but is 25% larger than the latter, with a proportionately longer bill, longer and heavier head, and bulkier body. Lesser crested tern has an orange-tinted bill, and in immature plumage it is much less variegated than greater crested. The greater crested tern is highly vocal, especially at its breeding grounds. The territorial advertising call is a loud, raucous, crow-like '. Other calls include a ' given at the nest by anxious or excited birds, and a hard '''' in flight. == Distribution and habitat ==

s, note size difference The greater crested tern occurs in tropical and warm temperate coastal parts of the Old World from South Africa around the Indian Ocean to the Pacific and Australia. The subspecies T. b. bergii (including the doubtfully distinct T. b. enigma) breeds in southern Africa from Namibia to Tanzania, and possibly on islands around Madagascar. There is then a break in the breeding distribution of the species until T. b. velox is met in Somalia, the Red Sea, and the Persian Gulf, and another discontinuity further east in southern India. In the Pacific Ocean, there are colonies of T. b. cristatus on numerous Pacific islands, including Kiribati, Fiji, Tonga, the Society Islands and the Tuamotus, as well as more widely on islets off the coasts of China, Taiwan, the Philippines, Indonesia, New Guinea, and Australia south to Tasmania. The nests are located on low‑lying sandy, rocky, or coral islands, sometimes amongst stunted shrubs, often without any shelter at all. New Zealand, North Korea, Jordan, In India, the greater crested tern is protected in the PM Sayeed Marine Birds Conservation Reserve. == Behaviour ==

Breeding The greater crested tern breeds in colonies, often in association with other seabirds. It is monogamous and the pair bond is maintained through the year and sometimes in consecutive breeding seasons. The colony size is related to the abundance of pelagic fish prey, This tern does not show site fidelity, frequently changing its nest site from year to year, , Western Australia|thumb|left A male greater crested tern establishes a small area of the colony in preparation for nesting, and initially pecks at any other tern entering his territory. If the intruder is another male, it retaliates in kind, and is normally vigorously repelled by the incumbent. A female entering the nest area reacts passively to the male's aggression, enabling him to recognise her sex and initiate pair formation by display, including head raising and bowing; this behaviour is frequently repeated during nesting to reinforce the bond between the pair. Terns also use fish as part of the courtship ritual. One bird flies around the colony with a fish in its beak, calling loudly; its partner may also fly, but the pair eventually settle and the gift is exchanged. The nest is a shallow scrape in the sand on open, flat or occasionally sloping ground. It is often unlined, but sometimes includes stones or cuttlefish bones. One, sometimes two, eggs are laid and incubated by both parents for 25 to 30 days prior to hatching. Egg laying is synchronised within a breeding colony and more tightly so within sub-colonies. Parents do not recognize their own eggs or newly hatched chicks, but are able to distinguish their chicks by the time they are two days-old, shortly before they begin to wander from the nest. The precocial chicks, which are very pale with black speckling, are brooded and fed by both parents, but may gather in crèches when older. The young terns fledge after 38 to 40 days, but remain dependent on the parents after leaving the colony until they are about four months old. , Philippines In South Africa, this species has adapted to breeding on the roofs of building, sometimes with Hartlaub's gull, which also shares the more typical nesting sites of the nominate subspecies. In 2000, 7.5% of the population of this subspecies bred on roofs. Artificial islands in salt pans and sewage works have also recently been colonised by this adaptable seabird. Smaller subcolonies with a relatively larger numbers of nests located on the perimeter are subject to more predation. Feeding Fish are the main food of the greater crested tern, found to make up nearly 90% of all prey items with the remainder including cephalopods, crustaceans and insects. The greater crested tern feeds mostly at sea by plunge diving to a depth of up to , or by dipping from the surface, and food is usually swallowed in mid-air. Birds may forage up to from land in the breeding season. Prey size ranges from in length and up to in weight. Shoaling pelagic fish such as anchovy and sardine are typical prey, Terns have red oil droplets in the cone cells of the retinas of their eyes. This improves contrast and sharpens distance vision, especially in hazy conditions. Birds that have to see through an air/water interface, such as terns and gulls, have more strongly coloured carotenoid pigments in the cone oil drops than other avian species. The improved eyesight helps terns to locate shoals of fish, although it is uncertain whether they are sighting the phytoplankton on which the fish feed, or observing other terns diving for food. Tern's eyes are not particularly ultraviolet sensitive, an adaptation more suited to terrestrial feeders like the gulls. == Status ==

The greater crested tern has a widespread distribution range, estimated at 1–10 million square kilometres (0.4–3.8 million square miles). The population has not been quantified, but it is not believed to approach the thresholds for either the size criterion (fewer than 10,000 mature individuals) or the population decline criterion (declining more than 30% in ten years or three generations) of the IUCN Red List. For these reasons, the species is evaluated as being of least concern at the global level. Parties to the Agreement are required to engage in a wide range of conservation strategies described in a detailed action plan. The plan is intended to address key issues such as species and habitat conservation, management of human activities, research, education, and implementation. == References ==