The greater crested tern (Thalasseus bergii), also called crested tern, swift tern, or great crested tern, is a tern in the family Laridae that nests in dense colonies on coastlines and islands in the tropical and subtropical Old World. Its four or five subspecies breed in the area from South Africa around the Indian Ocean to the central Pacific and Australia, all populations dispersing widely from the breeding range after nesting. This large tern is closely related to other species in its genus, but can be distinguished by its darker, battleship-grey upperparts, white forehead even in full breeding plumage, and greenish-tinged yellow (not orange or black) bill colour.
The greater crested tern has dark grey upperparts, white underparts, a yellow bill, and a shaggy black crest that recedes in winter. Its young have a distinctive appearance, with strongly patterned grey, brown and white plumage, and rely on their parents for food for several months after they have fledged. Like all members of the genus Thalasseus, the greater crested tern feeds by plunge diving for fish, usually in marine environments; the male offers fish to the female as part of the courtship ritual.
This is an adaptable species that has learned to follow fishing boats for jettisoned bycatch, and to use unusual nest sites such as the roofs of buildings and artificial islands in salt pans and sewage works. Its eggs and young are taken by gulls and ibises, and human activities such as fishing, shooting and egg harvesting have caused local population declines. There are no global conservation concerns for this bird, which has a stable total population of more than 500,000 individuals.
Taxonomy
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.
!width=15% | Image
!width=25% | Breeding range
!width=25% | Distinctive features
!width=25% | Population estimates
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|T. b. bergii
|20,000 individuals (inc 6,336 breeding pairs in South Africa and up to 1,682 pairs in Namibia)<br />(Clancey, 1979)
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|Zambezi delta, Mozambique, south to Durban, South Africa
|Palest subspecies
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Description
thumb|T. b. cristatus immature, French Polynesia
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 .
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
thumb|T. b. cristatus displaying
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,
Greater crested tern chick with parents on [[Penguin Island (Western Australia)|Penguin Island, 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.
thumb|Breeding plumage in [[New South Wales]]
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.
thumb|A nesting colony in [[Tubbataha Reef, Philippines]]
thumb|Egg, Collection [[Museum Wiesbaden]]
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
thumb|Greater crested tern with prey caught via a surface dip
thumb|Greater crested tern with a fish in its beak
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
Bibliography
- Cooper, John (2006) "Potential impacts of marine fisheries on migratory waterbirds of the Afrotropical Region: a study in progress" (PDF extract) in Boere, C.A.; Galbraith G.C.; Stroud D.A. (eds) Waterbirds around the world. Edinburgh: The Stationery Office,
External links
- (Greater crested tern = ) Swift tern - Species text in The Atlas of Southern African Birds