The greater crested tern[3] (Thalasseus bergii), also called crested tern or swift 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 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 the royal and lesser crested terns, but can be distinguished by its size and bill colour.
The greater crested tern has 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, family Sternidae, 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.[4]
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,[6] after mitochondrialDNA 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.[7]
The greater crested tern's closest relatives within its genus appear to be the lesser crested tern (T.bengalensis), and the royal tern (T. maximus).[7] The DNA study did not include the critically endangeredChinese crested tern (T. bernsteini) but, as that bird was formerly considered to be conspecific with the greater crested tern as a synonym of the subspecies T. b. cristatus, it is presumably also very closely related.[8]
The greater crested tern has four or five geographical races (depending on authority), differing mainly in the colour of the upperparts and bill. These are listed below in taxonomic sequence. A similar number of other potential subspecies have been proposed, but are not considered valid.[10]
The greater crested tern is a large tern with a long (5.4–6.5cm or 2.1–2.6in) yellow bill, black legs, and a glossy black crest that is noticeably shaggy at its rear. The breeding adult of the nominate subspeciesT. b. bergii is 46–49cm (18–19.5in) long, with a 125–130cm (49–51in) wing-span; this subspecies weighs 325–397g (11.4–14.0oz).[13] 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.[16]
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.[13]
The northern subspecies T. b. velox and T. b. thalassina are in breeding plumage from May to September or October, whereas the relevant period for the two southern African races is from December to April. For T. b. cristata, 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.[13]
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.[16] Lesser crested tern has an orange-tinted bill, and in immature plumage it is much less variegated than greater crested.[12]
The greater crested tern is highly vocal, especially at its breeding grounds. The territorial advertising call is a loud, raucous, crow-likekerrak. Other calls include a korrkorrkorr given at the nest by anxious or excited birds, and a hard wep wep in flight.[16]
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 and T. b. enigma breed in Southern Africa from Namibia to Tanzania, and possibly on islands around Madagascar. There is then a break in the breeding distribution of this species until Somalia and the Red Sea, and another discontinuity further east in southern India.[10]
The nests are located on low‑lying sandy, rocky, or coral islands, sometimes amongst stunted shrubs, often without any shelter at all.[16] When not breeding, the greater crested tern will roost or rest on open shores, less often on boats, pilings, harbour buildings and raised salt mounds in lagoons. It is rarely seen on tidal creeks or inland waters.[14]
All populations of greater crested tern disperse after breeding. When Southern African birds leave colonies in Namibia and Western Cape Province, most adults move east to the Indian Ocean coastline of South Africa. Many young birds also travel east, sometimes more than 2,000km (1,200mi), but others move northwards along the western coast. T. b. thalassina winters on the east African coast north to Kenya and Somalia and may move as far south as Durban. Populations of T. b. velox breeding from the Persian Gulf eastwards appear to be sedentary or dispersive rather than truly migratory, but those breeding in the Red Sea winter south along the east African coast to Kenya.[14]T. b. cristata mostly stays within 400km (250mi) of its colonies, but some birds wander up to around 1,000km (620mi).[19] This species has occurred as a vagrant to Hawaii,[20]New Zealand,[21][22]North Korea,[21]Jordan,[21] and Israel.[16]
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.[24] The colony size is related to the abundance of pelagic fish prey,[14] and the largest documented colony, with 13,000 to 15,000 pairs, is in the Gulf of Carpentaria in northern Australia, a region which also supports major colonies of other seabirds. Since nesting in this area follows the summer monsoonal flooding, it is presumably a response to fish stocks rising, probably due to river run-off providing extra nutrient to the Gulf.[25] This tern does not show site fidelity, frequently changing its nest site from year to year,[16] sometimes by more than 200km (120mi).[19]
Greater crested tern chick with parents on Penguin Island, Western Australia
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.[26]
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 30days prior to hatching.[14] The eggs are cream with blackish streaks.[27] Egg laying is synchronised within a breeding colony[28] and more tightly so within sub-colonies.[29] 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.[30] 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 40days, but remain dependent on the parents after leaving the colony until they are about four months old.[14]
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 race. In 2000, 7.5%of the population of this subspecies bred on roofs.[31] Artificial islands in salt pans and sewage works have also recently been colonised by this adaptable seabird.[10]
Adult terns have few predators, but in Namibia immature birds are often robbed of their food by kelp gulls, and that species, along with Hartlaub's gull, silver gull and sacred ibis, has been observed feeding on eggs or nestlings, especially when colonies are disturbed.[10][32] Smaller subcolonies with a relatively larger numbers of nests located on the perimeter are subject to more predation.[29] In Australia, predation by cats and dogs, and occasional deaths by shooting or collisions with cars, wires or light-towers have been documented.[9]
Nominate subspecies roosting with Sandwich terns in South Africa
Commercial fisheries can have both positive and negative effects on the greater crested tern. Juvenile survival rates are improved where trawler discards provide extra food, and huge population increases in the southeastern Gulf of Carpentaria are thought to have been due to the development of a large prawn trawl fishery.[33] Conversely, purse-seine fishing reduces the available food supply, and sizeable fluctuations in the numbers of great crested terns breeding in the Western Cape of South Africa are significantly related to changes in the abundance of pelagic fish, which are intensively exploited by purse-seine fishing.[32] Terns may be killed or injured by collisions with trawl warps, trapped in trawls or discarded gear, or hooked by longline fishing, but, unlike albatrosses and petrels, there is little evidence that overall numbers are significantly affected.[14]
An unusual incident was the incapacitation of 103 terns off Robben Island, South Africa by marine foam, generated by a combination of wave action, kelpmucilage and phytoplankton. After treatment, 90%of the birds were fit to be released.[34]
Feeding
Greater crested tern with prey caught via a surface dipGreater crested tern with a fish in its beak
The greater crested tern feeds mostly at sea by plunge diving to a depth of up to 1m (3.3ft), or by dipping from the surface, and food is usually swallowed in mid-air. Birds may forage up to 10km (6.2mi) from land in the breeding season. Prey size ranges from 7–138mm (0.28–5.43in) in length and up to 30g (1.1oz) in weight. Shoaling pelagic fish such as anchovy and sardine are typical prey,[14] but bottom-living species are taken as discards from commercial fishing. This tern actively follows trawlers, including at night, and during the fishing season trawl discards can constitute 70% of its diet.[33] Prawn fishing is particularly productive in providing extra food, since prawns usually represent only 10–20% of the catch, the remaining being bycatch, mainly fish such as cardinalfish and gobies.[33]
A study of an area of the Great Barrier Reef where the number of breeding great crested terns has grown ten-fold, probably due to extra food from trawl by-catch, suggested that lesser crested and sooty terns have moved away and now breed on a part of the reef where fishing is banned. It is possible that the large increase in the number of greater crested terns may have affected other species through competition for food and nesting sites.[35]
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.[36] Birds that have to see through an air/water interface, such as terns and gulls, have more strongly coloured carotenoidpigments in the cone oil drops than other avian species.[37] 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.[38] Tern's eyes are not particularly ultraviolet sensitive, an adaptation more suited to terrestrial feeders like the gulls.[39]
Status
The greater crested tern has a widespread distribution range, estimated at 1–10million square kilometres (0.4–3.8million 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.[1] However, there are concerns for populations in some areas such as the Gulf of Thailand where the species no longer breeds, and in Indonesia where egg harvesting has caused declines.[10]
All subspecies except T. b. cristata are covered under the Agreement on the Conservation of African-Eurasian Migratory Waterbirds (AEWA).[40] 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.[41]
Related Research Articles
Gulls, or colloquially seagulls, are seabirds of the family Laridae in the suborder Lari. They are most closely related to the terns and skimmers and distantly related to auks, and even more distantly to waders. Until the 21st century, most gulls were placed in the genus Larus, but that arrangement is now considered polyphyletic, leading to the resurrection of several genera. An older name for gulls is mews, which is cognate with German Möwe, Danish måge, Swedish mås, Dutch meeuw, Norwegian måke/måse, and French mouette, and can still be found in certain regional dialects.
Terns are seabirds in the family Laridae that have a worldwide distribution and are normally found near the sea, rivers, or wetlands. Terns are treated as a subgroup of the family Laridae which includes gulls and skimmers and consists of eleven genera. They are slender, lightly built birds with long, forked tails, narrow wings, long bills, and relatively short legs. Most species are pale grey above and white below, with a contrasting black cap to the head, but the marsh terns, the Inca tern, and some noddies have dark plumage for at least part of the year. The sexes are identical in appearance, but young birds are readily distinguishable from adults. Terns have a non-breeding plumage, which usually involves a white forehead and much-reduced black cap.
The Arctic tern is a tern in the family Laridae. This bird has a circumpolar breeding distribution covering the Arctic and sub-Arctic regions of Europe, Asia, and North America. The species is strongly migratory, seeing two summers each year as it migrates along a convoluted route from its northern breeding grounds to the Antarctic coast for the southern summer and back again about six months later. Recent studies have shown average annual round-trip lengths of about 70,900 km (44,100 mi) for birds nesting in Iceland and Greenland and about 48,700 km (30,300 mi) for birds nesting in the Netherlands. These are by far the longest migrations known in the animal kingdom. The Arctic tern nests once every one to three years.
The common tern is a seabird in the family Laridae. This bird has a circumpolar distribution, its four subspecies breeding in temperate and subarctic regions of Europe, Asia and North America. It is strongly migratory, wintering in coastal tropical and subtropical regions. Breeding adults have light grey upperparts, white to very light grey underparts, a black cap, orange-red legs, and a narrow pointed bill. Depending on the subspecies, the bill may be mostly red with a black tip or all black. There are several similar species, including the partly sympatric Arctic tern, which can be separated on plumage details, leg and bill colour, or vocalisations.
The little tern is a seabird of the family Laridae. It was first described by the German naturalist Peter Simon Pallas in 1764 and given the binomial name Sterna albifrons. It was moved to the genus Sternula when the genus Sterna was restricted to the larger typical terns. The genus name Sternula is a diminutive of Sterna, 'tern', while the specific name albifrons is from Latin albus, 'white', and frons, 'forehead'.
The Caspian tern is a species of tern, with a subcosmopolitan but scattered distribution. Despite its extensive range, it is monotypic of its genus, and has no accepted subspecies. The genus name is from Ancient Greek hudros, "water", and Latin progne, "swallow". The specific caspia is from Latin and, like the English name, refers to the Caspian Sea.
The Sandwich tern is a tern in the family Laridae. It is very closely related to the lesser crested tern, Chinese crested tern, Cabot's tern, and elegant tern and has been known to interbreed with both elegant and lesser crested. It breeds in the Palearctic from Europe to the Caspian Sea and winters in South Africa, India, and Sri Lanka.
The roseate tern is a species of tern in the family Laridae. The genus name Sterna is derived from Old English "stearn", "tern", and the specific dougallii refers to Scottish physician and collector Dr Peter McDougall (1777–1814). "Roseate" refers to the bird's pink breast in breeding plumage.
The gull-billed tern, formerly Sterna nilotica, is a tern in the family Laridae. It is widely distributed and breeds in scattered localities in Europe, Asia, northwest Africa, and the Americas. The Australian gull-billed tern was previously considered a subspecies.
The lesser crested tern is a tern in the family Laridae.
The royal tern is a tern in the family Laridae. The species is endemic to the Americas, though vagrants have been identified in Europe.
Forster's tern is a tern in the family Laridae. The genus name Sterna is derived from Old English "stearn", "tern", and forsteri commemorates the naturalist Johann Reinhold Forster.
The Aleutian tern is a migratory bird living in the subarctic region of the globe most of the year. It is frequently associated with the Arctic tern, which it closely resembles. While both species have a black cap, the Aleutian tern may be distinguished by its white forehead. During breeding season, the Arctic terns have bright red bills, feet, and legs while those of the Aleutian terns are black.
The Chinese crested tern is a tern in the family Laridae. It is the county bird of Lienchiang County, Fuchien.
The black noddy, also known as white-capped noddy, is a species of tern in the family Laridae. It is a medium-sized seabird with black plumage and a white cap that closely resembles the lesser noddy with which it was at one time considered conspecific. The black noddy has slightly darker plumage and dark rather than pale lores.
The Antarctic tern is a seabird in the family Laridae. It ranges throughout the southern oceans and is found on small islands around Antarctica as well as on the shores of the mainland. Its diet consists primarily of small fish and crustaceans. It is very similar in appearance to the closely related Arctic tern, but it is stockier, and it is in its breeding plumage in the southern summer, when the Arctic tern has shed old feathers to get its non-breeding plumage. The Antarctic tern does not migrate like the Arctic tern does, but it can still be found on a very large range. This tern species is actually more closely related to the South American tern.
Cabot's tern is a species of bird in subfamily Sterninae of the family Laridae, the gulls, terns, and skimmers. It is found in the eastern U.S. and Middle America, the Caribbean, Trinidad and Tobago, and in every mainland South American country except Bolivia and Paraguay.
The West African crested tern is a bird species in the family Laridae. Until 2020 it was considered a subspecies of the New World royal tern, Thalasseus maximus.
↑ Adapted from Bridge et al. (2005). This study did not include the Chinese crested tern
↑ This genus had originally been created by Heinrich Boie in 1822, but had been abandoned until the Bridge (2005) study confirmed the need for a separate genus for the crested terns.
↑ Skerrett, Adrian; Bullock, I.; Disley, T. (2001). Birds of Seychelles. London: Christopher Helm. p.230. ISBN0-7136-3973-3.
↑ Pratt, H. Douglas; Bruner, P.; Berrett, D. (1987). The Birds of Hawaii and the Tropical Pacific. Princeton: Princeton University Press. p.178. ISBN0-691-08402-5.
1 2 Carrick, R.; Wheeler, W. R.; Murray, M. D. (1957). "Seasonal dispersal and mortality in the Silver Gull, Larus novae-hollandiae Stephens, and Crested tern, Sterna bergii Lichstein, in Australia". Wildlife Research. 2 (2): 116–144. doi:10.1071/CWR9570116.
↑ Walker, T. A. (1992). "A record Crested Tern Sterna bergii colony and concentrated breeding by seabirds in the Gulf of Carpentaria". Emu. 92 (3): 152–156. Bibcode:1992EmuAO..92..152W. doi:10.1071/MU9920152.
↑ Fisher, James; Lockley, R. M. (1989). Sea‑Birds (Collins New Naturalist series). London: Bloomsbury Books. pp.155–156. ISBN1-870630-88-2.
↑ "Crested Tern". Fact Sheets. Australian Museum. Archived from the original on 13 November 2008. Retrieved 5 July 2008.
↑ Dunlop, J. N. (1987). "Social-behavior and colony formation in a population of crested terns, Sterna bergii, in southwestern Australia". Australian Wildlife Research. 14 (4): 529–540. doi:10.1071/WR9870529.
↑ Davies, S. J. J. F.; Carrick, R (1962). "On the ability of crested terns, Sterna bergii, to recognize their own chicks". Australian Journal of Zoology. 10 (2): 171–177. doi:10.1071/ZO9620171.
↑ Parsons, Nola J.; Tjørve, Kathy M. C.; Underhill, Les G.; Strauss, Venessa (April 2006). "The rehabilitation of Swift Terns Sterna bergii incapacitated by marine foam on Robben Island, South Africa". Ostrich: Journal of African Ornithology. 77 (1–2): 95–98. Bibcode:2006Ostri..77...95P. doi:10.2989/00306520609485514. S2CID86179144.
↑ Blaber, S. J. M.; Milton, D. A.; Farmer, M. J.; Smith, G. C. (1998). "Seabird breeding populations on the far northern Great Barrier Reef, Australia: trends and influences". Emu. 98 (1): 44–57. Bibcode:1998EmuAO..98...44B. doi:10.1071/MU98006.
↑ Sinclair, Sandra (1985). How Animals See: Other Visions of Our World. Beckenham, Kent: Croom Helm. pp.93–95. ISBN0-7099-3336-3.
↑ Varela, F. J.; Palacios, A. G.; Goldsmith T. M. (1993) "Vision, Brain, and Behavior in Birds" in Harris, Philip; Bischof, Hans-Joachim Vision, Brain, and Behavior in Birds: a comparative review Cambridge, Massachusetts: MIT Press 77–94 ISBN0-262-24036-X
↑ Lythgoe, J. N. (1979). The Ecology of Vision. Oxford: Clarendon Press. pp.180–183. ISBN0-19-854529-0.
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