Greater crested tern
Greater crested tern | |
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Breeding plumage T. b. cristata displaying | |
Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Chordata |
Class: | Aves |
Order: | Charadriiformes |
Family: | Laridae |
Genus: | Thalasseus |
Species: | T. bergii
|
Binomial name | |
Thalasseus bergii (
Lichtenstein, MHC , 1823) | |
Approximate breeding range Wintering range | |
Synonyms[2] | |
Sterna bergii Lichtenstein, 1823 |
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
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Relationships in the genus Thalasseus[5] |
The greater crested tern was originally described as Sterna bergii by German naturalist
The greater crested tern's closest relatives within its genus appear to be the
The generic name of the greater crested tern is derived from Greek Thalassa, "sea", and the species epithet bergii commemorates Carl Heinrich Bergius, a Prussian pharmacist and botanist who collected the first specimens of this tern near Cape Town.[9]
The greater crested tern has about five geographical races, 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]
Subspecies[11] | Breeding range | Distinctive features | Population estimates |
---|---|---|---|
T. b. bergii[12] (Lichtenstein, 1823) |
Coasts of South Africa and Namibia | Dark grey above, slightly larger than thalassina, least white on head[13] | 20,000 individuals (inc 6,336 breeding pairs in South Africa and up to 1,682 pairs in Namibia)[14] |
T. b. enigma[12][15] (Clancey, 1979) |
Zambezi delta, Mozambique, south to Durban, South Africa | Palest subspecies[10] | 8,000–10,000 individuals in Madagascar and Mozambique[14] |
T. b. cristata[13] (Stephens, 1826) |
Eastern Indian Ocean, Australia and western Pacific Ocean | Like bergii, with tail, rump and back concolorous. paler in Australia | 500,000+ individuals in Australia[10] |
T. b. thalassina[13] (Stresemann, 1914) |
Western Indian Ocean | Small and pale, larger and less pale in south of range | 2,550–4,500 individuals in Eastern Africa and Seychelles[14] |
T. b. velox[13] (Cretzschmar, 1827) |
Red Sea, Persian Gulf, northern Indian Ocean | Largest, heaviest, darkest and longest-billed subspecies | 33,000 in Middle East (inc 4,000 pairs Oman and 3,500 pairs on islands off Saudi Arabia)[10] |
Description
The greater crested tern is a large tern with a long (5.4–6.5 cm or 2.1–2.6 in) 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 46–49 cm (18–19.5 in) long, with a 125–130 cm (49–51 in) wing-span; this subspecies weighs 325–397 g (11.4–14.0 oz).[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
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-like kerrak. Other calls include a korrkorrkorr given at the nest by anxious or excited birds, and a hard wep wep in flight.[16]
Distribution and habitat
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 greater crested tern breeds on many islands in the Indian Ocean including
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
In India, the Greater crested tern is protected in the PM Sayeed Marine Birds Conservation Reserve.[23]
Behaviour
Breeding
The greater crested tern breeds in
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
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
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, kelp mucilage and phytoplankton. After treatment, 90% of the birds were fit to be released.[34]
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
The greater crested tern feeds mostly at sea by plunge diving to a depth of up to 1 m (3.3 ft), or by dipping from the surface, and food is usually swallowed in mid-air. Birds may forage up to 10 km (6.2 mi) from land in the breeding season. Prey size ranges from 7–138 mm (0.28–5.43 in) in length and up to 30 g (1.1 oz) in weight. Shoaling
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 carotenoid pigments 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 1–10 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.[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 (
References
- ^ . Retrieved 19 November 2021.
- ^ "Greater crested tern". Avibase.
- ^ Gill, F; Donsker, D (eds.). "IOC World Bird Names (v 2.11)". International Ornithologists' Union. Archived from the original on 2013-12-05. Retrieved 28 February 2012
- ^ Snow & Perrin (1998) 764
- ^ 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.
- ^ PMID 15804415.
- ^ Mees, G. F. (1975). "Identiteit en status van Sterna bernsteini Schlegel". Ardea (in Dutch). 63: 78–86.
- ^ a b Higgins & Davies (1996) 605–609
- ^ a b c d e f g h Del Hoyo et al. (1996) 648
- ^ Parentheses indicate originally described under a different name
- ^ a b c Harrison (1988) 383
- ^ a b c d e f g Olsen & Larsson (1995) 35–42
- ^ a b c d e f g h i j k Cooper (2006) 760–764
- ^ Not considered by Olsen & Larsson (1995)
- ^ a b c d e f Snow & Perrin (1998) 770–771
- ISBN 0-7136-3973-3.
- ISBN 0-691-08402-5.
- ^ .
- ISBN 1-891276-00-X.
- ^ a b c "Great Crested Tern Sterna bergii". BirdLife Species Factsheet (additional data). BirdLife International. Retrieved 7 July 2008.
- ^ Robertson C.J.R.; Medway, D. G. "New Zealand recognized bird names (NZRBN)" (PDF). Ornithological Society of New Zealand. Archived from the original (PDF) on 2008-10-14.
- ^ KA Shaji (13 May 2020). "Lakshadweep gets world's first sea cucumber conservation reserve to curb smuggling into China". Scroll. Retrieved 23 July 2020.
- ^ Higgins & Davies (1996) 610–611
- .
- ISBN 1-870630-88-2.
- ^ "Crested Tern". Fact Sheets. Australian Museum. Archived from the original on 13 November 2008. Retrieved 5 July 2008.
- .
- ^ .
- .
- ^ Crawford, R. J. M.; Dyer B. (2000). "Swift Terns Sterna bergii breeding on roofs and at other new localities in southern Africa" (PDF). Marine Ornithology. 28: 123–124.
- ^ a b le Roux, Janine. "Swift Tern Sterna bergii". Avian Demography Unit, University of Cape Town. Archived from the original on 24 December 2012. Retrieved 5 July 2008.
- ^ .
- S2CID 86179144.
- doi:10.1071/MU98006.
- ISBN 0-7099-3336-3.
- ISBN 0-262-24036-X
- ISBN 0-19-854529-0.
- PMID 17148194.
- ^ "Annex 2: Waterbird species to which the Agreement applies" (PDF). Agreement on the conservation of African-Eurasian migratory Waterbirds (AEWA). UNEP/ AEWA Secretariat. Archived from the original (PDF) on 28 July 2011. Retrieved 4 July 2008.
- ^ "Introduction". African-Eurasian Waterbird Agreement. UNEP/ AEWA Secretariat. Archived from the original on 11 February 2012. Retrieved 4 July 2008.
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, ISBN 0-11-497333-4
- Del Hoyo, Josep; Elliott, Andrew; Sargatal, Jordi, eds. (1996). ISBN 84-87334-20-2.
- Harrison, Peter (1988). Seabirds. London: Christopher Helm. ISBN 0-7470-1410-8.
- Higgins, P. J.; Davies, S. J. J. F., eds. (1996). ISBN 0-19-553070-5.
- Olsen, Klaus Malling; Larsson, Hans (1995). Terns of Europe and North America. London: Christopher Helm. ISBN 0-7136-4056-1.
- Snow, David; Perrins, Christopher M., eds. (1998). ISBN 0-19-854099-X.
External links
- (Greater crested tern = ) Swift tern - Species text in The Atlas of Southern African Birds