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Greater Crested Tern
Breeding plumage T. b. cristata
Conservation status
Least Concern  (IUCN 3.1)
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Aves
Order:	Charadriiformes
Family:	Sternidae
Genus:	Thalasseus
Species:	T. bergii
Binomial name
Thalasseus bergii (Lichtenstein, 1823)
Approximate breeding range   Wintering range

The Greater Crested Tern, Crested Tern, or Swift Tern (Thalasseus bergii), is a seabird of the tern family Sternidae. It breeds in dense colonies on coasts and islands in the tropical and subtropical Old World from South Africa around the Indian Ocean to the western Pacific and Australia. All populations of the five subspecies disperse more widely when not breeding.

The Greater Crested Tern is a large tern with grey upperparts, white underparts, a yellow bill and a shaggy black crest which becomes less extensive in winter. Young birds are distinctive, with plumage strongly patterned in grey, brown and white. This species is closely related to the Royal and Lesser Crested Terns, but can be distinguished from these by its bill colour and size. It is highly vocal, especially at its breeding grounds, with a variety of loud calls. The one or two eggs are laid in a bare ground scrape, and the young remain dependent on the parents long after they have left the breeding site.

Like all Thalasseus terns, Greater Crested feeds by plunge diving for fish, usually in marine environments, and the offering of fish by the male to the female is part of the courtship display. Commercial fisheries have a mixed impact on this tern. Netting of surface species can reduce the availability of pelagic fish, but discards from trawlers augment the tern's diet and improve the survival of young birds.

This is an adaptable species which has learned to follow fishing boats for offal, and to utilise 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 other disturbance have caused local losses. There is no evidence of a significant overall decline in numbers, and a world population of more than half a million individuals means that there are no major global conservation issues with this bird.

Taxonomy

The terns, family Sternidae, are small to medium-sized seabirds, gull-like in appearance, but usually with a more delicate, lighter build and shorter, weaker legs. They have long, pointed wings, which gives them a fast buoyant flight, and often a deeply forked tail. Most species are grey above and white below, and have a black cap which is reduced or flecked with white in the winter.

The Greater Crested Tern was originally described as Sterna bergii by Martin Lichtenstein in 1823, but was recently moved to its current genus Thalasseus (Boie, 1822) following mitochondrial DNA studies which confirmed that the three types of head pattern (white crown, black cap, and black cap with a white blaze on the forehead) found amongst the terns corresponded to distinct clades.

All the terns in the genus Thalasseus are relatively large and have shaggy crests, and Greater Crested's closest relatives within its genus appear to be the Lesser Crested Tern, T. bengalensis, and the Royal Tern, T. maximus. The DNA study did not include the critically endangered Chinese Crested Tern T. bernsteini, but since prior to the clarification of its specific status it was originally considered to be synonymous with the Crested Tern subspecies T. b. cristatus, it is presumably also a very close relative.

The binomial name of the Greater Crested Tern is derived from Greek Thalassa, "sea" and the specific bergi, which commemorates Carl Heinrich Bergius, a Prussian pharmacist and botanist who collected the first specimens of this tern near Cape Town.

An early Pliocene fossil bone fragment from the northeastern United States closely resembles a modern Royal Tern. It may be an unexpectedly early (3.7–4.8 million years before present) specimen of that species, or an ancestral member of the crested tern group.

The Greater Crested Tern has five generally recognised geographical races, differing mainly in the shade of the upperparts and bill colour. At least as many other subspecies have been proposed but are not now considered valid.

Description

This is a large tern with a long (5.4–6.5 cm, 2.1–2.6 in) yellow bill, black legs and a glossy black crest which is noticeably shaggy at its rear. The breeding adult of the nominate subspecies T. b. bergii is 46–49 cm (18–19 in) long, with a 125–130 cm (49–51 in) wing-span and weighs from 325–397 g (11.4–14.0 oz).. 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 into a peppered black crest and mask.

The sexes are identical, but juvenile birds are distinctive, with a head pattern like the winter adult but upperparts strongly patterned in grey, brown and white. The closed wings in particular appear to have dark bars. After moulting, they resemble the adult, but have a variegated wing pattern with a dark bar on the inner flight feathers.

This tern is highly vocal, especially at its breeding grounds. Its advertising call is a loud, raucous, crow-like kerrak. Other calls include a korrkorrkorr when anxious or excited at the nest and a hard wep wep in flight.

The northern subspecies T. b. velox and T. b. velox are in breeding plumage from between May or June to September or October, whereas for the southern African subspecies the relevant period is from December to April. For T. b. cristata, the moult depends on location. Birds from Australia and Oceania are in breeding plumage from September to about April, whereas 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 Lesser Crested Tern, but is 25% larger than that species, with a proportionately longer bill, longer and heavier head, and bulkier body. Lesser Crested has an orange-tinted bill, and in immature plumage is much less variegated than Greater Crested.

Distribution and habitat

This tern occurs in tropical and warm temperate coastal parts of the Old World from South Africa around the Indian Ocean to the western Pacific and Australia. It is sometimes seen in lagoons and estuaries, but hardly ever on tidal creeks or inland waters. It breeds mainly on low‑lying sandy, rocky, or coral islands, sometimes among stunted vegetation, but often without any shelter at all. When not nesting, the Greater Crested Tern will roost or rest on open shores, or less often on boats, pilings, harbour buildings and raised salt mounds in lagoons.

All populations disperse when not breeding. When Southern African birds leave breeding sites 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 2,000 km (1,200 mi), but some move substantial distances north. T. b. thalassina winters along 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. T. b. cristata mostly stays within 400 km (260 mi) of its colonies, but some birds wander up to around 1,000 km (600 mi). This species has occurred as a vagrant to Hawaii, the Cook Islands, New Zealand, North Korea, Jordan, and Israel.

Behaviour

Breeding

The Greater Crested Tern breeds in colonies, often in association with other seabirds. It is monogamous and the pair bond is maintained during the year and sometimes in subsequent breeding seasons. The colony size is related to the abundance of pelagic fish prey, and the largest documented colony, with 13,000-15,000 pairs, is in the Gulf of Carpentaria, Australia. That area also supports major colonies of other seabirds, and since nesting follows the summer monsoonal flooding, it is presumably a response to fish stocks rising due to nutrient enrichment of the Gulf by river runoff. This tern frequently changes its nest site from year to year, sometimes by more than 200 km (130 miles).

When terns nest, it takes a while for the pair to become established. The male initially pecks at any bird entering his territory, and will repel any intruding male. Once a bond with a female has been established, it is reinforced by display, including head raising and bowing. Terns also use fish as part of the courtship ritual. One bird flies around the colony with a fish in its beak, calling loudly. The partner may also fly, but the pair eventually settle and the fish may be 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 25–30 days to hatching. The eggs are cream with blackish streaks, and laying is synchronised within a breeding colony.Parents do not recognize their own eggs or newly hatched chicks, but are able to distinguish their own 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 3–40 days, but remain dependent on the parents after leaving the colony until they are about four months old.

In South Africa, this species has adapted to breeding on the roofs of building, along 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. Artificial islands in salt pans and sewage works have also recently been colonised by this adaptable seabird.

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. In Australia, predation by cats and dogs, and occasional deaths by shooting or collisions with cars, wires or light-towers have also been documented.

Commercial fisheries can have both positive and negative effects on this species. Juvenile survival rates are improved where trawler discards provide extra food, and the huge population increases in population in the southeast Gulf of Carpentaria are thought to have been due to the development of a large prawn trawl fishery. Conversely, purse-seine fishery reduces the available food supply, and large fluctuations in 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 a purse-seine fishery. Terns may be and 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 the populations are significantly affected.

An unusual incident was the incapacitation of 103 terns off South Africa by marine foam, generated by a combination of wave action, kelp mucilage and phytoplankton. After treatment, 90% were fit to be released

Feeding

Fish are the main food of the Greater Crested Tern, making up 86% of all prey items consumed in a study in the Western Cape. Other prey included cephalopods, crustaceans and insects. Unusual vertebrate food items include agamid lizards and green turtle hatchlings.

The Great Crested Tern feeds mostly at sea by plunge diving to a depth of up one metre (3 ft), or by dipping from the surface, and food is usually swallowed in mid-air. Birds may forage up to 10 km (6 mi) from land in the breeding season. Prey size ranged from 7–138 mm (0.27–5.4 in) in length and up to 30 g (1.1 oz) in weight. Shoaling pelagic fish such as anchovy and sardine are typical prey, 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.

A study of an area of the Great Barrier Reef where the number of breeding Great Crested Terns has grown by an order of magnitude, 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 due to competition for food and nesting sites.

Terns have red oil droplets in the cones of their retinas. This improves contrast and sharpens distance vision, especially in hazy conditions, and birds that have to look through an air/water interface have more deeply coloured carotenoid pigments in the oil drops than other species. This helps them to locate shoals of fish, although it is uncertain whether they are sighting phytoplankton that the fish feed on, or seeing other terns feeding. Unlike the gulls, they are not ultraviolet sensitive, an adaptation more suited to terrestrial feeding.

Status

This species has a large range, with an estimated global extent of 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 (less than 10,000 mature individuals in conjunction with subpopulation qualifiers) 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 Least Concern. Numbers in Indonesia are declining due to egg harvesting, and this species no longer breeds in the Gulf of Thailand. Some population estimates for subspecies are available.

The Greater Crested Tern is among the taxa to which the Agreement on the Conservation of African-Eurasian Migratory Waterbirds (AEWA) applies. Parties to the Agreement are required to engage in a wide range of conservation actions which are describes in a detailed action plan. This plan should address key issues such as species and habitat conservation, management of human activities, research, education, and implementation.

Footnotes

1. ^ Template:IUCN2006 Database entry includes justification for why this species is of least concern
2. Snow & Perrin (1998) 764
3. ^ Bridge, Eli S. (2005). "A phylogenetic framework for the terns (Sternini) inferred from mtDNA sequences: implications for taxonomy and plumage evolution" (PDF). Molecular Phylogenetics and Evolution. 35: 459–469. doi:10.1016/j.ympev.2004.12.010. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
4. Adapted from Bridge et al (2005). The study did not include the critically endangered Chinese Crested Tern
5. Template:Nl icon Mees, G. F. (1975). "Identiteit en status van Sterna bernsteini Schlegel". Ardea. 63: 78–86. {{cite journal}}: Cite has empty unknown parameters: |1= and |2= (help)
6. ^ Higgins & Davies (1996) 605–609
7. Olson, S., Rasmussen, P.C., "Miocene and Pliocene birds from the Lee Creek Mine, North Carolina" in Ray, C. E. & Bohaska, D. J. (2001). "Geology and Paleontology of the Lee Creek Mine, North Carolina, III." Smithsonian Contributions to Paleobiology, 90. Smithsonian Institution Press, Washington D.C. 233–365.
8. ^ Del Hoyo et al (1996) 648
9. Parentheses indicate originally described under a different name
10. ^ Harrison (1988) 383
11. ^ Olsen & Larsson (1995) 35–42
12. Not considered by Olsen & Larsson (1995)
13. ^ Snow & Perrin (1998) 770–771
14. ^ Cooper (2006) 760–764
15. ^ Carrick, R. (1957). "Seasonal dispersal and mortality in the Silver Gull, Larus novae-hollandiae Stephens, and Crested tern, Sterna bergii Lichstein, in Australia". Wildlife Research. 2: 116–144. {{cite journal}}: Cite has empty unknown parameter: |1= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
16. American Ornithologists' Union (1998). Check-list of North American Birds (7th edition ed.). Washington, D.C.: American Ornithologists' Union. p. 198. ISBN 1-891276-00-X. {{cite book}}: |edition= has extra text (help); |format= requires |url= (help); Cite has empty unknown parameter: |1= (help); External link in |title= (help)
17. McCormack, Gerald. "Crested Tern". Cook Islands Biodiversity Database, Version 2007.2. Cook Islands Natural Heritage Trust, Rarotonga. {{cite web}}: Cite has empty unknown parameter: |1= (help) Retrieved 4 July 2008
18. ^ "Great Crested Tern Sterna bergii". BirdLife Species Factsheet (additional data). BirdLife International. {{cite web}}: Cite has empty unknown parameter: |1= (help) Retrieved 7 July 2008
19. Robertson C.J.R.; Medway, D. G. "New Zealand recognized bird names (NZRBN)" (PDF). Ornithological Society of New Zealand. {{cite web}}: Cite has empty unknown parameter: |1= (help)CS1 maint: multiple names: authors list (link)
20. 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.
21. Fisher, James (1989). Sea‑Birds (Collins New Naturalist series). London, Bloomsbury Books. pp. 155–156. ISBN 1870630 88 2. {{cite book}}: Cite has empty unknown parameters: |1= and |2= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
22. "Crested Tern". Fact Sheets. Australian Museum. {{cite web}}: Cite has empty unknown parameter: |1= (help) Retrieved 5 July 2008
23. Dunlop, J. N. (1987). "Social-behavior and colony formation in a population of crested terns, Sterna bergii, in southwestern australia". Australian Wildlife Research. 14: 529–540. {{cite journal}}: Cite has empty unknown parameter: |1= (help)
24. Davies, S. J. J. F. (1962). "On the ability of crested terns, Sterna bergii, to recognize their own chicks". Australian Journal of Zoology. 10: 171–177. doi:10.1071/ZO9620171. {{cite journal}}: Cite has empty unknown parameters: |1= and |month= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
25. Crawford, R. J. M. (2000). "Swift Terns Sterna bergii breeding on roofs and at other new localities in southern Africa" (PDF). Marine Ornithology. 28: 123–124. {{cite journal}}: Cite has empty unknown parameter: |1= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
26. ^ le Roux, Janine. "Swift Tern Sterna bergii". Avian Demography Unit, University of Cape Town. {{cite web}}: Cite has empty unknown parameter: |1= (help) Retrieved 5 July 2008
27. ^ Blaber, S. J. M. (1995). "Trawl discards in the diets of tropical seabirds of the northern Great Barrier Reef, Australia" (PDF). Marine Ecology Progress Series. 127: 1–13. doi:10.3354/meps127001. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help); Unknown parameter |month= ignored (help)
28. Parsons, Nola J. (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. {{cite journal}}: Cite has empty unknown parameter: |1= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help); Unknown parameter |month= ignored (help)
29. Blaber, S. J. M. (1998). "Seabird breeding populations on the far northern Great Barrier Reef, Australia: trends and influences". Emu. 98: 44–57. {{cite journal}}: Cite has empty unknown parameters: |1= and |2= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
30. Sinclair, Sandra (1985). How Animals See: Other Visions of Our World. Beckenham, Kent: Croom Helm. pp. 93–95. ISBN 0709933363. {{cite book}}: Cite has empty unknown parameters: |1= and |2= (help)
31. 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 MIT Press 77–94 ISBN 026224036X
32. Lythgoe, J. N. (1979). The Ecology of Vision. Oxford: Clarendon Press. pp. 180–183. ISBN 0198545290. {{cite book}}: Cite has empty unknown parameters: |1=, |2=, and |coauthors= (help)
33. Håstad, Olle (2005). "Ultraviolet vision and foraging in dip and plunge diving birds". Biology Letters. 1 (3): 306–309. PMID 17148194. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help); Unknown parameter |month= ignored (help)
34. "Annex 2: Waterbird species to which the Agreement applies" (PDF). Agreement on the conservation of African-Eurasian migratory Waterbirds (AEWA). UNEP/ AEWA Secretariat. {{cite web}}: Cite has empty unknown parameter: |1= (help) Retrieved 4 July 2008
35. "Introduction". African-Eurasian Waterbird Agreement. UNEP/ AEWA Secretariat. {{cite web}}: Cite has empty unknown parameter: |1= (help) Retrieved 4 July 2008

References

• 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 0114973334
• Del Hoyo, Josep (1996). Handbook of the Birds of the World: Hoatzin to Auks v. 3. Lynx Edicions. ISBN 8487334202. {{cite book}}: Cite has empty unknown parameters: |1= and |2= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
• Harrison, Peter (1988). Seabirds. Christopher Helm, London. ISBN 0-7470-1410-8. {{cite book}}: Cite has empty unknown parameters: |1=, |2=, and |coauthors= (help)</ref>
• Higgins, P. J. (1996). Handbook of Australian, New Zealand and Antarctic Birds: Volume 3: Snipe to Pigeons. Oxford University Press, Melbourne. ISBN 0-19-553070-5. {{cite book}}: Cite has empty unknown parameters: |1= and |2= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
• Olsen, Klaus Malling (1995). Terns of Europe and North America. London: Christopher Helm. ISBN 0-7136-4056-1. {{cite book}}: Cite has empty unknown parameters: |1= and |2= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
• Snow, David (1998). The Birds of the Western Palearctic (BWP) concise edition (2 volumes). Oxford: Oxford University Press. ISBN 019854099X. {{cite book}}: Cite has empty unknown parameter: |1= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)

• IUCN Red List least concern species
• Birds of Western Australia
• Birds of South Australia
• Birds of Tasmania
• Thalasseus