Greater Crested Tern (Thalasseus bergii) | Text

Greater Crested Tern Thalasseus bergii

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Justification

Justification of Red List category
This species has an extremely large range, and hence does not approach the thresholds for Vulnerable under the range size criterion (extent of occurrence <20,000 km² combined with a declining or fluctuating range size, habitat extent/quality, or population size and a small number of locations or severe fragmentation). The population trend appears to be stable, and hence the species does not approach the thresholds for Vulnerable under the population trend criterion (>30% decline over ten years or three generations). The population size is very large, and hence does not approach the thresholds for Vulnerable under the population size criterion (<10,000 mature individuals with a continuing decline estimated to be >10% in ten years or three generations, or with a specified population structure). For these reasons the species is evaluated as Least Concern.

Population justification
The global population is estimated to number c.150,000-1,100,000 individuals (Wetlands International 2006), while national population estimates include: c.100-10,000 breeding pairs and c.50-1,000 individuals on migration in China; c.10,000-100,000 breeding pairs and c.1,000-10,000 individuals on migration in Taiwan and c.100-10,000 breeding pairs and c.50-1,000 individuals on migration in Japan (Brazil 2009).

Trend justification
The overall population trend is stable, although some populations have unknown trends (Delany and Scott 2006).

Distribution and population

This species can be found found on islands and coastlines of the tropical and subtropical Old World, ranging from the Atlantic Coast of South Africa, south around the Cape and continuing along the coast of Africa and Asia almost without break to south-east Asia and Australia. It can also be found on Madagascar, islands of the western Indian ocean and islands of the western and central Pacific Ocean. Outside the breeding season it can be found at sea throughout this range, with the exception of the central Indian Ocean (del Hoyo et al. 1996).

Ecology

Behaviour Many populations remain sedentary in their breeding areas or disperse locally (del Hoyo et al. 1996), although some are more migratory (Urban et al. 1986). The species breeds in large dense colonies, or in small groups of less than 10 pairs amidst colonies of other species (e.g. King Gull Larus hartlaubii or Silver Gull Larus novaehollandiae) (del Hoyo et al. 1996). It usually forages singly (Urban et al. 1986) or in small groups (del Hoyo et al. 1996) but several hundred individuals may gather at roost sites (Langrand 1990). Habitat The species inhabits tropical and subtropical coastlines, foraging in the shallow waters of lagoons (Higgins and Davies 1996, del Hoyo et al. 1996), coral reefs (del Hoyo et al. 1996), estuaries (Urban et al. 1986, del Hoyo et al. 1996), bays, harbours and inlets (Higgins and Davies 1996), along sandy, rocky, coral (del Hoyo et al. 1996) or muddy shores, on rocky outcrops in open sea, in mangrove swamps (Langrand 1990) and also far out to sea on open water (del Hoyo et al. 1996). It shows a preference for nesting on offshore islands (Urban et al. 1986, del Hoyo et al. 1996), low-lying coral reefs, sandy or rocky coastal islets, coastal spits, lagoon mudflats (del Hoyo et al. 1996), and artificial islets in saltpans and sewage works (Urban et al. 1986, del Hoyo et al. 1996) within 3 km of the coast (del Hoyo et al. 1996). Diet Its diet consists predominantly of pelagic fish 10-50 cm long (Urban et al. 1986, del Hoyo et al. 1996) although it will also take cephalopods (e.g. squid), crustaceans (del Hoyo et al. 1996) (e.g. crabs (del Hoyo et al. 1996) and prawns (Higgins and Davies 1996)), insects and hatchling turtles opportunistically (del Hoyo et al. 1996). Breeding site The nest is a shallow scrape in bare sand, rock or coral (del Hoyo et al. 1996) in flat open sites (Urban et al. 1986) on offshore islands (Urban et al. 1986, del Hoyo et al. 1996), low-lying coral reefs, sandy or rocky coastal islets, coastal spits, lagoon mudflats (del Hoyo et al. 1996) or islets in saltpans and sewage works (Urban et al. 1986, del Hoyo et al. 1996). The species nests in dense colonies (Urban et al. 1986) with neighbouring nests very close together (rims may be touching) (del Hoyo et al. 1996), and usually forages within 3 km of the breeding colony (del Hoyo et al. 1996).

Threats

The species is vulnerable to human disturbance, which may cause the flushing of adult birds allowing increased egg predation by gulls and ibis (Gochfield et al. 2018). It may be experiencing declines in Indonesia and South Africa due to egg collection (Gochfield et al. 2018).

Acknowledgements

Text account compilers
Butchart, S., Ashpole, J, Ekstrom, J., Malpas, L., Martin, R., Stuart, A., Calvert, R.

Recommended citation
BirdLife International (2024) Species factsheet: Greater Crested Tern Thalasseus bergii. Downloaded from https://datazone.birdlife.org/species/factsheet/greater-crested-tern-thalasseus-bergii on 23/12/2024.
Recommended citation for factsheets for more than one species: BirdLife International (2024) IUCN Red List for birds. Downloaded from https://datazone.birdlife.org/species/search on 23/12/2024.