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LC
Bridled Tern Sterna anaethetus

Justification
This species has a very 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 is not known, but the population is not believed to be decreasing sufficiently rapidly to approach the thresholds 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.

Taxonomic source(s)
AERC TAC. 2003. AERC TAC Checklist of bird taxa occurring in Western Palearctic region, 15th Draft. Available at: http://www.aerc.eu/DOCS/Bird_taxa_of _the_WP15.xls.
AOU. 1998. Check-list of North American birds. American Ornithologists' Union, Washington, D.C.
Christidis, L.; Boles, W. E. 1994. The taxonomy and species of birds of Australia and its territories. Royal Australasian Ornithologists' Union, Melbourne.
Christidis, L.; Boles, W. E. 2008. Systematics and taxonomy of Australian birds. CSIRO Publishing, Collingwood, Australia.
Cramp, S.; Perrins, C. M. 1977-1994. Handbook of the birds of Europe, the Middle East and Africa. The birds of the western Palearctic. Oxford University Press, Oxford.
Dowsett, R. J.; Forbes-Watson, A. D. 1993. Checklist of birds of the Afrotropical and Malagasy regions. Tauraco Press, Li
SACC. 2006. A classification of the bird species of South America. Available at: http://www.museum.lsu.edu/~Remsen/SACCBaseline.html.
Sibley, C. G.; Monroe, B. L. 1990. Distribution and taxonomy of birds of the world. Yale University Press, New Haven, USA.
Stotz, D. F.; Fitzpatrick, J. W.; Parker, T. A.; Moskovits, D. K. 1996. Neotropical birds: ecology and conservation. University of Chicago Press, Chicago.
Turbott, E. G. 1990. Checklist of the birds of New Zealand. Ornithological Society of New Zealand, Wellington.

Taxonomic note
The BirdLife Taxonomic Working Group is aware that phylogenetic analyses have been published which have proposed generic rearrangements which may affect this species, but prefers to wait until work by other taxonomists reveals how these changes affect the entire groups involved.

Synonym(s)
Onychoprion anaethetus AOU checklist (1998 + supplements), Onychoprion anaethetus Christidis and Boles (2008), Onychoprion anaethetus , Onychoprion anaethetus anaethetus Christidis and Boles (2008)

Distribution and population
The Bridled Tern is a bird of the tropical oceans. It breeds off the Pacific and Atlantic coast of Central America including the Carribean, off small areas of western Africa, around Arabia and eastern Africa down to South Africa, off the coast of India, and in much of south-east Asia and Australasia excluding southern Australia and New Zealand (del Hoyo et al. 1996).

Population justification
The global population is estimated to number c.610,000-1,500,000 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.10,000-1 million breeding pairs and > c.1,000 individuals on migration in Taiwan and < c.100 breeding pairs and < c.50 individuals on migration in Japan (Brazil 2009).

Trend justification
The overall population trend is uncertain, as some populations are decreasing, while others are increasing or have unknown trends (Wetlands International 2006).

Ecology
Behaviour Most populations are migratory and dispersive (Higgins and Davies 1996, Haney et al. 1999) and abandon their breeding sites at the end of the breeding season to overwinter at sea (Haney et al. 1999). Its detailed migratory movements are largely unknown however (del Hoyo et al. 1996) and some populations in the Indian Ocean are entirely sedentary or only partially migratory (Haney et al. 1999). The timing of breeding varies geographically, most populations breeding annually in groups of 2-30 pairs (sometimes up to 400-2,000 pairs) (del Hoyo et al. 1996) that are not strictly colonial but involve solitary pairs congregating in suitable habitat (Haney et al. 1999). When nesting the species often associates with nesting Sterna fuscata or Sterna bergii (del Hoyo et al. 1996). After breeding the adults and newly fledged young leave the breeding colonies in loose flocks (Higgins and Davies 1996) and migrate alone, in small groups of 10-12 individuals or more rarely in larger groups of up to 200 individuals (Haney et al. 1999). Outside of the breeding season the species is thought to occur singly (Higgins and Davies 1996). Habitat The species inhabits offshore tropical and subtropical seas (Higgins and Davies 1996, del Hoyo et al. 1996). Breeding It breeds on the periphery of vegetated coastal and continental (Haney et al. 1999) coral, rock or rubble islands and beaches (Higgins and Davies 1996, del Hoyo et al. 1996, Haney et al. 1999), volcanic stacks and exposed reefs (Haney et al. 1999), foraging inshore and up to 50 km offshore (although mostly within 15 km of land) (del Hoyo et al. 1996) and feeding from the surface of the water or up to 20 cm below it (Higgins and Davies 1996). Non-breeding Away from the breeding grounds the species is entirely pelagic and often associates with patches of macroalgae (e.g. Sargassum spp.) or flotsam (Haney et al. 1999) which it uses for perching (del Hoyo et al. 1996). Its marine distribution is therefore linked to small- and medium-scale oceanographic features where water circulation aggregates such floating matter into patches (Haney et al. 1999). Diet Its diet consists predominantly of squid and surface-schooling fish less than 6 cm long as well as crustaceans and occasionally aquatic insects (del Hoyo et al. 1996) or molluscs (Higgins and Davies 1996). Breeding site The nest is a scrape or depression in shingle or sand (Higgins and Davies 1996) that may be freshly excavated or re-used from a previous season (Higgins and Davies 1996). Nests are placed in a variety of concealed locations (Higgins and Davies 1996, del Hoyo et al. 1996) around the rim of oceanic islands (del Hoyo et al. 1996), including natural cavities amongst rocks or coral rubble, in vegetation (Higgins and Davies 1996, del Hoyo et al. 1996) (up to 75 % ground cover) (Higgins and Davies 1996), in a crevice or cave up to 1.5 m deep, under a cliff ledge or on the ground beneath low bushes or shrubs (Higgins and Davies 1996). The species is not strictly colonial but solitary pairs usually congregate in suitable habitats (Haney et al. 1999) with neighbouring nests spaced according to nest-site availability (usually 1-5 m apart, minimum 30 cm) (del Hoyo et al. 1996). Management information The species will become habituated to human presence in sites exposed to long term visitation, especially where human movements are predictable, groups sizes are kept consistent and human behaviour is reliable (Haney et al. 1999). Additional measures to reduce human disturbance of nesting colonies includes the erection of barriers and signs, the provision of walkways, and the supervision and education of vistors (Haney et al. 1999).

Threats
The species is vulnerable to the effects of oil spills and is highly vulnerable to the accidental introduction of domestic cats Felis catus to offshore breeding islands (Haney et al. 1999). It has also been known to abandon breeding colonies when subject to severe human disturbance (although at sites exposed to long-term visitation it may become habituated to continuous and predictable human presence and activity) (Haney et al. 1999). Utilisation Eggs are harvested for subsistence in the Bahamas and the West Indies, and eggs and chicks are harvested on some islands in the Pacific by local residents and coastal shipping crews (Haney et al. 1999).

References
Brazil, M. 2009. Birds of East Asia: eastern China, Taiwan, Korea, Japan, eastern Russia. Christopher Helm, London.

del Hoyo, J.; Elliott, A.; Sargatal, J. 1996. Handbook of the Birds of the World, vol. 3: Hoatzin to Auks. Lynx Edicions, Barcelona, Spain.

Delany, S.; Scott, D. 2006. Waterbird population estimates. Wetlands International, Wageningen, The Netherlands.

Haney, J.C.; Lee, D. S.; Morris, R.D. 1999. Bridled Tern, Sterna anaethetus. (Charadriiformes : Laridae). In: Poole, A.; Gill (ed.), The Birds of North America, The Birds of North America, Inc., Philadelphia, PA.

Higgins, P. J.; Davies, S. J. J. F. 1996. Handbook of Australian, New Zealand and Antarctic birds vol 3: snipe to pigeons. Oxford University Press, Oxford.

Text account compilers
Calvert, R., Malpas, L., Butchart, S., Ekstrom, J.

IUCN Red List evaluators
Butchart, S., Symes, A.

Recommended citation
BirdLife International (2014) Species factsheet: Sterna anaethetus. Downloaded from http://www.birdlife.org on 10/07/2014. Recommended citation for factsheets for more than one species: BirdLife International (2014) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 10/07/2014.

This information is based upon, and updates, the information published in BirdLife International (2000) Threatened birds of the world. Barcelona and Cambridge, UK: Lynx Edicions and BirdLife International, BirdLife International (2004) Threatened birds of the world 2004 CD-ROM and BirdLife International (2008) Threatened birds of the world 2008 CD-ROM. These sources provide the information for species accounts for the birds on the IUCN Red List.

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To contribute to discussions on the evaluation of the IUCN Red List status of Globally Threatened Birds, please visit BirdLife's Globally Threatened Bird Forums.

Additional resources for this species

ARKive species - Bridled tern (Sterna anaethetus) 0

Key facts
Current IUCN Red List category Least Concern
Family Laridae (Gulls and terns)
Species name author Scopoli, 1786
Population size mature individuals
Population trend Unknown
Distribution size (breeding/resident) 516,000 km2
Country endemic? No
Links to further information
- Additional Information on this species
- Climate change species distributions