Sterna anaethetus

Taxonomy

Assessment Information

Geographic Range

Habitat and Ecology

Threats

Bibliography

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Taxonomy [top]

Kingdom	Phylum	Class	Order	Family
ANIMALIA	CHORDATA	AVES	CHARADRIIFORMES	LARIDAE

Scientific Name:

Sterna anaethetus

Species Authority:

Scopoli, 1786

Common Name/s:

English	–	Bridled Tern
French	–	Sterne bridée

Assessment Information [top]

Red List Category & Criteria:

Least Concern ver 3.1

Year Published:

2009

Assessor/s:

BirdLife International

Reviewer/s:

Bird, J., Butchart, S.

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.

History:

2008	–	Least Concern
2004	–	Least Concern

Geographic Range [top]

Range Description:	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 Zealand¹.
Countries:	Native: Anguilla; Antigua and Barbuda; Aruba; Australia; Bahamas; Bahrain; Barbados; Belize; Bermuda; Brunei Darussalam; Cambodia; Canada; Cayman Islands; China; Colombia; Comoros; Costa Rica; Cuba; Djibouti; Dominica; Dominican Republic; Ecuador; Egypt; El Salvador; Equatorial Guinea; Eritrea; Fiji; Grenada; Guadeloupe; Guinea-Bissau; Guyana; Haiti; Honduras; India; Indonesia; Iran, Islamic Republic of; Iraq; Israel; Jamaica; Japan; Jordan; Kenya; Kuwait; Madagascar; Malaysia; Maldives; Martinique; Mauritania; Mauritius; Mexico; Montserrat; Mozambique; Netherlands Antilles; New Caledonia; Nicaragua; Nigeria; Oman; Pakistan; Palau; Panama; Papua New Guinea; Philippines; Puerto Rico; Qatar; Saint Kitts and Nevis; Saint Lucia; Saint Martin (French part); Saint Vincent and the Grenadines; Sao Tomé and Principe; Saudi Arabia; Senegal; Seychelles; Singapore; Solomon Islands; Somalia; South Africa; Sri Lanka; Sudan; Taiwan, Province of China; Tanzania, United Republic of; Thailand; Timor-Leste; Togo; Trinidad and Tobago; Turks and Caicos Islands; United Arab Emirates; United States; Venezuela; Viet Nam; Virgin Islands, British; Virgin Islands, U.S.; Wallis and Futuna; Western Sahara; Yemen Vagrant: Chile; Cocos (Keeling) Islands; Denmark; Ghana; Greece; Liberia; New Zealand; United Kingdom; Vanuatu Present - origin uncertain: Benin; Christmas Island; French Southern Territories (the); Gambia; Guatemala; Guinea; Mayotte; Myanmar; Réunion; Sierra Leone
Range Map:	Click here to open the map viewer and explore range.

Habitat and Ecology [top]

Habitat and Ecology:	Behaviour Most populations are migratory and dispersive^{2, 3} and abandon their breeding sites at the end of the breeding season to overwinter at sea³. Its detailed migratory movements are largely unknown however¹ and some populations in the Indian Ocean are entirely sedentary or only partially migratory³. The timing of breeding varies geographically, most populations breeding annually in groups of 2-30 pairs (sometimes up to 400-2,000 pairs)¹ that are not strictly colonial but involve solitary pairs congregating in suitable habitat³. When nesting the species often associates with nesting Sterna fuscata or Sterna bergii¹. After breeding the adults and newly fledged young leave the breeding colonies in loose flocks² and migrate alone, in small groups of 10-12 individuals or more rarely in larger groups of up to 200 individuals³. Outside of the breeding season the species is thought to occur singly². Habitat The species inhabits offshore tropical and subtropical seas^{1, 2}. Breeding It breeds on the periphery of vegetated coastal and continental³ coral, rock or rubble islands and beaches^{1, 2, 3}, volcanic stacks and exposed reefs³, foraging inshore and up to 50 km offshore (although mostly within 15 km of land)¹ and feeding from the surface of the water or up to 20 cm below it². Non-breeding Away from the breeding grounds the species is entirely pelagic and often associates with patches of macroalgae (e.g. Sargassum spp.) or flotsam³ which it uses for perching¹. Its marine distribution is therefore linked to small- and medium-scale oceanographic features where water circulation aggregates such floating matter into patches³. Diet Its diet consists predominantly of squid and surface-schooling fish less than 6 cm long as well as crustaceans and occasionally aquatic insects¹ or molluscs². Breeding site The nest is a scrape or depression in shingle or sand² that may be freshly excavated or re-used from a previous season². Nests are placed in a variety of concealed locations^{1, 2} around the rim of oceanic islands¹, including natural cavities amongst rocks or coral rubble, in vegetation^{1, 2} (up to 75 % ground cover)², in a crevice or cave up to 1.5 m deep, under a cliff ledge or on the ground beneath low bushes or shrubs². The species is not strictly colonial but solitary pairs usually congregate in suitable habitats³ with neighbouring nests spaced according to nest-site availability (usually 1-5 m apart, minimum 30 cm)¹. 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 reliable³. 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 vistors³.
Systems:	Terrestrial; Marine

Habitat and Ecology:

Behaviour Most populations are migratory and dispersive^{2, 3} and abandon their breeding sites at the end of the breeding season to overwinter at sea³. Its detailed migratory movements are largely unknown however¹ and some populations in the Indian Ocean are entirely sedentary or only partially migratory³. The timing of breeding varies geographically, most populations breeding annually in groups of 2-30 pairs (sometimes up to 400-2,000 pairs)¹ that are not strictly colonial but involve solitary pairs congregating in suitable habitat³. When nesting the species often associates with nesting Sterna fuscata or Sterna bergii¹. After breeding the adults and newly fledged young leave the breeding colonies in loose flocks² and migrate alone, in small groups of 10-12 individuals or more rarely in larger groups of up to 200 individuals³. Outside of the breeding season the species is thought to occur singly². Habitat The species inhabits offshore tropical and subtropical seas^{1, 2}. Breeding It breeds on the periphery of vegetated coastal and continental³ coral, rock or rubble islands and beaches^{1, 2, 3}, volcanic stacks and exposed reefs³, foraging inshore and up to 50 km offshore (although mostly within 15 km of land)¹ and feeding from the surface of the water or up to 20 cm below it². Non-breeding Away from the breeding grounds the species is entirely pelagic and often associates with patches of macroalgae (e.g. Sargassum spp.) or flotsam³ which it uses for perching¹. Its marine distribution is therefore linked to small- and medium-scale oceanographic features where water circulation aggregates such floating matter into patches³. Diet Its diet consists predominantly of squid and surface-schooling fish less than 6 cm long as well as crustaceans and occasionally aquatic insects¹ or molluscs². Breeding site The nest is a scrape or depression in shingle or sand² that may be freshly excavated or re-used from a previous season². Nests are placed in a variety of concealed locations^{1, 2} around the rim of oceanic islands¹, including natural cavities amongst rocks or coral rubble, in vegetation^{1, 2} (up to 75 % ground cover)², in a crevice or cave up to 1.5 m deep, under a cliff ledge or on the ground beneath low bushes or shrubs². The species is not strictly colonial but solitary pairs usually congregate in suitable habitats³ with neighbouring nests spaced according to nest-site availability (usually 1-5 m apart, minimum 30 cm)¹. 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 reliable³. 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 vistors³.

Systems:

Terrestrial; Marine

Threats [top]

Major Threat(s):	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 islands³. 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)³. 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 crews³.

Major Threat(s):

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 islands³. 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)³. 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 crews³.

Bibliography [top]

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

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

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.

Citation:	BirdLife International 2009. Sterna anaethetus. In: IUCN 2011. IUCN Red List of Threatened Species. Version 2011.2. <www.iucnredlist.org>. Downloaded on 21 May 2012.
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