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Bridled Tern    Sterna anaethetus melanoptera


bridled tern

At a glance

  • 8900 pairs in the wider Caribbean
  • Pelagic, dark-backed, tropical and subtropical tern
  • Similar in appearance but smaller than the more common Sooty Tern
  • Nest in sheltered crevices so they are harder to detect than other terns. 
  • Typically feed solitarily along oceanic fronts over Sargassum

  • Main threats are introduced predators and loss of habitat


Index



Identification

Adults

Medium-sized dark tern (14-15 inches, 35-38 cm, total length). The combination of dark back and white underparts distinguishes the Bridled from all other terns in the region except the Sooty Tern. Sooty Terns are slightly larger (17-18 inches) and stockier. Distinctive field marks for the Bridled Tern include: (1) a white collar that very nearly separates the dark back from dark head, (2) a contrast between the very dark head and less dark back and top of the wings, (3) the Bridled Tern's white forehead extends behind the eye, and (4) underwings with more extensive white than the Sooty Tern's. At sea, they fly lower than Sooty Terns and are seldom seen in flocks.

Juveniles

Similar but markings are less defined, backs paler and mottled. Juvenile Sooty Terns have dark bellies and no white on the face or neck.

Voice

'Yep,' 'Yerk,' or repeated 'ah-ah-ah . . .' in comparison to the 'wide-a-wake' or 'wacky-wack' of the Sooty Tern.

Likely locations

As a breeding species distributed throughout the Bahamas, Central American Coast, sporadically in the Greater Antilies, and throughout most of the Lesser Antilles. Non-breeding birds occur in Caribbean, Gulf of Mexico and regularly in Gulf Stream north to the Carolinas. Occasionally further north along the North American coast. Commonly displaced by storms.

Alternate Common Names

French: Sterne à collier, Sterne bridée, Oiseau fou, Touaou, Dongue

Spanish: Charran embridado, Charran monja, Charran sombrio collarino, Golandrina-marina embridada, Gaviotin embridado, Bubi, Gaviota, Gaviota oscura, Gaviota monja (Spanish).

Systematics


One of four species of dark-backed pelagic terns (Gray-backed Tern, Sooty Tern, and Aleutian Tern). Though generally compared to the Sooty Tern, Bridled Terns are more structurally similar to Gray-backed Terns in skeletal and external characteristics. They exhibit conspicuous differences from Sooty Terns in the downy and the first basic plumages.

Sterna a. melanoptera breeds in the Bahamas, the Greater and Lesser Antilles, along the northern coast of South America, on the Caribbean coast of Central America, off of Africa near Mauritania, and in the Gulf of Guinea. Based on slight variations in coloration and tail patterns, four to six races have been recognized. Some authors have suggested that the sub-specific name recognita proposed by Mathews in 1912, be for Caribbean birds and that melanoptera be reserved for populations in the eastern Atlantic. A thorough taxonomic revision is needed.

Distribution

Resides in subtropical and tropical Atlantic, Indian, and western Pacific Oceans and in a number of adjacent seas. They are displaced by the Gray-backed Tern in the Eastern Pacific. In the West Indies region this tern breeds from Florida Keys and throughout the Bahamas, southward into and the Greater and Lesser Antilles. Also on scattered  offshore islands and cays along the Central and South American coasts. Occurs at sea around nesting sites and post breeding birds move into the Gulf of Mexico and Caribbean. In the summer Bridled Terns occur regularly as far north as the Carolinas, following the Gulf Stream. The current population is thought to be around 8900 pairs.

Bridled Terns in the West Indies

Biology

At Sea

This species has not been well studied in the Western Atlantic, and there is little information for Bridled Terns at sea. Migrates away from nesting areas and typically remains far at sea over deep waters during the non-breeding season. Off the Southeastern United States they forage along the outer continental shelf and in the Gulf Stream. Sea surface temperatures vary considerably across the shelf edge but their foraging activity seems confined to zones where average sea surface temperatures are 26-27 C. They are typically high flying and solitary though they sometimes gather in small groups over food rich areas. Most foraging occurs along oceanic fronts. This species and Audubon's Shearwaters are Sargassum specialists and obtain a large proportion of their prey from the pelagic alga mats that accumulate along current edges. During foraging bouts they dip from heights of 3-10 meters. Occasionally, they make brief plunge-dives but rarely submerge entirely. Bridled Terns feed primarily on small fishes (10-22 mm) and crustaceans.

Their contrasting back and white coloration and high flying foraging makes them visible above the horizon.

At Nesting Sites

Breeding islands often small sand cays subject to frequent erosion. In West Indies, nesting habitat mostly restricted to small, vegetated offshore cays and rocks as a result of introduced predators and human activity on larger islands. Breeding birds forage 2-30 km from breeding sites. Nest under large rocks, in limestone solution holes, or deep within overhanging ledges on rock faces, often on the periphery of the islands. Nest is a depression in sand, or pebble substrate. Lay a single egg. Incubation 26-33 days, fledging normally 55-62 days. Unlike Sooty Terns, fledglings do not form creches. Parents continue to feed young after fledgling and adults have been observed feeding young after northward dispersal in Gulf Stream off the Carolinas.

Current Population

Below estimates include counts of terns from multiple years. Because they move nesting sites each year, these esimates represent the number of nesting places in the West Indies, but the true breeding population is probably less than half of these numbers. This oversampling is an unresolved problem for the atlas. The current best estimate of the population is 8900 pairs.

Location Colonies Low Estimate High Estimate
Florida Keys 1 0 3
Bahamas 54 2225 2572
Cuba 14 620 657
Turks and Caicos 11 2375 2393
Cayman Islands 1 16 16
British Virgin Islands 22 20 200
Navassa 1 5 10
Dominican Republic 8 4 70
United States Virgin Islands 12 397 930
Puerto Rico 15 161 250
Anguilla 6 405 450
St. Martin 1 5 10
St. Maarten 2 21 21
St. Barts 1 75 100
Jamaica 12 177 2145
Saba 3 52 70
St. Kitts and Nevis 1 50 75
Antigua and Barbuda 3 16 16
Guadeloupe 11 534 594
Dominica 1 0 10
Martinique 2 51 75
St. Lucia 3 21 30
St. Vincent and the Grenadines 2 25 60
Venezuela 4 421 448
Aruba 1 122 122
Grenada 7 7 160
Tobago 2 10 96
Totals 201 7815 11583

Conservation Status 

Rare and declining, but not specifically protected.

Conservation Needs

Bridled Terns often nest at the same sites as Sooty Terns and other seabirds. These large composite colonies, often consisting of other species of 'egg birds' need to be patrolled during the early breeding season. Educational signage for visitors to islands containing nesting colonies would be useful and could contain contact information for reporting violations. For sites where ecotourism is a factor walkways could direct visitors along paths that would keep them away from key areas. While a large percentage of the regions' population occurs at a few large colony sites, groups on small isolated cays are important too and should be monitored on a regular basis.

Like other ground nesting seabirds, visits to colonies should be at times (during the day and season) that have the least impact on the birds. The expanding Laughing Gull populations in the region are of concern, as these gulls regularly take chicks from unattended nests. Gull control should be considered for sites where populations are obviously declining. In some cases, conservation issues may still linger from earlier human activities; human exploitation of adults, eggs and chicks has been prolonged since perhaps 5,000 yr BP, with a sudden increase after European contact. Guano mining between 1850 and 1890 devastated many seabird colonies.

Like other West Indian seabirds, Bridled Tern populations are probably regulated by the amount of available predator-free nesting habitat in the vicinity of dependable foraging areas. Efforts to expand the number of nesting sites for this species would be welcome. Exotic mammalian predator control is a key issue. In the Bahamas and Caribbean, small, uninhabited islands are traditionally used for keeping goats and pigs. These animals overbrowse the native vegetation and prevent successful nesting by Bridled Terns, other seabirds, and other native island fauna. 

Introduced predators or other animals that disturb the natural vegetation (rats, monkeys, mongoose, raccoons, cats, dogs, pigs, sheep, goats, and cattle) negatively affect this species. Rats and house cats are the biggest problems at known nesting sites. On one small island in the Bahamas, Bridled Terns nested only on the half of the island with native vegetation and shunned the half of the island that was dominated by Australian Pines. 

The other concerns expressed for the regions' seabird fauna can also apply to this species and conservation decisions need to be made on a site-to-site basis. In all cases, these birds are now restricted to undeveloped islands and cays.

At sea, this tern is not recorded as a by-catch species. They do, however, ingest floating plastics that remain in their digestive tracts and can cause choking. Birds affected by oil spills (as a result of the Gulf War in Middle East) are documented to abandon nests and have lowered breeding success in the year following the spill. The fact that the birds do not settle directly on the water somewhat reduces risk of oiling, but, apparently, they are still affected by surface oil when foraging. Bridled Terns have higher concentrations of heavy metals than other Western North Atlantic terns.

Photos

Selected References

Chardine, J. W., and R. D. Morris. 1987. Trapping and color banding Brown Noddy and Bridled Tern adults at the breeding colony. Colonial Waterbirds 10: 100-102.

Chardine, J. W., R. D. Morris, J. F. Parnell, and J. Pierce. 2000. Status and conservation priorities for Laughing Gulls, Gull-billed Terns, Royal Terns and Bridled Terns in the West Indies. Pages 65-79 in Status and Conservation of West Indian Seabirds. Society Caribbean Ornithology (E. A. Schreiber and D. S. Lee, eds), Special Publication Number 1. 225 pp.

Haney, J. C., D. S. Lee, and R. D. Morris. Bridled Tern (Sterna anaethetus). In The Birds of North America. No. 468 (A. Poole and F. Gill, eds.). The Birds of North America, Inc., Philadelphia, PA.
Compiled by: Dave Lee and Will Mackin, 

West Indian Breeding Seabird Atlas by Will Mackin and David Lee is licensed under a Creative Commons Attribution-Noncommercial-No Derivative Works 3.0 United States License. Based on work at www.wicbirds.net. Permissions beyond the scope of this license may be available at www.wicbirds.net.

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February 18, 2009
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