[order] Charadriiformes | [family] Sternidae | [latin] Onychoprion anaethetus | [UK] Bridled Tern | [FR] Sterne bridée | [DE] Zügelseeschwalbe | [ES] Charrán embridado | [IT] Sterna dalle redini | [NL] Brilstern

Brilstern determination

copyright: P. Nash

Semi-pelagic tern with long, deeply forked tail and long, narrow wings. 37 cm, 130 g, wingspan 75-80 cm. forehead patch white and triangular, extending short way behind eye. Crown and nape black, occasionally with narrow paler hindneck collar. Upperparts dark grey-brown, paler on rump and tail, with white outer tail feathers. Upperwing dark grey-brown, with conspicuous pale leading edge. Whitish below, with grey breast. Bill and legs black. Shorter-winged than S. fuscata, with paler upperparts contrasting with black crown, and narrow white eyebrow extending further behind eye. Unlike S. fuscata, bill tomia not serrated, and webbing of toes more deeply incised which may facilitate perching on vegetation. Races differ mainly in darkness of plumage, amount of white in outer tail, and size. melanoptera somewhat paler above, with greyish collar between nape and upperparts.

Tropical, subtropical, and maritime, but differs from Sooty Tern in being primarily offshore rather than pelagic. Breeds on islands and, in some areas, on mainland, nesting under bushes on sand and coral islets, but using a crevice under a ledge or the floor of a cave on limestone islands or stacks. At sea commonly rests on floating objects, not on water.

This species is a rare vagrant to western Europe. Accidental in Britain and Ireland: 5 found dead 1931-58, 3 alive 1979-82. Also a wing was found in Britain, April 1977. Some unsubstantiated observations from continental Europe.

Primarily small fish, planktonic invertebrates including crustaceans and molluscs, and the water-bug Halobates. Feeds chiefly from surface, mainly by hovering and dipping-to-surface. Takes prey chased to surface by schools of predatory fish.

This species has a large range, with an estimated global Extent of Occurrence of 100,000-1,000,000 km². The global population size has not been quantified, but the species is not believed to approach the thresholds for the population size criterion of the IUCN Red List (i.e. less than 10,000 mature individuals in conjunction with appropriate decline rates and subpopulation qualifiers), even though the species is described as 'uncommon' in at least parts of its range (Stotz et al. 1996). Global population trends have not been quantified, but the species is not believed to approach the thresholds for the population decline criterion of the IUCN Red List (i.e. declining more than 30% in ten years or three generations). For these reasons, the species is evaluated as Least Concern. [conservation status from birdlife.org]

Breeds May-June in West Africa and Caribbean, June-August in East Africa and Madagascar, October-December in Australia. Colonies up to 200 pairs, often not synchronous. Nests are often distributed in vegetation, rock or rubble around rim or island. Most of the birds retain nest-site from year to year, even with different mates. Nest contents normally concealed under rock or vegetaion, reducing predation. 1 egg, incubation 28-30 days. Chick finely mottled, resembles that of S. dougallii. Young fledge after 53-63 days. First breeding usually at 4 years.

Little known. If not migratory out of breeding season, at least highly dispersive, leading offshore existence. None present in west Palearctic breeding areas in winter.