Justification of Red List Category
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.
The global population is estimated to number 610,000-1,500,000 individuals (Delany and Scott 2006). This roughly equates to 400,000-1,000,000 mature individuals. National population estimates include 100-10,000 breeding pairs and 50-1,000 individuals on migration in China; 10,000-1 million breeding pairs and > 1,000 individuals on migration in Taiwan (China) and < 100 breeding pairs and < 50 individuals on migration in Japan (Brazil 2009).
The overall population trend is uncertain, as some populations are decreasing, while others are increasing or have unknown trends (Delany and Scott 2006).
Bridled Tern is a bird of the tropical oceans. It breeds off the Pacific and Atlantic coast of Central America including the Caribbean, 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).
Behaviour Most populations are migratory and dispersive and abandon their breeding sites at the end of the breeding season to overwinter at sea (Higgins and Davies 1996, Haney et al. 1999). Its detailed migratory movements are largely unknown however, and some populations in the Indian Ocean are entirely sedentary or only partially migratory (del Hoyo et al. 1996, 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) that are not strictly colonial but involve solitary pairs congregating in suitable habitat (del Hoyo et al. 1996, Haney et al. 1999). When nesting the species often associates with nesting Sooty Tern (Onychoprion fuscatus) or Greater Crested Tern (Thalasseus bergii) (del Hoyo et al. 1996). After breeding, the adults and newly fledged young leave the breeding colonies in loose flocks and migrate alone, in small groups of 10-12 individuals or more rarely in larger groups of up to 200 individuals (Higgins and Davies 1996, 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 coral, rock or rubble islands and beaches, volcanic stacks and exposed reefs, foraging inshore and up to 50 km offshore (although mostly within 15 km of land) and feeding from the surface of the water or up to 20 cm below it (Higgins and Davies 1996, del Hoyo et al. 1996, Haney et al. 1999). 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 which it uses for perching (del Hoyo et al. 1996, Haney et al. 1999). 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 or molluscs (del Hoyo et al. 1996, Higgins and Davies 1996).
Breeding site The nest is a scrape or depression in shingle or sand 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 around the rim of oceanic islands (del Hoyo et al. 1996), including natural cavities amongst rocks or coral rubble, in vegetation (up to 75 % ground cover), 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, del Hoyo et al. 1996). The species is not strictly colonial but solitary pairs usually congregate in suitable habitats with neighbouring nests spaced according to nest-site availability (usually 1-5 m apart, minimum 30 cm) (del Hoyo et al. 1996, Haney et al. 1999).
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 visitors (Haney et al. 1999).
Bridled Tern is known to have abandoned 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). 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). Neither of these are considered to represent a significant threat to the population.
Text account compilers
Everest, J., Bennett, S., Malpas, L., Martin, R., Calvert, R., Ekstrom, J., Butchart, S.
BirdLife International (2020) Species factsheet: Onychoprion anaethetus. Downloaded from http://www.birdlife.org on 25/01/2020. Recommended citation for factsheets for more than one species: BirdLife International (2020) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 25/01/2020.