Justification of Red List Category
This species has an extremely 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 c.200,000-220,000 individuals (Wetlands International 2015). The European population is estimated at 2,300-2,900 pairs, which equates to 4,500-5,800 mature individuals (BirdLife International 2015). National population estimates include: c.100-10,000 breeding pairs and c.50-1,000 individuals on migration in China; c.100-100,000 breeding pairs and c.50-10,000 individuals on migration in Taiwan and c.100-10,000 breeding pairs and c.50-1,000 individuals on migration in Japan (Brazil 2009).
The overall population trend is uncertain, as some populations are decreasing, while others are increasing or stable (Wetlands International 2015). The European population is estimated to be increasing (BirdLife International 2015).
This species breeds in widely but sparsely distributed colonies along the east coast and offshore islands of Canada, U.S.A., from Honduras to Venezuela, possibly to Brazil, the Caribbean (including the Bahamas, Greater and Lesser Antilles and the West Indies), U.K., France, Ireland, Portugal (Azores, Salvages and perhaps Madeira), Spain (Canary Islands), South Africa, Kenya, Somalia, Madagascar, Oman, Seychelles, St Brandon and the Mascarene Islands (Mauritius), Maldives, Chagos (British Indian Ocean Territory), Andaman and Nicobar Islands (India), Sri Lanka, Ryukyu Islands (Japan), Indonesia, Fiji, Solomon Islands, New Guinea (Papua New Guinea), New Caledonia (to France) and Australia (del Hoyo et al. 1996, Snow and Perrins 1998). The population in North America underwent a significant decline in 40 years, decreasing from 8,500 pairs in the early 1930s to 2,500 in 1978 (Brown and Nettleship 1984). Numbers, however, appear to have stabilised at 3,000 pairs (del Hoyo et al. 1996). In Nova Scotia, the decline was from 200 to 32 pairs (Brown and Nettleship 1984). Between 1969 and 1992, the U.K. population declined from 1,018 pairs to 57, and pairs in Ireland dropped from 1,435 to 454 (Sprunt 1984). In 1995, however, over 1,700 pairs bred in Europe and the latest estimate is 2,300-2,900 pairs breeding in Europe (BirdLife International 2015). The French population is 9-58 pairs (BirdLife International 2015) which may be a decline from c.500 in 1973. The large Azores population has fluctuated between 550 and 1,028 pairs from 1989/90 to 1995 (Snow and Perrins 1998) with the most recent estimate at c. 840-1,350 pairs (BirdLife International 2015). The population in the Republic of Ireland is estimated at 1,300 pairs (BirdLife International 2015). The tropical Indian Ocean may be the most secure region for this species (Feare 1984).
Behaviour This is a migratory coastal seabird that feeds by plunge diving. It dives from a greater height than other terns. The species breeds in large, dense single- or mixed-species colonies that may contain several thousands of pairs (del Hoyo et al. 1996). It remains gregarious throughout the year, roosting in large groups (Urban et al. 1986, Snow and Perrins 1998) and feeding singly, in small loose groups (Snow and Perrins 1998) or in flocks of many hundreds of individuals (Urban et al. 1986, Snow and Perrins 1998). It is regularly found in mixed species flocks with Lesser Noddy (Anous tenuirostris) and White Tern (Gygis alba) (Ramos 2000). When mixing with the former in conjunction with predatory fish, breeding success was markedly better (Ramos 2000). Large, dense foraging flocks are associated with higher rates of chick feeding (Ramos 2000). Habitat The species nests on sand-dunes, sand-spits, shingle beaches, reefs (Snow and Perrins 1998), saltmarshes and rocky, sandy or coral islands (del Hoyo et al. 1996), showing a preference for densely vegetated sites in temperate regions but sparsely vegetated sites in the tropics (del Hoyo et al. 1996). It also shows a preference for nest sites close to clear, shallow, sandy fishing grounds (Snow and Perrins 1998) in tidal bays and sheltered inshore waters (Snow and Perrins 1998). Throughout the year the species often rests and forages in sheltered estuaries, creeks (Urban et al. 1986), inshore waters and up to several kilometres offshore (del Hoyo et al. 1996), moving to warm tropical coasts after breeding (Snow and Perrins 1998). Diet This species is a specialist forager, and takes a small prey spectrum compared to Common Tern at the same sites (Birdlife International 2000). Its diet consists predominantly of small pelagic fish (Urban et al. 1986, del Hoyo et al. 1996), particularly sandeel (Birdlife International 2000, Newton and Crowe 2000) and sprat (Birdlife International 2000) and sometimes clupeids (Birdlife International 2000, Newton and Crowe 2000) and gadoids (Newton and Crowe 2000), although it will also take insects and marine invertebrates (del Hoyo et al. 1996) such as crustaceans (Urban et al. 1986). Sandeel are particularly important during chick rearing (Newton and Crowe 2000). In Puerto Rico, adult Roseate Terns fed primarily on dwarf herrings (Jenkinsia lamprotaenia) and anchovies (Anchoa spp.), and chicks were mostly fed dwarf herrings and sardines (Harengula and Opisthonema spp.); few anchovies were fed to chicks (Shealer 1998). Breeding site The nest is a bare scrape in sand, shingle or coral rubble (del Hoyo et al. 1996), preferably in sites surrounded by walls and rocks (Newton and Crowe 2000) or in the shelter of vegetation (in temperate regions) (Richards 1990, Snow and Perrins 1998), also in crevices between and under rocks, or in the entrances to rabbit or Puffin burrows (Snow and Perrins 1998). Foraging range At various colonies in New York, USA, birds were observed to forage at sites up to 30 km away from their breeding colony, although at most sites, most birds foraged within 10 km (Birdlife International 2000). Similarly, in Massachusetts, USA, birds foraged at up to 30 km from the breeding colony (Shealer 1996). However, in Puerto Rico birds fed within 2 km of the colony (Shealer 1998). In Ireland, birds at Lady's Island Lake tended to forage about 5 km from the colony at a site 3 km offshore (Newton and Crowe 2000). At Rockabill, Ireland, during chick rearing, birds fed within 10 km of the colony in offshore, relatively deep water (20 - 30 m), but during incubation and post-fledging they appeared to be travelling tens of kilometres to feed over sandbanks to the south (Newton and Crowe 2000). The species may be either coastal or more pelagic in nature, depending on the colony location (Newton and Crowe 2000). Throughout their range they forage in habitats where prey availability is high. Temperate populations feed over tide rips (Birdlife International 2000), shoals (Birdlife International 2000, Environment Canada 2006), inlets (Birdlife International 2000), upwelling areas, and predatory fish that force prey to the surface (Ramos 2000). In Puerto Rico, Roseate Terns feed primarily in deep, open water, and rely heavily on predatory fish to drive prey fish to the surface (Shealer 1998). In the Caribbean, they feed primarily over shoals of predatory fish or along reef margins (Birdlife International 2000). In North American parts of the north-west Atlantic, Roseate Terns appear to use one of two strategies: either foraging over tide-rips, sand shoals and sandbars (Safina 1990, Shealer 1996, Gochfeld et al. 1998), in some cases up to 20-30 km from the colony, or more pelagically in deeper water over schools of predatory fish which flush prey fish species to the surface (Shealer 1996, Gochfeld et al. 1998). Where predatory fish are not relied upon, the birds forage over sandy substrates (Sheer and Kress 1994) in water under 10 m deep (Safina 1990, Shealer and Kress 1994). On Aride Island, Seychelles, birds concentrated their foraging along the coastline exposed to prevailing winds (Ramos 2000).
Trapping of adults and egg collecting continues in colonies in East Africa and the Caribbean, while trapping of adults in wintering areas is frequent in West Africa and north east South America (Gochfeld et al. 2018). The species is also thought to suffer from bycatch mortality in the Seychelles, where a large decline coincided with a considerable increase in long-line activity (Gochfeld et al. 2018). Predation can have significant effects, including complete breeding failure at some Azores colonies (Avery et al. 1995). Habitat loss in northern Europe is not a major problem but has caused the local extinction of some colonies, as have extreme weather events (Avery et al. 1995).
Conservation Actions Underway
Breeding birds in Europe are fully protected by national and international law but at sea outside of European territorial waters or in the coastal waters of other countries protection is limited (Tucker and Heath 1994). An action plan for the recovery of the European population was launched in 1987 (Avery 1987) and most of its recommendations have been implemented (Avery et al. 1995). An International Species Action Plan was published in 1999 (Newbery 1999). Habitat restoration work was carried out on Praia Islet in the Azores Archipelago, including rabbit eradication, control of soil erosion, reintroduction of native plants and installation of nest boxes for the species (Bried et al. 2009).
Text account compilers
Wheatley, H., Harding, M., Hatchett, J., Ashpole, J, Malpas, L., Butchart, S., Martin, R., Stuart, A., Ekstrom, J.
BirdLife International (2020) Species factsheet: Sterna dougallii. Downloaded from http://www.birdlife.org on 24/11/2020. Recommended citation for factsheets for more than one species: BirdLife International (2020) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 24/11/2020.