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). Despite the fact that the population trend appears to be decreasing, the decline is not believed to be sufficiently rapid to approach the thresholds for Vulnerable under the population trend criterion (>30% decline over ten years or three generations). The population size is extremely 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 >2,000,000 individuals (Wetlands International 2015). The European population is estimated at 564,000-906,000 pairs, which equates to 1,130,000-1,810,000 mature individuals (BirdLife International 2015). The population in Russia has been estimated at c.10,000-100,000 breeding pairs and c.1,000-10,000 individuals on migration (Brazil 2009).
The overall population trend is decreasing (Wetlands International 2015). In Europe, the population size is estimated to be decreasing by less than 25% in 40.2 years (three generations) (BirdLife International 2015).
This species has a circumpolar range, breeding in the Arctic and subarctic regions of Europe, Asia and North America as far south as Brittany (France) and Massachusetts (U.S.A.). It is a transequatorial migrant, and can be found wintering throughout the Southern Ocean to the edge of the Antarctic ice and the southern tips of South America and Africa (del Hoyo et al. 1996). Overall population trends are unknown.
Behaviour The species is a strong migrant and makes exceptional long-distance movements offshore or along western continental coastlines between its high Arctic breeding grounds and Antarctic wintering grounds (del Hoyo et al. 1996, Melville and Shortridge 2006). It breeds between May and July (although the exact timing varies with temperature and food availability) in solitary pairs or colonies of a few to several hundred pairs (usually 2-25) (del Hoyo et al. 1996), and remains gregarious throughout the year especially when roosting, foraging and on passage (Higgins and Davies 1996, Snow and Perrins 1998). The species generally feeds within 3 km of breeding colonies, but may occasionally forage up to 50 km away (del Hoyo et al. 1996). On its wintering grounds in Antarctica, it may also forage in association with Antarctic Minke Whale Balaenoptera bonaerensis in the open ocean north of the pack-ice zone (Higgins and Davies 1996).
Habitat Breeding The species breeds along northern coastlines and on inshore islands, as well as inland on tundra and forest-tundra (Flint et al. 1984, del Hoyo et al. 1996, Snow and Perrins 1998). It shows a preference for habitats with a vegetation cover of less than 40 %, nesting on sand or shingle beaches, ridges and spits, rocky ground and small islands in lakes and coastal lagoons (Flint et al. 1984, Richards 1990, del Hoyo et al. 1996, Snow and Perrins 1998). It may also nest on islets or banks along rivers, on swampy tundra and peatlands with bog hummocks and reed-covered flats, or on inland heaths, rough pastures, meadows and sedge grassland not far from water (Flint et al. 1984, del Hoyo et al. 1996, Snow and Perrins 1998). The species also forages offshore, in ice-filled coastal bays or over wet tundra (del Hoyo et al. 1996). Non-breeding On passage, it largely flies over open ocean, resting at sea on kelp, logs or flotsam, but may occur inland or along coastlines on beaches, reefs and spits (Higgins and Davies 1996, Snow and Perrins 1998). During the winter, the species is pelagic, foraging at the edges of pack-ice, icebergs and ice-floes near shore (especially in channels between ice-floes) and up to 24 km offshore, often in association with Antarctic Minke Whale Balaenoptera bonaerensis (Higgins and Davies 1996, del Hoyo et al. 1996). It also roosts on ice-floes and icebergs during this season (Higgins and Davies 1996).
Diet Its diet consists predominantly of fish as well as crustaceans (especially planktonic species), molluscs, insects (e.g. caterpillars, Chironomidae) and earthworms (del Hoyo et al. 1996). It will also take berries in the early spring on arrival on its breeding grounds, but does not readily switch to other prey items when preferred prey supplies fail (del Hoyo et al. 1996).
Breeding site The nest is a shallow scrape in sand, shingle or turf on beaches, ridges and spits, rocky ground, small islands in lakes, coastal lagoons and rivers, swampy tundra and peatlands with bog hummocks and reed-covered flats, or on inland heaths, rough pastures, meadows and sedge grassland not far from water (Flint et al. 1984, Richards 1990, del Hoyo et al. 1996, Snow and Perrins 1998). It will also nest on artificial structures (del Hoyo et al. 1996).
Arctic Tern colonies are threatened by invasive American Mink Neovison vison, which have been shown to reduce tern reproductive success through nest predation and increase the risks taken by adults in defending the nest (Nordström et al. 2004). It has been shown, however, that control measures are helping to reduce predation (Mavor et al. 2005) and the long-term removal of mink reduces risk taking by adults during nest defence (Nordström et al. 2004). Arctic Terns also suffer a loss of reproductive success due to egging, which is ongoing in some areas. In some areas, such as Belcher Island and Resurrection Bay, Alaska, egging may have caused the abandonment of colonies (Gilchrist and Robertson 1999, Griswold 2016), with the latter occurring after the reported harvesting of every Arctic Tern egg present in the colony (Griswold 2016).
Arctic Terns are projected to lose 20-50% of their habitat due to the temperature changes linked to a doubling of CO2 (BirdLife International unpublished). Sandeel is a key prey item, and major breeding failures are caused by food shortage (Schreiber and Kissling 2005). Increasing sea temperatures in the North Sea have been linked to reduced sandeel recruitment, likely causing a population decline (Vigfusdottir et al. 2013). Low sandeel populations have been linked with breeding failure through abandonment of clutches and chick starvation (Vigfusdottir 2012). Climate change already seeing shifts in copepod Calanus finmarchicus, underpinning much of food web (Frederiksen et al. 2013), which will likely have knock-on effects on sandeels. Further, in some areas, sandeel populations are decreasing due to overfishing. Dispersal ability and carrying capacity of northern areas will dictate population dynamics of Arctic Tern. Models predict that in the UK, only the Orkney Islands, the Shetland Islands and the northerly tip of Scotland will maintain Arctic Tern breeding colonies (Daunt and Mitchell 2013).
Conservation Actions Underway
The following information refers to the species's European range only: CMS Appendix II. EU Birds Directive Annex I. Bern Convention Appendix II. In the Baltic Sea, removing feral American Mink (Neovison vison) from a large archipelago with many small islands resulted in an increase in the breeding density of this species in the area (Nordström et al. 2003).
Conservation Actions Proposed
The following information refers to the species's European range only: Gull control measures may be practiced successfully at some sites to reduce predation and displacement, especially when carried out in conjunction with the use of recordings and models to induce recolonisation of nesting terns (Buckley and Buckley 1984). The species also benefits from the removal of American Mink (Nordström et al. 2003).
Text account compilers
Butchart, S., Calvert, R., Ashpole, J, Ekstrom, J., Hibble, R., Malpas, L., Martin, R., Stuart, A.
BirdLife International (2019) Species factsheet: Sterna paradisaea. Downloaded from http://www.birdlife.org on 19/07/2019. Recommended citation for factsheets for more than one species: BirdLife International (2019) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 19/07/2019.