Justification 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.

Family/Sub-family Laridae

Species name author Linnaeus, 1758

Taxonomic source(s) AERC TAC (2003), AOU checklist (1998 + supplements), Christidis and Boles (1994), Christidis and Boles (2008), Cramp and Simmons (1977-1994), Dowsett and Forbes-Watson (1993), SACC (2005 + updates), Sibley and Monroe (1990, 1993), Stotz et al. (1996), Turbott (1990)

Taxonomic note The BirdLife Taxonomic Working Group is aware that phylogenetic analyses have been published which have proposed generic rearrangements which may affect this species, but prefers to wait until work by other taxonomists reveals how these changes affect the entire groups involved.

Population estimate

Population trend

Range estimate (breeding/resident)

Country endemic?

1,600,000 - 4,600,000

unset

29,200,000 km²

No

Range & population The Common Tern has a circumpolar distribution and can be found breeding in most of Europe, Asia and North America except the extreme north and south. It winters further south, being found along the coast and inland of South America down to the Falkland Islands (Islas Malvinas), along the coast of Africa excluding the north, along parts of the Arabian Peninsula and the whole coast of India, and throughout much of south-east Asia and Australasia (excluding New Zealand)¹.

Important Bird Areas Click here to view map showing IBAs where species is recorded and triggers any of the IBA criteria.

Ecology: Behaviour This species is a strongly migratory coastal seabird^1,3. It breeds between April and June in solitary pairs or colonially in groups of up to several thousand pairs (inland colonies often smaller and more widely-dispersed than coastal ones)¹. Palearctic breeders migrate south after breeding between August and October, returning to the breeding grounds in March or April¹. The species may moult on arrival in its the wintering grounds (e.g. the Caribbean), during which it may become vulnerable to human exploitation⁹. It is gregarious throughout the year³ and shoals of fish may attract dense feeding flocks, although it otherwise feeds singly or in small loose groups^1,3. In some cases, it is territorial; in Massachusetts, feeding territories usually consist of linear strips of shoreline which are occupied and defended regularly by both members of a pair. The prevalence of territoriality at feeding sites varies between colonies and is related to food species availability¹⁵. Most individuals forage 5-10 km from breeding colonies, occasionally feeding at sea 15 km offshore¹. Common Terns forage over fresh water as well as marine habitats, and often follow predatory fish , waiting for panicking baitfish to surface. They sometimes forage in mixed-species flocks together with other terns^26,28. Habitat Breeding The species breeds in a wide variety of habitats in coastal and inland areas from sea-level to heights of greater than 4,000 m¹. Along the coast it shows a preference for nesting on flat rock surfaces on inshore islands³, open shingle and sandy beaches, dunes and spits^1,3, vegetated inter-dune areas, sandy, rocky, shell-strewn or well-vegetated islands in estuaries¹ and coastal lagoons³, saltmarshes^2,3, mainland peninsulas³ and grassy plateaus atop coastal cliffs¹. Inland it may nest in similar habitats including sand or shingle lakes shores², shingle banks in rivers³, sandy, rocky, shell-strewn or well-vegetated islands in lakes and rivers^1,3, sand- or gravel-pits^2,3, marshes, ponds, grassy areas and patches of dredged soil³. Non-breeding The species winters on sheltered coastal waters⁴, estuaries and along large rivers, occupying harbours, jetties, piers, beaches¹ and coastal wetlands including lagoons, rivers, lakes, swamps and saltworks, mangroves and saltmarshes⁴. During this season it roosts on unvegetated sandy beaches, shores of estuaries or lagoons, sandbars and rocky shores⁴. Diet The species is opportunistic, its diet consisting predominantly of small fish and occasionally planktonic crustaceans and insects¹. Breeding site The nest is a shallow depression on open substrates with little or no vegetation placed near a vertical object (e.g. rock, shell, plant or artefact) to provide shelter for chicks and to facilitate nest identification¹. Nest sites include the edges of bare sand amongst vegetation, rocks or logs, open areas on the margins of vegetation on beaches, the edges of mats of vegetation in marshes¹, and grassy or rocky substrates on rocky islets¹. The species will also readily nest on artificial rafts¹. Foraging range Studies have observed foraging birds at up to 37 km from the nearest colony^16,17. However, although Common Terns tend to range further than Roseate Terns¹⁷, most are observed within 10 km of a breeding colony^18,19,20. At various sites, visual observations e.g. of direction of flight also suggest that most feeding trips are to sites within 10 km of the colony^{17,21,22,23,24}, although birds in these studies were recorded at a maximum of 18 km from the colony²¹. Radio-tagging at the Wadden Sea has shown that birds forage at a mean radius of at least 6.3 km²⁰, although the total distance travelled by individuals on these foraging trips is much greater, around 26-30 km. In Massachusetts, feeding territories tend to be located in shallow water up to 75 m from shore, and may be at least 8.5 km away from the breeding colony¹⁵. In terms of the preferred type of foraging habitat, studies show that flocks prefer eutrophic, shallow bays^17,25, as well as areas where depth changes abruptly and tidal currents are strong, which presumably produces much upwelling and mixing of cold, deep water with warmer surface water²⁶. Birds lower the rate at which they attempt to forage when windspeed is high²⁷.

Threats During the breeding season the species is vulnerable to human disturbance at nesting colonies^{6, 8} (e.g. from off-road vehicles, recreation, motor-boats, personal watercraft and dogs)^{1, 9, 10, 11}, and to the flooding of nest sites as a result of naturally fluctuating water levels^{1, 8, 11}. On its breeding grounds the species is also threatened by habitat loss as a result of coastal developement^{1, 6, 8, 11}, erosion¹¹, vegetation overgrowth (rapid vegetation succession encroaching upon nesting habitats)^{1, 6, 11}, and chemical pollution (which may also result in eggshell thinning)^{1, 6, 11}. It suffers predation at nesting colonies from rats (especially on islands)^{1, 8} and from expanding populations of large gull species^{1, 7} such as Herring Gulls Larus argentatus⁸ (gulls may also prevent the species from nesting in the area by colonising it first)^{1, 6}. The species is susceptible to avian influenza so may be threatened by future outbreaks of the virus⁵. Utilisation The species is harvested semi-commercially on its wintering grounds in the Caribbean^{1, 9}.

Conservation measures underway Management techniques used to increase the breeding numbers and reproductive success of the species in the Great Lakes region of North America include creating artificial nesting sites, vegetation management, enhancement of existing nesting habitat, using models and vocalisation to attract breeding pairs and predator control (e.g. mammal-exclusion, destruction of gull nests, direct predator removal, or preventative measures against gull nesting)⁶. A conservation scheme for the protection of gull and tern breeding colonies in coastal lagoons and deltas (e.g. Po Delta, Italy) involves protection from human disturbance, prevention of erosion of islet complexes, habitat maintenance and the creation of new islets for nest sites¹². The scheme particularly specifies that bare islets with 30-100 % cover of low vegetation (sward heights less than 20 cm) should be maintained or created as nesting sites¹². Artificial nesting rafts have proved effective in promoting breeding success in areas where there is a lack of suitable nesting habitat or where human disturbance is a particular threat (1996)¹. Using fire to expose the ground surface in areas where vegetation succession is proceeding too far towards closed vegetation stages has been successful in some areas¹¹. Culling predatory gulls can be an effective management tool to enhance breeding productivity¹³, although some management plans recommend non-lethal harassment techniques that target gulls (e.g. egg and nest destruction, conspicuous human observers, gull displacement walks, and pyrotechnics) to reduce predation on nesting colonies rather than culling¹⁴.

References 1. del Hoyo et al. (1996). 2. Richards (1990). 3. Snow and Perrins (1998). 4. Higgins and Davies (1996). 5. Melville and Shortridge (2006). 6. Blokpoel and Scharf (1991). 7. Brown and Nettleship (1984). 8. Buckley and Buckley (1984). 9. van Halewyn and Norton (1984). 10. Burger (1998). 11. Hyde (1997). 12. Fasola and Canova (1996). 13. Guillemette and Brousseau (2001). 14. Donehower et al. (2007). 15. Nisbet (1983). 16. Cramp (1985). 17. BirdLife International (2000). 18. Wanless et al. (1998). 19. Newton and Crowe (1999). 20. Becker et al. (1993). 21. Duffy (1986). 22. Burness et al. (1994). 23. Hopkins and Wiley (1972). 24. Uttley et al. (1989). 25. Lemmetyinen (1973). 26. Safina (1990). 27. Taylor (1983). 28. Brenninkmeijer et al. (2002).

Further web sources of information

Detailed species account from Birds in Europe: population estimates, trends and conservation status (BirdLife International 2004)

Text account compilers Stuart Butchart (BirdLife International), Rob Calvert (BirdLife International), Jonathan Ekstrom (BirdLife International), Jess Hatchett (BirdLife International), Lucy Malpas (BirdLife International)

IUCN Red List evaluators Jeremy Bird (BirdLife International), Stuart Butchart (BirdLife International)

Recommended citation BirdLife International (2010) Species factsheet: Sterna hirundo. Downloaded from http://www.birdlife.org on 30/7/2010

This information is based upon, and updates, the information published in BirdLife International (2000) Threatened birds of the world. Barcelona and Cambridge, UK: Lynx Edicions and BirdLife International, BirdLife International (2004) Threatened birds of the world 2004 CD-ROM and BirdLife International (2008) Threatened birds of the world 2008 CD-ROM. These sources provide the information for species accounts for the birds on the IUCN Red List.

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