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 unclear but it is not thought 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 c.1,600,000-3,600,000 individuals (Wetlands International 2015). The European population is estimated at 316,000-605,000 breeding pairs (BirdLife International 2015).
The overall population trend is unclear. Some populations may be stable, while others have unknown trends (Wetlands International 2015). In North America, the species has undergone a large and statistically significant decrease over the last 40 years (-70.4% decline over 40 years, equating to a -26.2% decline per decade; data from Breeding Bird Survey and/or Christmas Bird Count: Butcher and Niven 2007). In Europe, the population is estimated to be increasing (BirdLife International 2015).
This species has a circumpolar distribution and can be found breeding in most of Europe, Asia and North America, except the in the extreme north. 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 northern Africa), along parts of the Arabian Peninsula, the coast of India and throughout much of south-east Asia and Australasia, excluding New Zealand (del Hoyo et al. 1996).
Behaviour This species is a strongly migratory coastal seabird (del Hoyo et al. 1996, Snow and Perrins 1998). It breeds between April and June in solitary pairs or colonially in groups of up to several thousand pairs (del Hoyo et al. 1996). Palearctic breeders migrate south after breeding between August and October, returning to the breeding grounds in March or April (del Hoyo et al. 1996). The species may moult on arrival in its the wintering grounds, when it may become vulnerable to human exploitation (van Halewyn and Norton 1984). It is gregarious throughout the year (Snow and Perrins 1998). Shoals of fish may attract dense feeding flocks, although it otherwise feeds singly or in small loose groups (del Hoyo et al. 1996, Snow and Perrins 1998). 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 availability (Nisbet 1983). Most individuals forage 5-10 km from breeding colonies, occasionally feeding at sea 15 km offshore (del Hoyo et al. 1996). 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 (Safina 1990, Brenninkmeijer et al. 2002).
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 m (del Hoyo et al. 1996). Along the coast, it shows a preference for nesting on flat rock surfaces on inshore islands, open shingle and sandy beaches, dunes and spits, vegetated inter-dune areas, sandy, rocky, shell-strewn or well-vegetated islands in estuaries and coastal lagoons, saltmarshes, mainland peninsulas and grassy plateaus atop coastal cliffs (Richards 1990, del Hoyo et al. 1996, Snow and Perrins 1998). Inland, it may nest in similar habitats, including sand or shingle lakes shores, shingle banks in rivers, sandy, rocky, shell-strewn or well-vegetated islands in lakes and rivers, sand- or gravel-pits, marshes, ponds, grassy areas and patches of dredged soil (Richards 1990, del Hoyo et al. 1996, Snow and Perrins 1998). Non-breeding The species winters on sheltered coastal waters, estuaries and along large rivers, occupying harbours, jetties, piers, beaches and coastal wetlands including lagoons, rivers, lakes, swamps and saltworks, mangroves and saltmarshes (del Hoyo et al. 1996, Higgins and Davies 1996). During this season, it roosts on unvegetated sandy beaches, shores of estuaries or lagoons, sandbars and rocky shores (Higgins and Davies 1996).
Diet The species is opportunistic, its diet consisting predominantly of small fish and occasionally planktonic crustaceans and insects (del Hoyo et al. 1996).
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 identification (del Hoyo et al. 1996). 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 marshes, and grassy or rocky substrates on rocky islets (del Hoyo et al. 1996). The species will also readily nest on artificial rafts (del Hoyo et al. 1996).
Foraging range Studies have observed foraging birds at up to 37 km from the nearest colony (Cramp 1985, BirdLife International 2000). However, although Common Terns tend to range further than Roseate Terns (BirdLife International 2000), most are observed within 10 km of a breeding colony (Becker et al. 1993, Wanless et al. 1998, Newton and Crowe 1999). 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 (Hopkins and Wiley 1972, Duffy 1986, Uttley et al. 1989, Burness et al. 1994, BirdLife International 2000), although birds in these studies were recorded at a maximum of 18 km from the colony (Duffy 1986). Radio-tagging at the Wadden Sea has shown that birds forage at a mean radius of at least 6.3 km (Becker et al. 1993), 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 colony (Lemmetyinen 1973, Nisbet 1983, BirdLife International 2000), 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 water (Safina 1990). Birds lower the rate at which they attempt to forage when windspeed is high (Taylor 1983).
During the breeding season, the species is vulnerable to human disturbance at nesting colonies (Buckley and Buckley 1984, Blokpoel and Scharf 1991), with sources of disturbance including off-road vehicles, recreation, motor-boats, personal watercraft and dogs (van Halewyn and Norton 1984, Hyde 1997, Burger 1998, Gochfeld et al. 2018). The flooding of nest sites as a result of naturally fluctuating water levels can also result in complete breeding failure of the effected colony (Buckley and Buckley 1984, Hyde 1997, Gochfeld et al. 2018). Previously, this species has experienced significant declines due to egging, hunting and the millinery trade (Gochfeld et al. 2018). Although in most places populations have recovered, hunting remains a significant problem in some areas, including parts of West Africa, where human predation results in a fledging success rate of only 12% (Gochfeld et al. 2018).
Conservation Actions 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) (Blokpoel and Scharf 1991). 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 sites (Fasola and Canova 1996). 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).
Conservation Actions Proposed
Maintain bare islets with 30-100 % cover of low vegetation (sward heights less than 20 cm) or artificially create some as nesting sites (Fasola and Canova 1996). Offer artificial nesting rafts to promote breeding in areas where there is a lack of suitable nesting habitat or where human disturbance is a particular threat (del Hoyo et al. 1996). Use controlled fire to expose the ground surface in areas where vegetation succession is proceeding too far towards closed vegetation stages (Hyde 1997). Enhance breeding productivity by deterring predatory gulls, either by culling or by the use of non-lethal harassment techniques, e.g. egg and nest destruction, conspicuous human observers, gull displacement walks, and pyrotechnics (Guillemette and Brousseau 2001, Donehower et al. 2007).
Text account compilers
Butchart, S., Calvert, R., Ekstrom, J., Ashpole, J, Hatchett, J., Malpas, L., Martin, R., Stuart, A.
BirdLife International (2019) Species factsheet: Sterna hirundo. Downloaded from http://www.birdlife.org on 17/11/2019. Recommended citation for factsheets for more than one species: BirdLife International (2019) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 17/11/2019.