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 appears to be increasing, and hence the species does not approach the thresholds for Vulnerable 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.250,000-470,000 individuals (Wetlands International 2015). The European population is estimated at 11,800-14,800 pairs, which equates to 23,600-29,600 mature individuals (BirdLife International 2015). National population estimates include: c.50-1,000 individuals on migration and c.50-1,000 wintering individuals in China; c.50-1,000 individuals on migration and c.50-1,000 wintering individuals in Taiwan; < c.50 individuals on migration and < c.50 wintering individuals in Japan and c.100-10,000 breeding pairs and c.50-1,000 individuals on migration in Russia (Brazil 2009).
The overall population trend is increasing, although some populations are decreasing, stable, or have unknown trends (Delany & Scott 2006). This species has undergone a large and statistically significant increase over the last 40 years in North America (266% increase over 40 years, equating to a 38.3% increase per decade; data from Breeding Bird Survey and/or Christmas Bird Count: Butcher and Niven 2007). The European population trend is estimated to be increasing (BirdLife International 2015).
This species has a cosmopolitan but scattered distribution. Their breeding habitat is large lakes and ocean coasts in North America (including the Great Lakes), and locally in Europe (mainly around the Baltic Sea and Black Sea), Asia, Africa, and Australasia (Australia and New Zealand). North American birds migrate to southern coasts, the West Indies and northernmost South America. European and Asian birds winter in the Old World tropics. African and Australasian birds are resident or disperse over short distances (del Hoyo et al. 1996).
Behaviour Although populations breeding near the equator are largely sedentary (Richards 1990), northern populations are strongly migratory and disperse after breeding before migrating southwards (del Hoyo et al. 1996). The species breeds between April and June (northern Hemisphere) or between September and December (southern Hemisphere) (del Hoyo et al. 1996) in large dense monospecific colonies or as single pairs or small groups (2-3 pairs) amidst large colonies of other species (Flint et al. 1984, del Hoyo et al. 1996, Snow and Perrins 1998). It is not a highly gregarious species outside of the breeding season but may aggregate into flocks on passage, and during the winter it may feed in loose congregations in rich fishing areas or at nightly roost sites (Urban et al.1986, del Hoyo et al. 1996, Snow and Perrins 1998). Habitat The breeding, passage and wintering habitats of this species are similar, although during the winter it is largely confined to the coast (Shuford and Craig 2002). It frequents sheltered sea coasts, estuaries, inlets, bays, harbours, coastal lagoons, saltmarshes and saltpans, also occurring inland on fresh or saline wetlands including large lakes, inland seas, large rivers, creeks, floodlands, reservoirs and sewage ponds (Flint et al. 1984, Martin and Randall 1987, Richards 1990, Higgins and Davies 1996, del Hoyo et al. 1996, Snow and Perrins 1998). When breeding the species shows a preference for nesting on sandy, shell-strewn or shingle beaches, sand-dunes, flat rock-surfaces, sheltered reefs or islands with sparse vegetation and flat or gently sloping margins surrounded by clear, shallow, undisturbed waters (Flint et al. 1984, Higgins and Davies 1996, del Hoyo et al. 1996, Snow and Perrins 1998). It also forms winter roosts on sandbars, mudflats and banks of shell (del Hoyo et al. 1996). Diet Its diet consists predominantly of fish 5-25 cm in length also including the eggs and young of other birds, carrion, aquatic invertebrates (e.g. crayfish), flying insects and earthworms (Flint et al. 1984, Urban et al. 1986, del Hoyo et al. 1996, Shuford and Craig 2002). Breeding site The nest is a shallow depression in the sand, gravel, shells, sparse vegetation or dried mud of ridges, beaches, dunes, flat rocky or stony islets, banks, islands or reefs in seas, lakes and large rivers, dredge spoil piles and islands in reservoirs (Flint et al. 1984, Richards 1990, Higgins and Davies 1996, del Hoyo et al. 1996, Snow and Perrins 1998). The species nests in large colonies or as single pairs or small groups amidst colonies of other species, neighbouring nests placed between 0.7 and 4 m part (del Hoyo et al. 1996). The species may forage up to 60 km from the site of the breeding colony (del Hoyo et al. 1996). Management information Management for this species should include a number of measures such as habitat and vegetation management, the use of artificial nest substrates, predator management (e.g. control of gull populations) and the minimisation of disturbance (Shuford and Craig 2002). Habitat and vegetation management may include the creation of artificial islands with calm water on their leeward side to allow the growth of submerged vegetation and fish spawning habitats, or alternatively the creation of floating artificial nesting-rafts (e.g. barges covered with sand) (Shuford and Craig 2002).
Human disturbance represents a significant threat to this species. Caspian Tern colonies are especially vulnerable to disturbance during the early courtship and incubation stages, during which human visitation results in the flushing of the whole colony, potentially leading to nest or colony abandonment (Cuthbert and Wires 1999 per Shuford and Craig 2002). The effect of disturbance during research activities has been well quantified, with activities such as ringing and cannon-netting representing a large proportion of the reproductive failure in colonies studied (Shuford and Craig 2002). Disturbance due to recreational activities are less well quantified but likely also significant (Shuford and Craig 2002). Much of the loss of reproductive success due to human disturbance occurs through gull predation of chicks during the few seconds of panic flight in which the eggs and chicks of a nest are left exposed (Shuford and Craig 2002). Historically, overexploitation has resulted in large population declines through the collection of eggs and adults for food and feathers (Shuford and Craig 2002). Shooting of birds is still thought to occur, but on a much smaller scale than in the past (Shuford and Craig 2002).
Conservation Actions Underway
The following information refers to the species's European range only: The species is listed on Appendix II of the Convention on Migratory Species. It is covered by the African Eurasian Waterbird Agreement. It is listed on Appendix II of the Bern Convention. In the EU it is listed on Annex I of the Birds Directive. There are 66 Important Bird Areas identified for this species within Europe. Within the EU there are 320 Special Protection Areas which include this species.
Conservation Actions Proposed
The following information refers to the species's European range only: Management of existing protected areas for site based threats. On board observer programmes to monitor bycatch events on fishing vessels and implement mitigation measures where appropriate.
Text account compilers
Ashpole, J, Martin, R., Stuart, A., Calvert, R., Ekstrom, J., Butchart, S., Malpas, L.
BirdLife International (2021) Species factsheet: Hydroprogne caspia. Downloaded from http://www.birdlife.org on 22/10/2021. Recommended citation for factsheets for more than one species: BirdLife International (2021) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 22/10/2021.