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 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 at 800,000-1,750,000 individuals (Wetlands International 2015). The European population is estimated at 74,400-154,000 pairs, which equates to 149,000-308,000 mature individuals (BirdLife International 2015).
The overall population trend is declining (Wetlands International 2015). In North America, this species has undergone a small or statistically insignificant decrease over the last 40 years (data from Breeding Bird Survey and/or Christmas Bird Count: Butcher and Niven 2007). The European population trend is unknown (BirdLife International 2015).
This species is found in the Old and New World. It ranges from southern Scandinavia to southern Spain, east through Europe and western Asia to central Mongolia. Individuals from this area predominately winter on the Atlantic coast of Africa, from the Western Sahara to South Africa. It is also found across much of Canada to northern regions of the U.S.A., with individuals wintering on the Pacific coast of Mexico, the Pacific and Atlantic coast of Central America and northern South America (del Hoyo et al. 1996).
Behaviour This species is strongly migratory and travels both over land and over sea (del Hoyo et al. 1996, Snow and Perrins 1998). It breeds between May and June in colonies, usually of less than 20 pairs (rarely more than 100 pairs) and often close to other species (Flint et al. 1984, del Hoyo et al. 1996, Snow and Perrins 1998). After breeding, it departs for its wintering grounds from July onwards, returning north again from late-March (Richards 1990, Snow and Perrins 1998). The species is gregarious throughout the year, foraging in groups of 2-20 during the breeding season and congregating in large flocks offshore on passage and in the winter over shoals of predatory fish (Richards 1990, del Hoyo et al. 1996, Snow and Perrins 1998).
Habitat Breeding The species breeds on fresh or brackish wetlands such as small pools, lakes, marshes, ditches, overgrown canals, quiet reaches of rivers, swampy meadows, peat bogs and rice-fields, showing a preference for well-vegetated areas with sparse, open emergent vegetation (e.g. Typha spp., sedge or reeds) and floating water-lilies, and with water 1-2 m deep (Flint et al. 1984, Richards 1990, del Hoyo et al. 1996). It generally avoids small marshland areas less than 4 ha in area (del Hoyo et al. 1996). Non-breeding On passage, the species frequents inland wetlands including pools, ditches, reservoirs, lakes and sewage farms, as well as coastal habitats and estuaries (del Hoyo et al. 1996, Snow and Perrins 1998). In winter, it is predominantly coastal, frequenting estuaries, saltmarshes, bays, coastlines and coastal lagoons as well as marine waters up to 400-600 km offshore (Urban et al. 1986, del Hoyo et al. 1996, Snow and Perrins 1998).
Diet Breeding Its breeding diet consists predominantly of insects (e.g. chironomids, Odonata, Ephemeroptera and Coleoptera) as well as small fish and amphibians (e.g. tadpoles and frogs) (del Hoyo et al. 1996, Snow and Perrins 1998). Non-breeding On passage and during the winter, the species's diet consists largely of marine fish, although insects and crustaceans may also be taken (Snow and Perrins 1998).
Breeding site The nest may be a low compressed mound of plant matter placed in very shallow water or on a floating mat of aquatic vegetation over water more than 50 cm deep (Flint et al. 1984, del Hoyo et al. 1996, Snow and Perrins 1998). The nest may also be a shallow scrape on the ground amongst marsh vegetation (Snow and Perrins 1998). The species breeds in small colonies and may forage up to 2-5 km from breeding sites (del Hoyo et al. 1996).
As Black Terns breed in wetlands on floating rafts of plant matter, the reduction of water level due to droughts and water abstraction for domestic and agricultural use represents a significant threat to the species due to loss of breeding habitat. The loss of rafts used for breeding represents a source of lost reproductive success, as shown by increases in breeding populations on the provisioning of artificial rafts (Gochfeld et al. 2018). Drought also has the potential to reduce reproductive success, as lower water levels allow the access of chicks by predators such as pigs. Encroachment of agriculture on wetlands has resulted in further loss and degradation of habitat. Introduction of the peacock bass to Lake Gutan in Panama resulted in the elimination of the Silverside, Meleniris chagresi, a significant prey species of the Black Tern, causing abandonment of this wintering site (Gochfeld et al. 2018).
Conservation Actions Underway
The following information refers to the species's European range only: The following information refers to the species's European range only: Bern Convention Appendix II. CMS Appendix II. The species has been included as a target species in two EU LIFE Projects. In the Netherlands the provision of anchored artificial nesting rafts has been partly successful as a conservation measure (van der Winden et al. 2004, 2005), especially in habitats where unstable nest substrates (such as floating water-lilies) result in poor breeding successes (van der Winden et al. 2004). In the Netherlands there have also been successful programmes to reduce disturbance and improve habitat quality in agricultural areas, which has benefited the species (van der Winden 2002).
Conservation Actions Proposed
The following information refers to the species's European range only: The application of glyphosate-based herbicides to combat and prevent the overgrowth of Typha spp. in wetlands may also benefit the species (Linz and Blixt 1997).
Text account compilers
Malpas, L., Ashpole, J, Ekstrom, J., Butchart, S., Martin, R., Stuart, A.
BirdLife International (2019) Species factsheet: Chlidonias niger. Downloaded from http://www.birdlife.org on 15/09/2019. Recommended citation for factsheets for more than one species: BirdLife International (2019) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 15/09/2019.