Justification of Red List Category
This species has a very 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 unknown, but is not believed to be declining sufficiently rapidly 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 population of S. v. sanctipauli is decreasing, but trends for the remainder of populations, and therefore the overall trend, is unknown (Delany and Scott 2006).
The Antarctic Tern can be found breeding on a large number of islands in the Southern Oceans and off the coast of Antarctica. Some birds from the south of its range have been found wintering on the coast of Argentina and South Africa.
Behaviour Breeding populations in the southern part of this species's range are migratory, post-breeding flocks migrating long distances to winter off the southern coasts of South America and South Africa (del Hoyo et al. 1996). Those populations that winter in South America arrive from mid April and depart again from mid-October, during which time the adults moult their flight feathers (del Hoyo et al. 1996). Some populations around Antarctica remain close to their breeding grounds all year round however and moult on ice-floes or icebergs on open water (del Hoyo et al. 1996). The species breeds between November and December although the exact timing varies depending on climate and food availability (del Hoyo et al. 1996). It usually nests in small loose colonies of 5-20 pairs, although it may often nest singly and has been known to nest in larger colonies of up to 1,000 pairs (del Hoyo et al. 1996). It forages in inshore waters singly or in small flocks (Urban et al. 1986, del Hoyo et al. 1996) and in the winter communal roosts of 10-1,200 individuals often form (Urban et al. 1986).
Habitat Breeding The species breeds on rocky areas very near the coast or a short distance inland (Higgins and Davies 1996), showing a strong preference for nesting sites that are inaccessible to ground predators (del Hoyo et al. 1996). Suitable nesting habitats include vegetated or unvegetated rocky islets, offshore stacks, coastal cliffs, gravel, rocky and sandy beaches and sparse scrubland (del Hoyo et al. 1996). The species forages in inshore waters up to 200 m from the shore and in coves, bays, inlets, harbours and off estuaries, especially where there are large forests of kelp (Higgins and Davies 1996). Non-breeding Outside of the breeding season the species moves to the nearest area of open water or to pelagic zones far from land (Higgins and Davies 1996) where it forms communal roosts on ice-floes and icebergs (Higgins and Davies 1996) and forages in patches of unfrozen inshore water or in open water along the edge of ice. More migratory populations also winter off the temperate southern coasts of South America and South Africa with adjacent cold water currents (Higgins and Davies 1996), inhabiting rocky headlands and beaches (del Hoyo et al. 1996).
Diet Its diet consists predominantly of small fish although it also takes polychaetes, molluscs (e.g. limpets), crustaceans (e.g. euphausiids and amphipods), insects and algae (del Hoyo et al. 1996, Higgins and Davies 1996).
Breeding site The species nests in natural depressions in rock or in shallow scrapes in soil, sand or vegetation (del Hoyo et al. 1996) that may be positioned on ledges or crevices of sheer cliffs, boulders at the base of cliffs, headlands, stacks, rocky islets, ridges, spits and peninsulas, rock fields by freshwater, and beaches of gravel, coarse shingle and sand (Higgins and Davies 1996).
On Macquarie Island, cat predation was thought to be restricting the population to offshore stacks for breeding (Schulz and Gales 2004, Garnett et al. 2010). The eradication of cats is likely to benefit the species.
Text account compilers
Ekstrom, J., Calvert, R., Malpas, L., Butchart, S., Martin, R., Stuart, A., Symes, A.
BirdLife International (2018) Species factsheet: Sterna vittata. Downloaded from http://www.birdlife.org on 11/12/2018. Recommended citation for factsheets for more than one species: BirdLife International (2018) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 11/12/2018.