Rationale (Changed due to Criteria application)

The regional population of Antarctic Tern Sterna vittata breeding on the Prince Edward Islands satisfies criterion D for classification as Endangered, with a regional population well below the threshold of less than 250 mature individuals.

Distribution

The Antarctic Tern breeds on Subantarctic and Southern Ocean islands and on the Antarctic Peninsula (Higgins and Davies 1996) including the Prince Edward Islands. Birds on the Prince Edward Islands breed on the more sheltered eastern coasts (Ryan et al. 2003). Indian Ocean breeding birds, of the nominate race S. v. vittata, are moderately common winter visitors to the coastline and offshore islands of Western and Eastern Cape provinces (Tree and Klages 2004). It is probable, but not known with certainty, that these groups include wintering birds from the Prince Edward Islands. Birds in South African waters occur in loose flocks up to the continental shelf edge, sometimes up to 150 km offshore (Brooke et al. 1988).

Habitat View more ecological information

Population

The global population has not been quantified (BirdLife International 2014). Berruti and Harris (1976) estimated there to be 10-30 pairs on Marion Island. Between 1996/1997 and 2008/2009, the highest estimate at Marion Island was of 10 pairs (Whittington et al. 2009). Ryan and Bester (2008) estimated there to be 25 pairs on Marion Island and five pairs on Prince Edward Island. Confidence in this regional population estimate is however low and an urgent investigation is required.

Population trend

The global population is suspected to be in decline (BirdLife International 2014). From 1974 to 1977, the regional population of Antarctic Terns at Marion Island was less than50 pairs (Williams et al. 1979) decreasing in 1979 to less than 20 pairs (Brooke 1984) and then increasing slightly to less than 25 pairs in the 1980s (Cooper and Brown 1990). Numbers fluctuated between 1996/1997-2008/2009 from zero to 10 pairs without any apparent trend (Whittington et al. 2009). Four pairs were recorded at the adjacent Prince Edward Island in 2008 (Whittington et al. 2009). Confidence in this regional trend estimate is low.

Threats

Subsequent to the removal of cats at Marion Island there are no known, explicit threats to the Antarctic Terns breeding at the Prince Edward Islands. However, the population has failed to recover following eradication of cats, suggesting that other threats are acting on this species. The naturally small population size, particularly at Prince Edward Island, is an intrinsic threat. Because this is apparently a natural situation, mitigating the threat from small population size is not recommended. Because this species migrates northwards for winter (including to South Africa's coastline), it is possible that it experiences threats at wintering sites; this possibility should be investigated.

Conservation

Underway

None specific to this species but various general conservation and management measures are in place at the Prince Edward Islands.

Proposed

Until the fundamental aspects of the species' ecology are better understood, no conservation measures are proposed.

Research

* Dispersal during winter is to unknown areas, and should be investigated. Stable isotope studies of feathers (grown during winter) would provide information about broad-scale wintering grounds and possibly also about foraging preferences (e.g. near-shore or pelagic).

* Dietary studies during breeding at Marion Island may elucidate mechanisms preventing the expansion of the regional population, and could also shed light on the causes for decreases in the regional population of the closely related Kerguelen Tern S. virgata.

* Are any of reproductive success, juvenile survival, recruitment rates, adult survival or other demographic parameters preventing the population from expanding, or is food availability, inter-specific competition, nesting requirements or territory size a limiting factor?