|Avian Demography Unit
Department of Statistical Sciences
University of Cape Town
Antarctic Tern Sterna vittata
The Antarctic Tern is a regular winter visitor to South Africa. The first birds arrive about April in the Western Cape and May in the Eastern Cape and numbers build up to a peak in August. Departure takes place mainly in September and October, and is complete by November, with only isolated birds remaining in South Africa for the summer months.
It feeds at sea, and is often seen as far as 150 km offshore. The distribution at sea extends from about Hondeklipbaai in the Northern Cape to about Cape Padrone at the eastern limit of Algoa Bay in the Eastern Cape. It roosts gregariously, with other species of gulls and terns. Regular roosts lie in a narrower range than the "at sea" records, and are between Lamberts Bay and Cape Agulhas and between Cape St Francis and Bird Island, Algoa Bay. Beyond this area it is a vagrant, with records from as far as Walvis Bay in Namibia and from KwaZulu-Natal. Most roosts are on the offshore islands; on the mainland it uses headlands with flat rocks and sheltered sandy beaches. Large mainland roosts have been recorded at Grootpaternosterpunt (32 44S, 17 54E), Bekbaai (32 49S, 17 53E), Mauritzbaai (32 59S, 17 53E), North Head, Saldanha Bay (33 03S, 17 55E), Kommetjie (34 08S, 18 19E), Danger Point (34 38S, 19 17E) and Cape Recife (34 02S 25 43E). Many other headlands along the coast have smaller roosts.
There are probably only three well-defined subspecies which breed on most subantarctic islands and the Antarctic Peninsula during the southern hemisphere summer. Birds migrate north, to the southern coast of South Africa, to the eastern coast of southern South America and to southern New Zealand; it is a vagrant in Australia. Two of the subspecies have been recorded in South Africa: the race vittata which breeds on islands in southern Indian Ocean, and the race sanctipauli which breeds on islands in the Tristan du Cunha group in the south Atlantic Ocean and the Amsterdam St Paul group in the Indian Ocean. But it is possible that birds from other parts of the range also visit South Africa.
The world population has been estimated at about 45 000 pairs. At least 15 000 birds regularly visit South Africa. This is based on the sum of counts at diurnal roosts; it is probably an underestimate, because the species is known to roost at sea. Roosts exceeding 4000 birds have been seen at Dyer Island and at Dassen Island. The largest roost is at Bird Island in Algoa Bay, where numbers build up to exceed 5000 birds in August.
Prior to 1998, 249 Antarctic Terns had been ringed. These have delivered five remarkable recoveries. An immature bird ringed at Dyer Island was controlled nearly 20 years later on Gough Island in the south Atlantic Ocean. An adult ringed at Dassen Island was recaptured at its nest on Kerguelen Island, south Indian Ocean. Two adults ringed on Dyer Island have been recovered: one on the island nearly 15 years later, and one on the mainland close by six years later. One first-year bird ringed at Cape Recife, Eastern Cape, was recovered 90 km to the west six weeks later. Full details of these recoveries may be found in the Review of Ring Recoveries of Waterbirds in Southern Africa
Between July 1998 and September 2000, several expeditions to ring Antarctic Terns have been made to Bird Island, Algoa Bay. more than 1000 terns have been ringed, and colour leg flags were also fitted to 600 of these birds. A request to look out for these birds was made in September 2000 to all researchers visiting Southern Ocean islands. Within three months, there was an exciting response. Eric Woehler of the Australian Antarctic Division saw one of these leg flagged birds between Christmas and New Year on Heard Island. This is at 53 S 73 E, in the southern Indian Ocean and 4300 km from Bird Island.
These birds will be returning to the South African coastline from May onwards. If you have access to a roost, please scan each bird for leg flags. If any sightings of these birds are made, report them to Dieter Oschadleus at the South African Bird Ringing Unit (SAFRING).
In fact, all sightings of Antarctic Terns, either at sea or at roosts, can usefully be reported to the ADU's Bird Sightings system.