Migration of black terns Chlidonias niger and common terns Sterna hirundo between south Sweden and the Atlantic coast of Africa

Abstract

Light-level geolocators were used to record the annual migration cycles of black terns Chlidonias niger (9 individuals, 11 journeys) and common terns Sterna hirundo (7 individuals, 11 journeys) breeding in southernmost Sweden. The black terns used two different non-breeding (Oct–Mar) regions along the Atlantic coast of Africa, either north of the equator between Senegal and Liberia (3 individuals) or south of the equator between Gabon and northern Namibia (5 individuals). All the common terns travelled to non-breeding quarters south of the equator, mainly along the coasts of Namibia and South Africa. One juvenile common tern was tracked during the first twenty months of its life. This bird spent its first northern winter in South Africa, after which it migrated north of the equator to spend the northern summer as a one-year-old non-breeder in tropical waters off Ghana, after which it returned to South Africa for its second northern winter. This record demonstrates that one-year-old terns may undertake extensive intra-African migration to distant over-summering areas. Comparing geolocator results from Swedish and Dutch black tern populations indicate that they have similar migration habits, with a possible tendency of relatively more individuals migrating south of the equator in the more northerly Swedish population (leap-frog migration). Comparing geolocator and ringing results among common tern populations indicates a fascinating and complex pattern of scale-dependent geographic segregation and intermixing along the coasts of Africa.