A postnuptial staging site for the Black Tern (Chlidonias niger) and White-winged Tern (C.leucopterus) in the Sivash, Ukraine | Ein Mauserplatz für Trauerseeschwalben (Chlidoniasniger) und Weißflügelseeschalben (C.Leucopterus) im Sivash, Ukraine

Numbers, biometrics and feeding ecology of two marsh tern species, the Black Tern (Chlidonias niger) and White-winged Tern (C.leucopterus) were studied in the Sivash, Ukraine in July–August 1996 and 1998. In 1998 the Sivash region was counted completely and, including the nearby Molochnyi Liman, an estimated 30,000 to 40,000 Black Terns and 50,000 to 60,000 White-winged Terns were present. This makes the Sivash region one of the most important postnuptial stopover sites for these marsh terns in Europe with respectively 15 % and 25 % of the total Eurasian population present at any one moment. Both species showed clear habitat segregation. Black Terns fed in deep brackish waters and White-winged Terns fed in terrestrial habitats as well as shallow fresh and brackish waters. Black Terns were feeding mainly on relatively large fish (of 6–12 cm), mostly on Big-scale Sand SmeltAtherina boyeri bonapartii. White-winged Terns were feeding on more diverse food types like large insects and small fish. In the Lucerne fields they were feeding on caterpillars and in the steppe areas on grasshoppers or spiders. In the very shallow lagoons they fed on young mullet but mainly in the evenings. Both species had high food intake rates as compared to terns feeding in other food-rich coastal wetlands. Both tern species moulted primaries as well as body feathers and they changed breeding plumage into non-breeding within three to four weeks. Even some juveniles changed their first body feathers into first winter plumage, an event unknown to date for both species in this period. The small sample of captured adult Black Terns showed a mass increase for active moulting terns resulting in higher body mass in later moult stages. The high body mass of marsh terns in August compared to the breeding season indicates fat storage prior to migration. The results support the theory that marsh terns are long non-stop migrants that use the Sivash as a last stopover before their flight to Africa.