Population trends among archipelago birds in the Krunnit sanctuary, northern Gulf of Bothina, in 1939–85

Abstract

Breeding pairs of ducks, alcids, waders, gulls and terns were censused on the Krunnit Islands, northern Gulf of Bothnia, in 24 summers between 1939-85. Thedata for ducks, gulls and tems are mainly based onnest finds, those for waders andalcids on the numbers of adult birds . Thesanctuary was founded in 1936 andeffective guarding started in the late 1940s. In 1939 the number of regularly nesting species was 28 and these were joined by Scolopax rusticola, Tringa glareola and Larus argentatus in the 1940s, and by Anaspenelope, L. marinus and L. rżdibundus in the 1950s. On the other hand, Cepphus grylle has almost disappeared. From the 1930s to the 1950s, the total bird numbers remained at the level of about 650 pairs, but by the 1980s they had increased to over 2000 pairs, chiefly due to the rapid growth of gull and tern numbers. The populations of Anas species varied irregularly, exceptfor that ofA. penelope, which clearly increased . Most waterfowl species (8 out of 12) showedan increase from the 1970s to the 1980s . Of the waders, Arenaria interpres and Actitis hypoleucos have increased, whereas Charadrius hiaticula, Calidris temminckii, Numenius arquata and Phalaropus lobatus have decreased . In 1939 only 30 pairs of gulls werebreeding on the Krunnit Islands . Larusfuscusshowedarapidincrease in the 1950s, L. canusin the early 1960s, L. argentatus in the late 1960s and L. ridibundus from the early 1970s onwards. Sterna caspia, S. hirundo and S. paradisaea also increased . Therole of this sanctuary in monitoring bird populations is discussed . Information about the feeding ecology of gulls is needed to elucidate the effect of increasing predation byL. argentatus andL. marinus in the coming years and the significance of the largeL. ridibundus population in competition for food among gull and tern species. Thebird populations of the Krunnit Islands are threatened bypossible immigration of the mink, raccoon dog and Arctic fox .