Changes in the archipelago bird populations of the Finnish Quark, Gull of Bothnia, from 1957–60 to 1990–91

Abstract

The article presents (mostly one-visit) censuses of 114 islands in the Finnish Quark in 1957-60 and 1991 . As a reference area we include data from censuses conducted at the bird sanctuary ValsörarnaNalassaaret in 1960 and 1990. The populations of 20 wellknown species almost doubled in 30 years; the change is fairly similar both in the 114 islands studied and in the reference area . The increase in numbers is largely attributable to the dominating species in the Finnish Quark, namely Sterna paradisaea (ca 2 770 pairs in the areas studied), Larus canus (ca 2 290 pairs), and Cepphus grylle (ca 2 250 pairs) . Increases have occurred also in low or formerly decreased populations, e.g . Alca torda, Anthus petrosus, and Stercorarius parasiticus. The establishment of Larus ridibundus even on offshore islands is reflected in the data. Marked population decreases are noted for Aythya marila, Charadrius hiaticula, Larus fuscus, Sterna caspia, and Sterna hirundo, out of which four species are listed as threatened in Finland. At Valsörarna, Tringa totanus and Arenaria interpres have also decreased significantly, but in the rest of the Quark we noted aminor decrease only. Totally, we present population data for ca 50 species, with reservations for census errors in some of them. Immediate follow-up studies are suggested for Björkögrunden, a protected area where the number ofarchipelago birds decreased ca 30%, and for the species with marked population decreases.