CHANGES IN THE ARCHIPELAGO BIRD POPULATIONS OF THE FINNISH QUARK, GULF OF BOTHNIA, FROM 1957-60 TO 1990-91

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 dat a from censuses conducted at the bird sanctuary Valsorarna/Valassaaret in 1960 and 1990. The populations of 20 well-known species almost dou bled 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 S terna paradisaea (ca 2 770 pairs in the areas studied), Larus canus (c a 2 290 pairs), and Cepphus grylle (ca 2 250 pairs). Increases have oc curred also in low or formerly decreased populations, e.g. Alca torda, Anthus petrosus, and Stercorarius parasiticus. The establishment of L arus ridibundus even on offshore islands is reflected in the data. Mar ked population decreases are noted for Aythya marila, Charadrius hiati cula, Larus fuscus, Sterna caspia, and Sterna hirundo, out of which fo ur species are listed as threatened in Finland. At Valsorarna, Tringa tetanus and Arenaria interpres have also decreased significantly, but in the rest of the Quark we noted a minor decrease only. Totally, we p resent population data for ca 50 species, with reservations for census errors in some of them. Immediate follow-up studies are suggested for Bjorkogrunden, a protected area where the number of archipelago birds decreased ca 30%, and for the species with marked population decrease s.