POPULATION FLUCTUATIONS OF WOODPECKER SPECIES ON THE BALTIC ISLAND OFAASLA, SW FINLAND

The results of the monitoring of woodpecker abundance on the island of Aasla (Finnish Baltic coast) during 1979-1995 are presented and analy sed. Population fluctuations on Aasla were similar to those in mainlan d Finland in the case of the Grey-headed Woodpecker Picus canus and th e Black Woodpecker Dryocopus martius. Breeding abundance of Black Wood peckers was negatively correlated with snow depth and positively corre lated with temperature in February. Also, breeding densities of Great Spotted Woodpecker Dendrocopos major and Lesser Spotted Woodpecker Den drocopos minor correlated positively with mean January and February te mperatures. We suggest that several mild winters in the second part of the study period (1988-1995), are the primary reasons for the continu ously high population densities of Grey-headed and Black Woodpeckers. We did not find any evidence indicating competition between Grey-heade d and Black Woodpeckers. We stress the need for creating foraging subs trates (e.g. standing dead wood) as food sources for woodpecker specie s in boreal forests under severe winter conditions.