Population fluctuations of woodpecker species on the Baltic island of Aasla, SW Finland

Abstrakti

The results of the monitoring of woodpecker abundance on the island ofAasla (Finnish Baltic coast) during 1979-1995 are presented and analysed. Population fluctuations on Aasla were similar to those in mainland Finland in the case of the Grey-headed Woodpecker Picus canus and the Black Woodpecker Dryocopus martius. Breeding abundance of Black Woodpeckers was negatively correlated with snow depth and positively correlated with temperature in February. Also, breeding densities ofGreat Spotted Woodpecker Dendrocopos major and Lesser Spotted Woodpecker Dendrocopos minor correlated positively with mean January and February temperatures . We suggest that several mild winters in the second part of the study period (1988-1995), are the primary reasons for the continuously high population densities of Grey-headed and Black Woodpeckers . We did not find any evidence indicating competition between Grey-headed and Black Woodpeckers. We stress the need for creating foraging substrates (e .g . standing dead wood) as food sources for woodpecker species in boreal forests under severe winter conditions .