Long-term monitoring of the great spotted woodpecker Dendrocopos major irruptions in the Baltic region and Karelia

Winter and summer counts of Great Spotted Woodpeckers Dendrocopos major in Karelia during 43 years (1970–2013) revealed significant fluctuations. Analysis of capture data in the Baltic region during 65 years (1948–2013) also showed large annual variation, from 0 to 1058 individuals. Winter and breeding period numbers of these birds in Karelia were positively related to summer and autumn numbers in the Baltic region. Numbers in both regions were significantly directly related to spruce seed crops in the preceding year and inversely related to the crops in the current year. Of 10 years with strongest irruptions of Great Spotted Woodpeckers in the Baltic region, five irruptions occurred in years with poor spruce seed crops and five in years with good crops. This fact puts into question the hypothesis that Great Spotted Woodpecker irruptions are caused by deficit of conifer seeds in the years of poor crops. Numbers of juveniles in the Baltic region showed no significant relationship with North Atlantic Oscillation index NAODJFM or with regional air temperatures. Timing of postfledging dispersal in the Baltic region underwent long-term change during the study period and was inversely related to spring air temperature in the region. The date of onset of postfledging movements was inversely related to late summer numbers of juveniles in the Baltic area. Our main conclusion is that after the years with good spruce seed crops numbers of Great Spotted Woodpeckers that survive winter and start breeding increases which causes production of a large number of juveniles that take place in irruptions in late summer and autumn. We suggest that irruptions are triggered not by food shortage, as commonly assumed, but some other trigger, possibly related to intraspecific competition and aggression under high population density, as known from other irregular migrants.