Nutcracker irruptions depend more on food abundance than population size

Abstract

Irruptive migration is a form of partial migration, where the part of the population that migrates varies significantly between years. The intensity of irruptions has been explained by variation in food availability, breeding success and breeding population size. We investigated factors affecting the irruptions by northern nutcrackers Nucifraga caryocatactes (hereafter nutcrackers) in northern Europe. We used long-term monitoring data on nutcrackers autumn migrating in SW Finland (Hanko) and SW Sweden (Falsterbo), the age ratio of nutcrackers in Finland, breeding population size estimates in Sweden, the annual pollen abundance variation of hazel nuts Corylus avellana in Finland and Sweden and annual nut crop size variation in SW Finland. Two subspecies of nutcrackers occur in the area: southern breeding birds belong to the nominate subspecies caryocatactes, whereas individuals of the macrorhynchos subspecies breed further north but also occur in irruptions from the east (Russia). The ringing numbers of the nominate subspecies at Hanko were negatively correlated with the nut crop size, but this connection was not found in macrorhynchos. The annual autumn total nutcracker migration numbers in Hanko and Falsterbo were negatively connected with hazel nut crop size and also hazel pollen abundance in Hanko. Proportion of young was positively connected with migration intensity in Hanko. The Swedish adult breeding population size did not show any connection with the migration numbers in Falsterbo. The role of the nut crop size was clearly stronger than the age ratio indicating that the lack of food is the key driver of the irruptions.