Epsilon Archive for Student Projects

Abstract

Breeding success for specialist avian predators in northern Fennoscandia is highly dependent on cyclic vole populations. The 3-4 year high amplitude vole cycles once regular to this region have shown significant long term declines in abundance since the 1970s. The vole decline in Sweden, detected by the National Environmental Monitoring Program, is suggested to have negative consequences on breeding performance for specialist raptors. Complete, long term monitoring data to show this on particularly northern owls are scarce. I compared Tengmalm’s owl to the European kestrel, an increasing generalist, in northern Sweden, and used these as model species for comparing with other northern raptors. Diet compilation for 15 raptors revealed the snowy, great grey, hawk, long and short-eared owls are also small mammal specialists and were placed together with Tengmalm’s owl in an ‘owl’ predator category; the hen harrier, common buzzard, kestrel, and eagle, tawny, Ural, and pygmy owls are generalists and were placed in a ‘kestrel’ predator category; and the rough-legged buzzard is an intermediate placed in a combined owl/kestrel category. I predicted trends for species based on degree of small mammal specialization and model predator category. I suggested those in the owl category should decline, those in the kestrel category should not decline, and rough-legged buzzard should probably decline. Population data for raptors from Sweden, including migration counts from Falsterbo and breeding density of Tengmalm’s owl and kestrel in Västerbotten County, were analyzed against the spring and previous autumn vole food supply indices from Västerbotten County. Migration counts of the common and rough-legged buzzards showed a significant positive relationship with the spring vole index. Breeding density of Tengmalm’s owl and kestrel showed significant positive relationships with the previous autumn vole index (and also spring for Tengmalm’s owl), while mean annual brood size for kestrels was only dependent on the spring vole index. For species with data available from Sweden, only the kestrel showed an increasing population trend. However, breeding density of Tengmalm’s owl, a vole specialist, has declined in northern Sweden since the 1980s. The specialists placed in the owl category are predicted show similar patterns as Tengmalm’s owl and are the most likely species that have been and will continue to be adversely affected by significant declines in vole abundance.