An experiment during a peak year. In Annales Zoologici Fennici (pp. 1-6). Finnish Zoological Publishing Board, formed by the Finnish Academy of Sciences, Societas Biologica Fennica Vanamo, Societas pro Fauna et Flora Fennica, and Societas Scientiarum Fennica. Per study in Finland, Ural owls took field and bank voles that averaged larger than the average weight of those captured in the field by biologist, i.e. the weight of those caught by owls was estimated at for field voles vs for bank voles, while those caught by biologists averaged for fields vs for banks. 56% of the field voles those caught by Ural owls were reproductively active, while 44% of bank voles were active as such. Helminths did not appear to make field voles more vulnerable to predation but male field voles were far more often caught by Ural owls than females, 76% of those caught in a study in Finland being male of those caught around vole nests, with 52% of the ones caught in fields being male. However, pregnant female field voles were found to be more vulnerable than other age female voles. More socially inclined and concentrated voles such as common voles (Microtus arvalis) tend to occur in more extensive open areas so forest-dwelling owls such as the Ural tend hunt them relatively scarcely, but only relative to many other predators. The largest known dietary study to date occurred in Finland, with 5995 prey items examined across several study years. According to this data, a much larger vole was preferred in the diet where available, the European water vole (Arvicola amphibius). With an estimated mean weight of , the water vole constituted an estimated 22.15% of the prey by number and 52.2% of the biomass, against 19.5% by number and 18% of the biomass being made up by field voles. The average estimated prey weight overall within the study was estimated at . Another, albeit smaller, central Finnish study showed field voles leading by number over water voles, 42.7% against 33.9%, among 1739 prey items respectively, but with the two species constituting 17.4% and 69.1% of the prey biomass. Southern Finnish studies showed sharp shifts in the import of vole prey due to their population cycles. In southern Finland, of 3351 prey items in a peak vole year, 58.43% by number and 42.8% by biomass of the prey was made of by field voles, followed by bank vole at