The Common Vole Microtus arvalis population in the peat district of central Friesland was high in the summer of 2004 and reached peak numbers in the early winter of 2004/05, attracting high numbers of Hen Harrier Circus cyaneus and Short-eared Owls Asia flammeus. In summer 2004, local raptors and owls functionally responded to the vole outbreak by producing large clutches or second layings (in Bam Owl Tyto alba even three, through November 2004). Based upon age and sex, at least 22 different Hen Harriers were identified at several roosts in central Friesland (peak numbers late December, with 18 at Sneekermeer) in the winter of 2004/05. All counts at roosts taken together, adult females outnumbered adult males by 37 to 7, with another 5 males in 2nd calender-year plumage; 15 birds were identified as juveniles. In 130 pellets, collected on 24 December 2004, only two birds figured among 165 prey items. All other prey remains consisted of small mammals, especially Common Vole Microtus arvalis (n=69, but 83 unidentified voles and voles/mice were most likely this species as well).