During systematic counts of geese departing from roosts in Central Friesland, from one hour before sunrise till 1.5-2 hours after sunrise in the period mid-October 2003 through mid-March 2004, a roost of Hen Harriers was detected in marshland surrounded by grasslands. The roost was used between 17 October and 17 April, first by two females (till 19 November), then steadily increasing to 5 females and 2 males (mid-December through early February), and again declining to a single female between 6 and 17 April. In 12 out of 22 geese counts during daytime, hunting Hen Harriers were seen between 19 November and 27 February; these observations were made within 250-7600 m of the roost. Pellets were collected on 3 April (n=91), and almost exclusively contained mammals (1 Snipe Gallinago gallinago among 110 prey items). The main prey was common vole Microtus arvalis, but at least ten other mammal species were recorded, including three species of shrews and several other Microtus-species (Table 1). The high presence of common voles in the pellets, and the fact that many Hen Harriers were female (in the neighbouring region of Drenthe females are present in some numbers only when voles are abundant; van Manen 1996), indicate that – at least in this part of the country – common voles must have been present in peak numbers. This was corroborated in spring and summer 2004, when vole-eating raptors and owls peaked in numbers and reproductive output.

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Romke Kleefstra, & Sip Veenstra. (2004). Prooikeuze van Blauwe Kiekendieven Circus cyaneus op een slaapplaats in het Sneekermeergebied in de winter van 2003/04. De Takkeling, 12(3), 199–204.