Overwinterende Blauwe Kiekendieven Circus cyaneus in De Onlanden

At a winter roost of Hen Harriers in a wetland in northern Drenthe and southern Groningen, numbers counted between 11 January and 3 April 2014 ranged from 1-14, mostly ringtails (adult females or juveniles, N=37), with some 2nd calender-year males (N=2) and adult males (N=11); 14 birds were not identified as to age or sex (Table 1). A collection of 211 observations of Hen Harriers from the public showed that this species was present in this region from early October through late April 2013/14, with peak numbers in December and January (Fig. 3). The roost was situated in former rough grassland that had been converted into wet herbage with Juncus effusus as the dominant vegetation (1.87 ha, Fig. 1). 409 pellets collected at the roost showed that voles were the major food source, with 87% of 771 identified prey items (Table 2). Of identified voles, Common Voles Microtus arvalis were represented by 58% of the sample, Field Voles M. agrestis by 42% (N=393). The remaining prey consisted of small numbers of Micromys minutus (N=5), Arvicola amphibius (N=3), Rattus norvegicus (N=3), Sorex araneus ((N=2), S. minutus (N=1), lagomorphs (N=1), and birds (N=78, 15 species but mostly Rallus aquaticus and Gallinago gallinago; Table 2). Voles were the dominant prey species throughout the winter, although Common Vole densities in De Onlanden had declined substantially after this region had been partly inundated in early 2012 (6.2 captures/100 catching nights before inundation, and 0.5 captures/100 catching nights after). High local densities of Sorex araneus and Neomys fodiens (10-15 captures/100 catching nights) were not reflected in the diet of wintering Hen Harriers.