Blauwe Kiekendieven Circus cyaneus op Friese slaapplaatsen in de muizenrijke winter van 2014/15

In the province of Friesland (northern Netherlands) Common Vole Microtus arvalis numbers reached a spectacular high in 2014. Consequently, breeding success of Marsh Harriers Circus aeruginosus, Buzzards Buteo buteo and Kestrels Falco tinnunculus was high, and the number of breeding pairs of Barn Owl Tyto alba doubled. After years of absence, Short-eared Owl Asio flammeus invaded the province and were found breeding in unexpected numbers, whereas numbers of Long-eared Owls Asiootus on roosts peaked during the winter of 2014/15. The Frisian Roost Survey Group organized counts of Hen Harriers on nocturnal roosts in the winter of 2014/15, to survey the impact of the vole outbreak on the number of wintering Hen Harriers (as compared to previous winters). A total of 21 natural sites amidst farmland were visited monthly during October- March, 16 of which contained a roost of Hen Harriers. Numbers increased from 23 individuals in October to a peak of 185 in February (Table 1, Figure 1). The largest roost was found along the Frisian Wadden Sea coast where 40-62% of all Hen Harriers spent the night. Adult males accounted for 37.8% in October, and their relative numbers remained rather stable throughout the winter; the remaining birds were females and juveniles (Fig. 2). In 161 pellets collected on roosts, Common Voles were the only prey species encountered. The peak of 185 Hen Harriers on nocturnal roosts in the province of Friesland in 2014/15 was higher than peak numbers recorded during previous provincial roost surveys, like in 1986-1990 (40-80) and in the winter of 2010/11 (106). However, Hen Harriers recorded during waterbird surveys indicated that the number of wintering Hen Harriers in Friesland must have been even higher (than during 2014/15) in the winters of 2005/06 and 2010/11 (Fig. 3). It is suggested that the extraordinary vole peak in 2014/15 did not boost numbers of Hen Harriers wintering in Friesland, unlike the irruption of Short-eared Owls. Rather, the vole peak resulted in high and stable numbers of Hen Harriers in Friesland throughout the winter, perhaps at the expense of roosts elsewhere in The Netherlands.