MANEN W VAN (1996) Demography and food of Hen Harriers Circus cyaneus wintering in Drenthe. LIMOSA 69 (1): 9-12.

Hen Harrier Circus cyaneus During five winters in the period 1989-95, Hen Harriers Circus cyaneus were counted at six winter roosts in the province of Drenthe (Tab. 1). Birds were aged and sexed and at some roosts prey items were identified by analysing pellets. In winters with large numbers of harrier, females and juveniles (ringtails) outnumbered adult (;O::2y) males, whereas in the winter with the smallest number of harriers, ringtails were absent (Tab. 2, Fig. 1). Ringtails appeared to be mostly juvenile males. The wintering harriers largely depended on Common Voles (90,9% of prey items) of which fluctuations were measured by analysing pellets of Long-eared Owls at nearby roosts. Proportions of Common Voles in the owl-diet correlated with the number of wintering Hen Harriers (Fig. 2). In the province of Drenthe, Common Voles occur mainly in grassland, where vegetation is short in winter. Male harriers are more agile than females and the nearly white adult males are less outlined against the sky than are ringtails. This renders adult males better hunters over short vegetations. Even when numbers of Common Voles are low they may form a profitable prey to adult male Hen Harriers. When vole numbers increase voles become profitable to juvenile males and juvenile and adult females. Females are probably the better hunters in areas with higher vegetation. In the province of Drenthe these habitats are mainly heath and peat where potential prey is scarce in winter.

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