Blauwe Kiekendieven Circus cyaneus en Velduilen Asio flammeus profiteren van tijdelijke Mid-Friese braaklegging

Each winter roosts of Hen Harriers are counted and visited in the central part of Friesland. In the spring of 2006 a new roost was found in a set-aside field on the isle of It Eilan. The set-aside abounded with Common Voles Microtus arvalis, as well as amphibians. In the winter of 2006/07 counts at roosts were carried out and pellets collected. This site also held a Short-eared Owl roost with a maximum of 12 individuals. Compared with another Hen Harrier roosts in central Frisian near Lake Sneekermeer, monitored since the winter of 2003/04, Hen Harriers using the roost It Eilan showed up already in October. The Sneekermeer roost came into use by late November and early December. Maximum numbers amounted to 13 individuals at It Eilân, and 11 at Sneekermeer. Average numbers per count were higher on It Eilan (8.0) than at Sneekermeer (5.6). The vole-ridden set-aside of about 80 ha at It Eilan also attracted high numbers of other vole-eating birds like Great Egret Casmerodius albus (max. 7), Grey Heron Ardea cinerea (max, 10), Buzzard Buteo buteo (max. 7) and Kestrel Falco tinnunculus (max. 4). Prey remains in Hen Harrier pellets consisted almost exclusively of small mammals, notably Common Voles, both at It Eilan (96% of 145 prey remains) and Sneekermeer (97% of 60 prey remains). An equally high Common Vole proportion was found in the pellets of Short-eared Owls at It Eilân, i.e. 96% of 155 prey items. The set-aside at It Eilan was a temporary phenomenon. The local rough herbage was mowed in summer 2007, to facilitate the construction of a swamp. Wintering Hen Harriers are therefore forced to look for other roosts. As expected, in the winter of 2007/08 two ‘new’ roosts were found in central Frisia.