Blauwe Kiekendieven Circus cyaneus op Mid-Friese slaapplaatsen in de winters van 2003/04-2010/11

Since the winter of 2003/04, numbers of Hen Harriers Circus cyaneus are being monitored at nocturnal roosts in the central part of the province of Friesland; those in the area of lake Sneekermeer are covered annually. The results of the first four seasons have already been published (Kleefstra & Veenstra 2002, Kleefstra 2005, Kleefstra & Spijkstra-Scholten 2008). This paper summarizes the results of all seasons, including trends in numbers, prey choice, demography, seasonal patterns and timing of arrival at the roosts. These data are compared with the results of national counts at roosts in the late 1980s. The highest numbers of Hen Harriers were present in the winters of 2004/05, 2007/08 and 2010/11 (Fig. 1). These winters were characterised by a good supply of Common Voles Microtus arvalis, which were the sole prey species found in pellets (Fig. 2). Due to the fact that the counts were carried out close to the roosts, the age- and sex-composition of the Hen Harriers was well studied (Appendix 2). Simplified to a division between ringtails (females and juveniles) and males, an average ratio of 75.2% ringtails (range 61.7 to 86.7%) and 24.8% males (range 13.3 to 38.3%) was found at the roosts (Fig. 3), which is close to the average of 21.7% males found during national roost counts in the late 1980s (Castelijns & Wouters 2011). Numbers peaked in January (Fig. 4), although adult females tended to peak in December, while adult males did so in February. The latter is probably associated with the later arrival of males in Friesland, as males tend to winter closer to the breeding areas than females unless adverse winter conditions force the birds further south. In the winters of 2007/08-2010/11, 59% of the Hen Harriers arrived at the roost after sundown, more specifically 5-15 minutes after sundown (Fig. 5). For female Hen Harriers, the number of individuals arriving before and after sundown was fifty-fifty. Among males, two thirds of the individuals arrived after sundown (66.7%). It is suggested to continue monitoring roosting harriers, preferably nationwide. This enables a comparison with data collected in the 1980s, and provides a monitoring scheme for a species in decline.