Blauwe Kiekendieven Circus cyaneus op Friese slaapplaatsen in de veldmuizenwinter van 2019/20

Just like in 2014, 2019 was a spectacular year for Common Voles Microtus arvalis in the Dutch province of Friesland. Monthly counts of Hen Harriers on nocturnal roosts in the winter season of 2019/20 (October-March) revealed that 29 out of 33 known roosts were occupied (Table 1). Although roosts with more than 10 Hen Harriers are often found in The Netherlands, roosts with more than 20 birds are exceptional. In Friesland in 2019/20, 14 of 30 occupied roosts held more than 10 Hen Harriers, 7 roosts >20, and 4 roosts >30 individuals (Table 1, Fig. 1). The average number of Hen Harriers per occupied roost was highest in December and January with respectively 13.4 and 13.3 individuals. In none of the months was the coverage of the roost survey 100% (Table 1). Taking into account roost coverage, monthly estimates were 50-60 in October, 160-170 in November, 290-310 in December, 280-300 in January, 190-210 in February and 120-140 in March. Peak numbers in December and January were higher than the peak numbers during previous provincial roost surveys, like in 1986-1990 (40-80), and in the winters of 2010/11 (106) and 2014/15 (185). In winter 2019/20, 671 individuals were identified as male or female. The proportion of Hen Harriers in male plumage was highest in October (35.7%), but then dropped below 10%, while no adult or 2nd calendar year males were observed in March (Fig. 2). In 142 pellets the remains of 131 Common Voles, 2 Bank Voles Myodes glareolus, 1 Field Vole Microtus agrestis, 9 Vole spec. Microtidae spec. and 1 Brown Rat Rattus norvegicus were found. Long-term counts of Hen Harriers at nocturnal roosts in Friesland show that numbers peak in vole-rich winters, especially during vole outbreaks, such as in 2014 and 2019. The question is how this will be in the near future. The winter of 2019/20 ended soaking wet, as were the winters later on. Climate scenarios predict increasingly wetter winters in The Netherlands, which unfavourably impact vole populations. And hence Hen Harriers?