KLEEFSTRA R., E. BRANDENBURG, E. DE GROOT, J. KRAMER & W. SPIJKSTRA-SCHOLTEN (2016) Harriers and sugar beets: discovery of a large night roost of Marsh Harrier Circus aeruginosus in arable land. LIMOSA 89 (2): 84-87.

Late August 2015 a large night roost of Marsh Harriers was found in a sugar beet field in the province of Friesland, the Netherlands. The estimated number of individuals was 100-120 birds. A count on 2 September showed that 82 individuals spent the night in the sugar beet field, while counts on 10 and 11 September showed that numbers had declined to 54 and 56, respectively, of which 60-65% juveniles and 20-35% adult females. Before the discovery of the roost in the sugar beet field, the harriers spent their summer nights in an adjacent wheat field until this crop was harvested. Also in the summer of 2014 at least tens of harriers apparently roosted in a wheat crop in the area. Night roosts of Marsh Harriers in arable land are not uncommon in the Netherlands, but have rarely been described. We suggest that the high number of Marsh Harriers in the Frisian arable fields is related to the outbreak of Common Voles Microtus arvalis in foraging areas around the roost. In 45 collected pellets we found the remains of 42 Common Voles. Preferred roosting habitat, such as reedbeds, is not available in close proximity to these vole-rich foraging grounds (about 16-20 km away).

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