Slaapplaatsen van Bruine Kiekendieven Circus aeruginosus in de Haarlemmermeer, nazomer 2006

In late August 2006 two roosts of Marsh Harriers were located in farmland in polder Haarlemmermeer in the western Netherlands. Both roosts were checked three times till 13 and 16 September respectively, when numbers had dwindled to almost zero. The roosts were detected by coincidence, one in a field with Brussels sprout, another in sugar beets. As Marsh Harriers do not breed in polder Haarlemmermeer, the birds must have orginated from areas in the vicinity. Usually, the birds arrived individually in the hour before sunset. Out of 34 Marsh Harriers, 22 were in first-year plumage, the remaining birds in adult or second calenderyear plumage. Adults were mostly seen during the August counts, but during later counts juveniles predominated. In The Netherlands, roosting of Marsh Harriers in farmland apparently has not been described before, although it has been noted in Hen Harriers Circus cyaneus.