Grauwe Kiekendieven Circus pygargus in Nederland in 2007

In 2007, the Montagu’s Harrier reached highest numbers in The Netherlands since the 1970s, i.e. 47 pairs of which 37 started nest building and 27 nests were successful (73 fledlings, with a secondary sexratio of 1,5). Mean onset of laying was 17 May, mean clutch size 3.9. Half of the nests had to be protected from harvesting in order to survive. Breeding in Lauwersmeer, the only site where Montagu’s Harriers still bred in semi-natural vegetation, ceased to occur for the first time; it is thought that grazing with large herbivores resulted here in the loss of nesting habitat. In the neighbouring part of Germany (Rheiderland, Niedersachsen) 11 pairs were located, of which 8 nested and 8 were successful (raising 20 fledglings, secondary sexratio 1.6). Here, egg laying started on average on 18 May. Pellet analysis and plucks near/on nests revealed that Common Vole Microtus arvalis was the main prey species in 2007, both in The Netherlands and in Rheiderland (Appendix 1), accounting in biomass for between 85% and 98% depending on region. A variety of birds, small mammals, insects and eggs was taken in addition to voles. The use of colour rings, wing tags, radio tags and satellite transmitters resulted in a plethora of data concerning habitat use, distribution, dispersal, migratory pathways and survival. African wintering sites of Dutch birds are targeted via a southwesterly course (France, Spain), and most birds end up in the western Sahel (Senegal, Mauritania). Few Dutch birds cross the Mediterranean (partly via Italy and Sardinia) and winter in the central Sahel (Niger, Mali), where they mingle with birds from the eastern European population. Flight paths are more or less similar in autumn and spring, refuting the idea of circular movements. Also contrary to common believe, the main food in West Africa consists of local Orthoptera species (like Ornithacris cavroisi), not migratory locusts. Protection in The Netherlands is now mainly focused on safe-guarding nest sites and providing suitable feeding grounds via set-aside programs. Radio-tagged birds showed a clear preference for feeding in set-aside, especially when mowing (grass, alfalfa) has not yet progressed. Freshly mowed fields attract Montagu’s Harriers that profit from the temporary exposure of voles. Home ranges in 2007 on average were smaller than usual, associated with high vole abundance.