Ibis (2007), 149, 575–586 © 2007 The Authors Journal compilation © 2007 British Ornithologists’ Union Blackwell Publishing Ltd Do voles make agricultural habitat attractive to Montagu’s Harrier Circus pygargus? BEN J. KOKS, 1 CHRISTIANE TRIERWEILER, 1,2 * ERIK G. VISSER, 1 COR DIJKSTRA 3 & JAN KOMDEUR 2 1 Dutch Montagu’s Harrier Foundation, PO Box 46, 9679 ZG Scheemda, Netherlands 2 Animal Ecology Group, Centre for Ecological and Evolutionary Studies, University of Groningen, PO Box 14, 9750 AA Haren, Netherlands 3 Behavioural Biology, University of Groningen, PO Box 14, 9750 AA Haren, Netherlands Loss and degradation of habitat threatens many bird populations. Recent rural land-use changes in the Netherlands have led to a shift in habitat use by breeding Montagu’s Harriers Circus pygargus. Since the 1990s, unprecedented numbers of this species have bred in farmland compared with numbers in natural habitat. Destruction of nests by agricultural operations, however, compromises breeding success. Between 1992 and 2005, the number of breeding pairs in the northeastern Netherlands was positively, though weakly, correlated with previous-year estimated abundance of voles, mostly Microtus arvalis. In good vole years, the onset of laying was earlier and mean clutch size was larger. Vole abundance was relatively higher in set-aside land and in high and dense vegetation. We suggest that agri-environmental schemes aimed at increasing the availability of voles in agricultural breeding areas may be an effective management tool for the conservation of Montagu’s Harriers in the northeastern Netherlands. In recent decades, many bird populations have declined in Europe and worldwide (Tucker & Heath 1994, Norris & Pain 2002). One of the principal causes of these declines has been habitat loss (e.g. Owens & Bennett 2000, Bruford 2002, Newton 2004). Large areas of natural habitat have been lost from Europe, where many bird species have become increasingly reliant on farmland habitats (Tucker 1997). One such species is Montagu’s Harrier Circus pygargus (Arroyo et al. 2002), the conservation status of which is vulnerable because more than half of its global population is found in Europe (Burfield & Van Bommel 2004). Montagu’s Harrier is included in Annex I of the European Birds Directive (79/409/ EEC), which lists species that are classified as particu- larly threatened and in need of special conservation measures. European Montagu’s Harriers are long-distance Palearctic migrants, traditionally breeding in lowland heaths, dunes, hay-meadows and (pseudo-) steppes (Clarke 1996, Leroux 2004). Since the 1990s, 70–90% of breeding pairs in Western Europe have bred in agricultural habitats (Arroyo et al. 2002). With intensification of land use, the percentage of Harriers breeding in farmland is also expected to increase in Eastern Europe. Montagu’s Harriers are opportunistic predators (Arroyo 1997) and, in vole-rich habitats, feed mainly on voles. Up to 90% of their diet in Western France (Butet & Leroux 1993, Salamolard et al. 2000) and around 60% in Dutch farmland consisted of voles (Koks et al. 2005). Montagu’s Harriers breeding in farmland, however, may experience lower breeding success than those in more natural habitats. Between 20 and 70% of nests of this ground-nesting species can be destroyed during harvesting activities (Corbacho et al. 1997, Koks et al. 2001, Millon et al. 2002). In the Nether- lands, lucerne (alfalfa) is mown repeatedly and early in the season (starting in May–June), which means that clutches in this crop do not survive and even breeding females can be killed during the harvest. Nests in early harvested winter barley and winter wheat are at risk as well. In the Netherlands, nests are protected by leaving a 10 × 10-m unharvested area around nests and placing electric fences around the remaining vegetation to exclude terrestrial predators. *Corresponding author. Email: christianetrierweiler@yahoo.com