The annual cycle of Montagu's Harrier <i>Circus pygargus</i>:Driven by voles and grasshoppers

Year-round research on European Montagu's Harriers should help to clarify what governs their numbers and distribution in breeding and wintering areas, in order to provide a firm basis for the conservation of this vulnerable species. During the breeding season, the Dutch Montagu's Harrier Foundation investigates distribution, numbers and breeding success, diet, prey abundance and habitat use. Set-aside land and field margins (agrienvironmental schemes) contain high numbers of voles, one of the most important prey species, and are used intensively by hunting harriers. The number of breeding pairs seems to be enhanced by increasing the area and quality of set-aside land through agri-environment schemes. Migratory movements of Montagu's Harriers are tracked using satellite telemetry. Harriers from NW Europe follow a western route via Gibraltar or a more central route via Italy towards wintering areas in the Sahel of West Africa. In spring, the harriers returned via the same routes and thus did not engage in loop migration. Research in the wintering quarters focuses on the spatial distribution of wintering harriers, food choice, prey abundance and habitat use. Non-migratory grasshoppers are an important prey in winter, at least in years when migratory grasshopper species do not swarm. Montagu's Harriers move southward during the winter which is hypothesized to be related to a seasonal south-ward shift in productivity and food availability. Satellite telemetry and colour-marking are also used to determine the connectivity between different breeding populations. Before and during their autumn migration, harriers seem to visit other breeding populations, which might be related to prospecting behaviour.