Pre-migratory movements of adult Montagu's Harriers Circus pygargus

Pre-migratory and migratory movements of Montagu's Harrier Circus pygargus, a long-distance migrant, are poorly documented by ring recoveries and observations. In the present study, we tagged 10 adult Montagu's Harriers in NE Spain with satellite transmitters and tracked their movements until their arrival on the wintering grounds. To identify the end of the breeding season and the onset of the migration based on our satellite-telemetry data we developed and applied a statistical approach, defining the time period between these two events as the pre-migratory stage. We then compared habitat preferences during the breeding season and the pre-migratory stage. The duration of the pre-migratory stage averaged 42 days, with harriers leaving the breeding areas between 4 June and 27 July, and the onset of the migration taking place between 25 July and 4 September. Staging areas used during pre-migration were located at higher altitudes than the breeding grounds, and were characterized by the presence of shrublands and cultivated areas, mainly cereals. Montagu's Harriers' use of these sites is likely to be related to food availability: at higher altitudes, peak abundance of Orthoptera, their main prey source, occurs later in the season than in the lowland breeding area. Pre-migratory movements may also play an important role in the search for suitable breeding sites for future use.