Nachttrek bij de Bruine Kiekendief Circus aeruginosus, of vroeg vertrekken?

Marsh Harriers can be seen migrating during the greater part of the day, sometimes already around sunrise. It has even been suggested that Marsh Harriers may migrate at night (Gatter 1984), although this autumn observation is circumstantial only. Here, it is suggested that early-morning passage may involve birds having roosted nearby. In the southwestern Netherlands, Marsh Harriers are known to roost communally in autumn, with 21-35 birds being present in September 1993-98 in the Braakman nature reserve in Zeeuws-Vlaanderen (pers. comm. Henk Castelijns). Whereas Marsh Harriers are largely independent of thermals during migration, hence their broad front passage of Europe including the Mediterranean, they may start migration in early morning, using active flight. This may also explain the records of small migrating flocks (originating from the same roost?), an unusual sight in a species that normally migrates singly.