During the summer of 2003, four times adult Hen Harriers (1x female, 3x male) were observed near Zwanenwater in the dimes of the province of Noord-Holland, about 18 km south of nearest known breeding site (on the Wadden Sea Island of Texel, where 17 breeding pairs were located in 2003, 7 of which on the southern part of the island). One of the males was seen carrying food (on 29 June; Table 1), but indications of breeding at Zwanenwater were lacking. Observers watching seabird migration at Den Helder, at the northernmost tip of the mainland of Noord-Holland, recorded eight flights (involving at least two different individuals) of males crossing Marsdiep, the 2.5 km wide stretch of water separating Texel from the mainland, from Texel towards the mainland; none of these males was carrying food (Table 2). Thirteen male flights in the opposite direction, from the mainland heading for Texel, were recorded between 2 May and 6 July; seven of these flights involved prey transportations (Table 3). It is therefore likely that breeding birds from Texel visit the mainland to forage during the breeding season, traversing apparently up to at least 18 km between nest site and foraging site. Such long flights, and prey transportations, are apparently responsible for many of the occasional records of alleged breeding near Zwanenwater on the mainland.

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Freek Kalsbeek. (2004). Blauwe Kiekendieven Circus cyaneus in het Zwanenwater en omgeving in het voorjaar en de vroege zomer van 2003: aanwijzingen voor een broedgeval?. De Takkeling, 12(2), 112–115.