WAL CA VAN DER, KEIZER R & VAN WIEREN SE (1999) Twenty-five years Hen Harriers Circus cyaneus on the island of Schiermonnikoog. LIMOSA 72 (1): 11-22.

The breeding ecology of the Hen Harrier Circus cyaneus has been studied on the island of Schiermonnikoog (Fig. 1) during the period 1973-97. In the first half of the study period (1973-85), the Hen Harrier was the most common harrier on the island (Fig. 2), and there were only a few breeding pairs of Marsh Harrier C. aeruginosus. From 1986 onwards, the population of Marsh Harriers has increased, probably due Ie the predation of nests on the mainland (Lauwersmeer) by the Red Fox. In 1989 a massive influx of Marsh Harriers occurred on the island; since then the Marsh Harrier is the most common harrier on the island. In the period after the arrival of the Marsh Harrier on the island (1986-97), the number of breeding pairs, clutch size and number of fledglings per successful pair of Hen Harrier is significant lower than in the period before the arrival of the Marsh Harrier (Tab. 1 and 2). After studying the occupation of nest sites by Hen- and Marsh Harriers during five years (Fig. 4), it is suggested that the Marsh Harrier is suppressing the Hen Harrier from the breeding sites.
      During the last five years of the study-period, the populations of harriers appear to stabilize, with the Hen Harrier occupying their old nest sites in the dune valleys and the Marsh Harrier occupying the marshy areas with reedbeds. The preference of vegetation-type is discussed. The Hen Harrier is less dependent on Reed-vegetation compared to the Marsh Harrier (Tab. 3). The succession of the vegetation on the island can be a threat for the Hen Harrier, because of afforestation of the breeding- and hunting-sites. Management of hydrology and stimulation of processes which slow down succession (grazing, mowing) are recommended.

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