ZIJLSTRA M (1987) Marsh Harriers Circus aeruginosus in Flevoland in winter. LIMOSA 60 (2): 57-62.

Since we know little about numbers and ecology of Marsh Harriers Circus aeruginosus wintering in the Netherlands, a study was undertaken in Flevoland during the winters of 1971/1972 - 1985/1986. After a period of few wintering Marsh Harriers, the numbers are increasing since 1976/1977. Birds in female plumage are observed more often than adult 55 (9: 1). Females hunt more often in marsh areas than 55 (tab. 2). During severe winters hunting takes place on the adjacent lake Markermeer (waterfowl, dead or injured) and in the extensively managed agricultural areas. Both in and outside the marsh area much carrion is eaten. Competition occurs with Rough-legged Buzzard Buteo lagopus, Buzzard Buteo buteo, Hen Harrier Circus cyaneus, Carrion Crow Corvus carone corone, and Hooded Crow Corvus carone cornix, Herring Gull Larus argentatus, and Great Black-backed Gull Larus Marinus. Because the numbers of wintering birds are related to the numbers of breeding birds it is suggested that wintering Marsh Harriers belong largely to the local breeding population. In winter average potential prey sizes exceed those in summer. This fact, in addition to competitive effects, suggests that 55 are less able to winter in Dutch marshes than ~~. Thus wintering was observed only in marshes in which no food shortage occurs during frost periods. So, at relatively high latitudes wintering Marsh Harriers may select their habitat at factors minimizing the risk of getting deprived of food during cold spells. All three known wintering sites of any importance in the Netherlands (fig. I) consist of large scale marshland, neighbouring rivers or large lakes, leaving many breeding sites unsuitable with respect to this criterion.

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