Verleden en toekomst van een dynamisch, oerhollands gebied Roofvogels als wintergasten in de Flevopolders

The author has regularly counted the birds of prey wintering in two Dutch IJsselmeerpolders over six winters (1978-1979 up to and including 1983-1984). These polders, Eastern Flevoland and Southern Flevoland, were drained in 1957 and 1968 respectively, and since then have been gradually reclaimed. Eastern Flevoland was largely cultivated for the benefit of agriculture (mainly arable farming) and human settlement by 1980. In Southern Flevoland, however, large uncultivated areas were still left, especially the Oostvaardersplassen (a very large marshland area) and a drier reed-covered area in the centre of the polder (figure 1). Birds of prey were counted along a 135 km census stretch running through these polders (figure 2), nine times per winter. The numbers of the four most common species (Common Buzzard. Common Kestrel, Hen Harrier and Rough-legged Buzzard) are presented graphically (figures 3 and 4). In these graphs, the numbers of raptors during a harsh winter, with low densities of the Common Vole Microtus arvalis (1978-1979), are compared to the numbers of raptors in a mild, vote-rich winter (1980-1981). The occurrence of scarce and rare raptors (including Merlin, Peregrine Falcon, White-tailed Eagle and even Golden Eagle) is presented in a separate table. Furthermore, the total numbers of birds of prey counted in the whole polder area (Eastern and Southern Flevoland) in the winters of 1977-1981 are shown in table 1. During the 1980’s, Southern Flevoland was also for the greater part cultivated. This process of cultivation is illustrated by some landscape photographs. In the period 1980-1995, some of the raptor species have declined in numbers: especially the White-tailed Eagle (from 2-4 to 0-2 wintering individuals), Marsh Harrier, Hen Harrier and Kestrel. Some species seem to stay more or less stable in numbers; the Peregrine is an example of this (on average, 4-6 individuals wintering annually). The Common Buzzard has become much more numerous as cultivation has proceeded and the polders have been furnished with woodlands and rows of trees. Fortunately, the Oostvaardersptassen (5500 ha) and some smaller areas have been saved as nature reserves. Table 3 shows the total numbers of raptors in the Oostvaardersplassen and its drier surroundings during the period 1986-1991. This particular area (figure 5) is of crucial interest with regard to the future of wintering birds of prey, including the scarcer species. The introduction of large herbivores (Konik Horses, Heck Cattle) cannot prevent the marginal areas from being overgrown by rough vegetation and scrub. This process makes the area much less valuable as a breeding habitat and hunting haunt for threatened bird species. Therefore, the most appropriate management model for the Oostvaardersplassen is a sub-natural core area of extensive marshland (3600 ha), surrounded by a drier zone with large-scale but low-intensity forms of semiagricultural land use, including grazing and sustainable arable farming. This marginal zone could support large populations of key bird species, including scarce birds of prey and owls. Moreover, it could also serve education and recreation purposes in this unique Dutch nature reserve on the former sea-bed.