Roofvogels in de Flevopolders in de winter

The last two polders reclaimed from the former Zuiderzee (nowadays: IJsselmeer) were Oostelijk Flevoland (in 1957) and Zuidelijk Flevoland (in 1968), respectively 54,000 and 43,000 ha in size. The genesis of both polders was closely studied, also biologically. In this paper the results of systematic winter counts of raptors have been collated for the period of 1970-1994 and put into perspective regarding land use changes and the concomitant variations in prey abundance (notably voles). By the time raptor counts started, in the early 1970s, Oostelijk Flevoland was already completely cultivated. The largest section was used for agriculture (mostly arable), with four towns interspersed and forests planted along the eastern edges. The genesis of Zuidelijk Flevoland was followed from the very beginning, including the pioneer stages and the gradual cultivation and conversion into farmland (central part), cities (Almere and Zeewolde), forests (mostly near Zeewolde) and nature reserves (Lepelaars- and Oostvaardersplassen). Farmland was first cropped with cereals, later on also with alfalfa, and by the early 1990s the polder had attained its present status. Monthly raptors counts in winter were standardized, following fixed routes by car with frequent stops to scan the terrain (with 7x50 binocular), at first in November- February, later on also in October and March (from 1981 onwards). For each field, land use and crop type were recorded. Simultaneously, relative vole abundance (i.e. mostly referring to Microtus arvalis) was tracked by counting active burrows in fixed plots of 10 m² in September, and by using 10 traps per habitat type for five consecutive days. Raptor numbers were very small in the early pioneer stages of Zuidelijk Flevoland (early 1970s) when numbers and diversity in Oostelijk Flevoland were much greater. Wintering raptor numbers in Zuidelijk Flevoland steadily increased as soon as cultivation started (from the mid-1970s onwards, when much land was covered with reedbeds or rough herbage), whereas numbers in Oostelijk Flevoland showed a steady decline in line with the polder being put to production (agricultural, forestry, towns). This shift in raptor abundance and composition was particularly evident in vole-eaters as Hen Harrier Circus cyaneus, Rough-legged Buzzard Buteo lagopus and Kestrel Falco tinnunculus: peak numbers in the early polder stages (up to polder ages of 15-25 years), then steep decline. Vole numbers boomed in the early stages of cultivation, when large areas were covered with reedbeds and rough herbage, and crops mainly consisted of cereals and alfalfa. Stubble attracted many vole-eating raptors and highest numbers of voles were recorded in rough grassland, rough herbage and stubble. As soon as land use intensified, and other crops came into production, vole numbers declined, and so did vole-eating raptors except Buzzard B. buteo. The latter showed a gradual and consistent increase in numbers with ageing of the polders, perhaps indicating that local breeding birds were involved that profited from planting and ageing of forests (increase in breeding population from the late 1970s onwards, first breeding in Oostelijk Flevoland in 1978). Also, Buzzards use an energetically ‘cheaper’ method of hunting (sit and wait) than Hen Harriers, Rough-legged Buzzards and Kestrels (flying and hovering). With poor vole numbers, the latter methods of hunting may not be profitable to sustain a living. The upsurge and subsequent decline in numbers of vole-eating raptors, as found in Flevoland during the full cycle from pioneer stages through complete cultivation, was typical of all polder schemes in the 20th century, as similar trends were recorded in Wieringermeer (reclaimed in 1930), Noordoostpolder (1942) and Eems-Dollard. The initial boost in food supply (seeds, voles) gradually lost its momentum during cultivation, when high-intensity farmland and towns eventually replaced the seed and vole bonanzas of the early stages of polder development.