Overwintering van Bruine Kiekendieven Circus aeruginosus in de IJsseldelta tussen 1925 en 2007

Using the field notes of C.G.B. ten Kate (1925-69), J.A.F. Koridon (1960-81) and J. Nap (1967-2007) collected in the IJssel valley, we reconstructed the trend of wintering Marsh Harriers in this mixed marsh- and farmland region in the central Netherlands. The IJssel valley used to be a dynamic ecosystem where fresh water of the river IJssel mixed with salt and brackish water of the Zuiderzee, a large inland sea connected with the Wadden Sea. This ecosystem underwent a drastic change when, in 1932, the Zuiderzee was cut off from the Wadden Sea. The salty and brackish Zuiderzee transformed into the freshwater Ijsselmeer, where three large polders were reclaimed successively in 1942 (Noordoostpolder), 1957 (Oostelijk Flevoland) and 1968 (Zuidelijk Flevoland); a smaller polder, Wieringermeer, was reclaimed in 1930. Each embankment led to a short upsurge in breeding numbers of Marsh Harriers (Fig. 3). The trend in the IJssel valley showed highest breeding numbers in the 1920s and early 1950s (Fig. 2), followed by a steady decline to <5 pairs in the late 1960s; numbers remained low in the decades to follow. The number of wintering Marsh Harriers in the IJssel valley reflects the vicissitudes of local breeding numbers (Fig. 1), suggesting a positive correlation between local abundance and wintering numbers. Also, highest numbers of wintering Marsh Harriers coincided with severe winters (Fig. 4), perhaps related to the higher availability of carrion in severe winters. In this respect, it is worth noting that the majority of wintering Marsh Harriers were in female-juvenile plumage, i.e. 98% in the first half of the 20th century (N=135) and 10% in 1967-2007 (N=29), perhaps related to the females – because of their larger size – being more adept at exploiting carrion. Contrastingly, wintering numbers were not correlated with rainfall in the Sahel in the previous winter (Fig. 5). The data from the IJssel valley suggest that wintering numbers are largely correlated with local breeding numbers in dynamic wetlands. Upsurges in wintering numbers elsewhere in The Netherlands, such as in the Flevopolders in the 1970s and 1980s (after reclamation of Zuidelijk Flevoland) and in Zeeuws-Vlaanderen (SW Netherlands) in the 1990s and 2000s, also coincided with high numbers of local breeding birds under conditions of high dynamics (recently reclaimed or tidal area) and large food resources (mainly waterbirds). Declines in local wintering numbers occur when local wetlands diminish in size and/or in dynamics, on top of industrialisation of farmland in the surrounding landscape (grassland converted to maize, desiccation and homogenisation of grassland, steep declines of Common Vole Microtus arvalis). Wintering in Flevoland and IJssel valley is now a rare phenomenon, in line with the changes mentioned, whereas wintering in Zeeuws-Vlaanderen has increased in frequency since the 1990s. Elsewhere in The Netherlands, wintering has occurred frequently in the past century, each time associated with highly dynamic ecosystems (especially Biesbosch, a brackish tidal area turned into polders).