Grauwe Kiekendieven Circus pygargus in Nederland in 2002

A complete survey of The Netherlands revealed 36 pairs, the distribution of which was similar to that in previous years (Fig. 1), i.e. a stronghold in the province of Groningen (28 pairs) and smaller clusters in Lauwersmeer (3 pairs) and Flevoland (5 pairs). Numbers in The Netherlands have been more or less stable since 1992 (Fig. 2). Clutch size of Montagu’s Flarriers in Groningen was on average 3.6 eggs, resulting in a mean number of 2.25 fledglings per successful nest. Most pairs nested in arable land on marine clay (where nests are protected), but a single pair used a conifer plantation near Westerwolde where 2 fledglings were raised (the first pair in Groningen using this habitat type, although the species had been breeding in forestry plantations in Flevoland in the 1980s). A single case of bigamy, with nests 4.6 km apart, was detected by means of a colour-ringed male. The pairs in Lauwersmeer breed in more or less natural habitats, the only site in The Netherlands where this is still the case (and where three harrier species, i.e. Circus aeruginosus, C. cyaneus and C. pygargus nest sympatrically). The three pairs raised 5 fledglings. Here, nesting success largely depends on the presence of the Mamerwaard where favourable foraging conditions coincide with high vole numbers (males commuting up to 7.5 km between nest and Mamerwaard to profit from these circumstances). Of 5 pairs in Zuidelijk Flevoland, only two raised a total of 6 fledglings. Farming is highly industrialized in this reclaimed polder (in 1968), resulting in a poor food supply. Urbanization is furthermore rampant, leading to habitat loss. Montagu’s Harriers already disappeared from Oostelijk Flevoland (reclaimed in 1957), probably for the same reasons (mainly agri-industry). In cooperation with German birdwatchers and institutions, research into Montagu’s Harriers in Niedersachsen (bordering the northern Netherlands) was intensified in 2002 (locating nests, collecting breeding parameters and prey remains, colour- ringing). It is thought that Dutch and German breeding birds belong to the same breeding population, and freely interchange. This aspect will receive further attention in the forthcoming years. Several breeding birds, captured at the nest, had been ringed in earlier years, showing natal philopatry as well as some dispersion from the natal site (Table 1). Several birds nested in habitats similar to those in which they were raised, but others switched into different habitats. One adult female, which had been trapped by flattened vegetation on the nest during a downpour, was caught by hand and turned out to be in her 8th calender-year (ringed); her sole surviving young was colour- ringed and relocated 300 km south of the nest site at a stop-over site during its autumn migration (Photo 4). Important prey species in The Netherlands were Alauda arvensis, Motacilla flava, Anthus pratensis and Microtus arvalis, but prey diversity was high (Appendix 1).