Een halve eeuw overwinterende Bruine Kiekendieven Circus aeruginosus in de noordelijke Delta

Based on binding trips and midwinter censuses in the northern delta of the southwestern Netherlands since the early 1950s, the average number of Marsh Harriers seen per trip in November-February inclusive is used to outline the trend in wintering numbers for the past 50 years. In the 1950s, most trips produced some Marsh Harriers, although rarely more than three birds in all. A steep drop in numbers was noted in the 1960s and early 1970s, coinciding with the use of organochlorines in farming (including abuse of pesticides to deliberately poison birds). After persistent pesticides had been banned, numbers increased again in the late 1970s, 1980s and early 1990s. This increase runs in parallel with the increase of the breeding population. The subsequent decline into the 2000s may be partly caused by some decline in the breeding population. It is hypothesised that increased food competition with Buzzards Buteo buteo may also play a role; the latter species has expanded its breeding range (including the colonisation of wetlands) and has shown a marked increase in numbers. Where the species occur in sympatry, Buzzards are dominant.