Trends bij de Bruine Kiekendief Circus aeruginosus als broedvogel in Zeeuws-Vlaanderen

From the 1950s up to and including the early 1970s, Marsh Harriers used to breed in Zeeuws-Vlaanderen (SW-Netherlands) with at most 2 pairs. The species showed an increase from the mid-1970s onwards, culminating at 145-160 pairs (0.19 pairs/ km2) in 2000-02 (of which 20-22 pairs in the intertidal marshes of Verdronken Land van Saeftinghe, the rest in inland polders), then declining to 70-75 pairs in 2009 (0.09 pairs/km²). The decline was restricted to pairs breeding in polders, as the population in Saeftinghe increased to 36 pairs in 2007. The initial increase must have been due to the ban on persistent pesticides in farmland (early 1970s), perhaps coinciding with a decline in human persecution. During the early 2000s, Marsh Harriers bred wherever reedbeds were available, including small, dry reedbeds and rough herbage. The drier and smallest reedbeds were the first to be deserted as soon as the Red Fox Vulpes vulpes invaded Zeeuws-Vlaanderen, but the impact of Foxes soon spread across other inland reedbeds as well. The effectiveness of Foxes as predators in dense reedbeds was unintentionally improved by campaigns to cull local geese numbers. To destroy geese nests, reedbeds are intensively searched in mid-March and early April, resulting in disturbance of Marsh Harriers during the settling period and in the creation of paths trampled amidst an otherwise dense and – for Foxes – often impenetrable reedbed. Culling geese is facilitated by the regional government, even though the adverse effects on other fauna are not fully understood. Although Foxes are also present in the intertidal marshes, the dynamic tidal ecosystem prevents permanent settlement and restricts the use of the area by Foxes. It is thought that the (lack of) safety against – mainly ground – predators explains the differential decline of Marsh Harriers in inland habitats compared to the increase and more stable numbers in the intertidal marshes. At present. Marsh Harriers breeding in inland polders do so exclusively in large and/or wet reedbeds and, increasingly, in cereals (confirmed for the first time in 2007). To improve breeding conditions for Marsh Harriers, reedbeds should be increased in size and flooded. Carrion Crows Corvus corone and Buzzards Buteo buteo, either as predator or kleptoparasite, can be deterred by removing trees within 300-500 m of reedbeds. Onset of laying, clutch size, egg size, brood size nor breeding success differed between Marsh Harriers breeding in inland and intertidal sites. In both sites combined, however, the proportion of pairs failing to fledge young has increased from 4-7% in 1995-97 to 30-49% in 2007-09. This trend is not accompanied by a change in clutch size, indicating that local conditions in the pre-laying period have not changed. Interestingly, the interval between early and late starters has substantially increased over the years, from 17-22 days in 1995-97 to 31-45 days in 2007-09. This change may indicate a larger proportion of pairs failing early in the incubation period and producing repeat layings after such failures, or a larger proportion of immature breedings birds (which were shown to delay onset of laying, produce fewer eggs and raise fewer young) in recent years.