Houden Drentse Eksters Pica pica het hoofd boven water?

In the 1970s and early 1980s the Magpie was a common breeding bird in farm- and woodland in the province of Drenthe. Probably as a result of increasing numbers of Goshawks Accipiter gentilis and Common Buzzards Buteo buteo, Magpies became extinct in woodland. Shortly afterwards, numbers in farmland also started to decrease substantially. The decline in farmland is possibly due to deteriorating feeding conditions (manure being injected, rather than spread over the fields; period of injection restricted), but may also have to do with the increase in numbers of the Carrion Crow Corvus corone and overall declining insect populations. In spring 1998 and 1999, we mapped occupied Magpie nests in an area of 8700 ha. This area is situated in the northern part of Drenthe and includes the town of Assen. Densities were highest in urban areas and all nests, even in farmland, were within 100 m of human settlements (Fig. 1, Table 1). We consider this to be an anti-predator strategy. In Assen Magpies preferred a densely packed mosaic of houses, gardens, little parks and ponds. Densities were smaller in industrial sites, even when sufficient suitable nesting trees were available for breeding. This habitat type lacks cover (bushes), which increases the predation risk for Magpies. Magpie nests were absent in downtown Assen, probably because feeding opportunities were scarce in this area of mainly concrete. Besides 160 pairs of Magpies, we counted (incompletely) 35 occupied nests of Carrion Crows in the built-up area of Assen. Their numbers are increasing, but a negative impact on Magpies is not yet recorded. The population of Magpies in Drenthe is estimated at c. 1200-1500 pairs in the late 1990s. This is considerably less than the estimated 8000-9000 pairs in 1990-95 (van den Brink et al. 1996).