Urbanisatie van Ekster Pica pica en Zwarte Kraai Corvus corone, gedreven door vervolging, voedsel of predatie?

In an area of 2500 ha between Assen and Rolde in the province of Drenthe, all mediumand large-sized nests were mapped in 1989, 2000 and 2012. During this period the predominantly rural landscape changed radically. Especially the surface area of arable land substantially declined in favour of an expansion of built-up area, industrial grassland and natural (unfertilised) meadow (Fig. 1, Table 1). The number of raptor nests initially increased (1989-2000), but then started to decrease agian (Fig. l).The number of Magpie nests at first remained stable, but sharply increased between 2000 and 2012. Rooks settled as a breeding bird between 1989 and 2000, but their nest numbers remained low. The number of nests of Carrion Crows showed a steady decrease. Magpies once were widespread on heathlands and in woodland and farmland, but by 1989, when the first nest survey was conducted, had already disappeared from heaths. Most Magpie nests in woodland were deserted or in poor shape, whereas in farmland densities were much lower in 1989 than in the previous decade (van Dijk & van Os 1982). In 2000, the few remaining woodland-breeding Magpies had vanished and densities in farmland remained low. In urban areas (old and newly built) the number of Magpie nests increased five- to sevenfold between 1989 and 2012 (Fig. 2). The minor increase in farmland between 2000 and 2012 was restricted to settlements in villages and near farm buildings. In 1989 the number of Carrion Crows in rural landscapes (including heaths and woodland) probably had never been higher (van Dijk &van Os 1982). Between 1989 and 2000 numbers in farmland decreased slightly, but numbers on heaths and in woodland plummeted. By 2012 the species had disappeared from heaths and had decreased fivefold in woodland compared to 1989 (Fig. 3). In farmland the number of nests in 2012 was about a third lower than in 1989. In urban and suburban areas numbers increased throughout the study period and highest nest densities of Carrion Crow are now found within city limits. During the study period, the proportion of Magpies and Carrion Crows building their nest in deciduous trees increased: in 1989, 2000 and 2012 resp. 55,100 and 99% in Magpie and 50, 64 and 90% in Carrion Crow. These figures are partly inflated as both species quitted breeding in woodland where most coniferous trees occur. However, nesting by Carrion Crows in trees covered with Common Ivy Hedera helix also decreased (occurrence in 1989, 2000 and 2012 resp: 10,7 and 0 times). Predation risk in the present breeding haunts (mostly urban) is most likely much smaller than in rural areas and woodland, perhaps accounting for nest sites being less well concealed nowadays. It is discussed whether increasing predation by Goshawks (Fig. 4) and Buzzards may have driven both species from rural habitats into cities and villages, despite some evidence that feeding conditions may be better in farmland than in cities: city crows show more feather anomalies linked to food deficiency than rural crows. Breeding of Carrion Crows in coniferous trees and deciduous trees covered with dense Common Ivy in 1989 (and before) may have been a successful strategy to outwit gamekeepers, but not raptors like Goshawk and Buzzard. As soon as these predators entered the scene, and especially when they encountered declining food stocks, Carrion Crows and Magpies shifted to the relative safety of cities and villages where shooting is forbidden and far fewer Goshawks and Buzzards fly around. Perhaps this also explains the conspicuous nesting in deciduous trees in cities.