Ups and downs van in Drenthe broedende Tapuiten Oenanthe oenanthe

Breeding bird surveys inDrenthe(2681 km2) in 1975-2001 initially showed increasing numbers of Northern Wheatear, but in the course of the 1990s numbers started to decline sharply (Table 1). In the 1970s, the species was still fairly widespread, with some 50% of the population breeding on heaths and the rest in isolated patches of heath, sandy outcrops, peat moors and fringes of small-scale farmland. Over the years the distribution became more and more restricted to large heaths (Figs. 1 and 2). Already before 1980, the farmland population had dwindled to almost zero. In general, trends in twenty regularly surveyed large heaths showed more or less stable numbers in the 1970s whereas increasing numbers were typical of the 1980s and early 1990s. Subsequently, numbers dropped and kept declining through 2000-2001. In the 1970s, the formerly poor and sandy heather was overgrown with Molinia and other dense vegetations and thickets, most likely due to ‘acid rain’. This resulted in habitat degradation and loss of nesting sites for Northern Wheatear. After introduction of cattle and sheep (increased grazing intensity), mowing, sod removal and cutting of thickets and trees on some heaths, local Wheatear populations started booming. During the same period, new breeding sites on sandy soil became available after deforestation. The maximum population in Drenthe in this period was estimated at 300 pairs. Due to local changes in management of heaths, like reduced grazing, some favourite breeding sites were steadily overtaken by taller and denser vegetations and were subsequently abandoned by Wheatears. In contrast, two recently deforestated areas showed a remarkable population increase (Fig. 4). Predation, recreation, mortality in the African wintering areas, disturbance by grazing cattle and rising water levels in nature reserves apparently had no impact on population size. The decreasing population of Rabbit Oryctolagus cuniculus may have had a large impact through the subsequent disappearance of severely cropped grassy patches on heaths and a decline in available burrows as nesting sites (Fig. 3). All together, the main reason for the decline of the Northern Wheatear in Drenthe seems to have been loss of suitable breeding habitat.