Zwanenzang van weidevogels in stroomdal van de Drentsche Aa: een reconstructie

After farming was introduced several thousands of years ago, the primeval forests in Drenthe were quickly converted into heaths. In river valleys deforestation took place as late as medieval times. Till well into the 20th century', meadows must have been a paradise for many species of meadow birds. Early in the 20th century, with the introduction of artificial fertilisers, heaths were turned into arable land and forests. Initially, this may have been profitable for some species that experienced an increase in food supply and breeding habitat. However, the same process also heralded the beginning of the end. Nevertheless, until 1970 farmers still complained about parts of the Drentsche Aa being too wet for hay production or for grazing cattle. During the last land reforms, mainly during 1960-75, the surrounding farmland was converted into the present large fields with draining systems of up to two meters deep. Part of the river valley could be saved by the State Forestry (Fig. 1), and in these parts the structure of the landscape largely remained untouched (Fig. 2). The hydrological system, however, could not be preserved and consequendy the high botanical and avifaunal diversity was lost. To reconstruct the meadow bird trends for the past 40 years, all' available quantitative data were used, the oldest source dating back to 1965. Sample size fluctuated considerably over the past decades (Fig. 3), only recendy covering more than half of the area. Also, methodology and intensity of fieldwork showed wide variations. Some species probably were already in declining by 1965 (Ruff, perhaps Black-tailed Godwit). Others were more tolerant towards drainage, or at first even profited from desiccation of formerly wet grasslands, like Lapwing, Curlew, Redshank and Oystercatcher. Continued drainage and removal of the top layer with heavy machinery led to invasion of bull rush since 1985. Presently, meadow bird species in both reserve and adjoining farmland have declined to historically low levels, or even disappeared.