Zwarte Stern Chlidonias niger bijna van de Drentse kaart geveegd

A reconstruction is made of the population trend of the Black Tern in the province of Drenthe during the 20th century. At the start of the 20th century, it was considered by several authors to be a ’common’ or ’rather common’ breeding bird. Anecdotal evidence suggests a wider distribution in the period of 1930-70 than in the 1970s. An incomplete census of Drenthe in 1970 revealed 322 pairs. Complete censuses are available for SW-Drenthe since the mid-1960s, where approximately 30% of the total population of Drenthe bred. Based on the ratio SW-Drenthe/Drenthe, therefore, it is estimated that the population in Drenthe must have reached some 850-1000 pairs in the 1960s and more than 700 pairs in 1970. A summation of all sites and known maximum number of breeding pairs per site, irrespective of years of occupation, indicate that Drenthe may have had a population of some 1100 breeding pairs in the period before 1965 (Appendix I). The first complete coverage of Drenthe was obtained in 1975-76 and revealed 245 pairs. A second complete survey was performed in 1980 (125 pairs). The population continued to decline till only 20-30 pairs were left in the 1990s (Fig. 1), restricted to 2-3 breeding sites. 63% of 94 colonies were deserted during the 1960s and 1970s, compared to only 21% in the years before 1960, but this discrepancy may be partly due to smaller numbers of observers before 1960. Colony size decreased by some 50% during the 20th century, especially since 1970 (Table 2). Black Terns used to breed in all kinds of shallow fresh water sites, especially fens and pools on wet heaths, in forests (fens) and in farmland (pools, canals). Foraging flights extended up to 5 km from the nesting sites (Fig. 2), mainly to canals, waterways, lake shores, wet heaths and botanically diverse meadows (rich in insects). Up to the late 1970s, droughts had a negative impact on numbers, and high water tables a positive impact. Since then, numbers did not fluctuate anymore in conjunction with water level but remained low throughout. The causes of the decline are not precisely known. Clearly, reclamation of heaths, loss of important plant communities, intensification of farming practices, drainage, lowering of the water table and water pollution must have had a detrimental effect on numbers. Recently, it was found that shortage of calcium may hamper the skeletal built-up of nestlings, causing fractures in wings and legs (Beintema 1995). It is not yet known whether this is due to lack of profitable fish. Reproductive success of Black Terns is poorly known in Drenthe, except for some years (Table 1).