In vijftig jaar tijd steeds meer overwinterende ganzen in ZW-Drenthe

Since at least 1890 the south-western part of the Province of Drenthe (52o50*N, 6o20’O) is a known wintering site for geese. Counts at foraging grounds in 1959-2008 revealed concentrations of geese in agricultural (Lolium) grasslands in the valleys of Vledder, Wapserveense and Steenwijker Aa and Dwingelderstroom/Oude Vaart and approximately 10% at arable land. Geese assembled at nocturnal roosts mainly at fens in heath of the Dwingelderveld (1 in Fig. 1) and Doldersummerand Wapserveld (2) and also outside the study area in wedands in the southwest near Meppel and in the north near Assen. Irregular roosting was observed in sandpits near Vledder (3) and Havelte (4). During periods of severe frost geese stayed at feeding grounds during the night. In 1959-78 the maximum number of Bean Geese never exceeded 1000 (Fig. 2). In the severe winters of 1978/79 and 1985/86 numbers peaked at 4860 and 9100. Since 1998 the population increased further to a maximum of31,000 during the mild winter of2005/06. Until 1987/88 Bean geese with characteristics of Taiga Bean Goose Anser fahalis predominated with 65-100%. The percentage of Taigas gradually declined to 24-27% in 1989-1998 and 4-5% in 1999-2008 in favour of Tundra Bean Geese A. serrirostris (Appendix 2). The maximum number of Taigas was found between 1978/79 (1794) and 1986/87 (3568) and lowered since then to some tens or some hundreds in 1999- 2008. The decline in numbers of wintering Taigas coincided with a huge growth in the number of wintering Tundra Bean Goose, especially since 1994/95. Before 1980 Greater White-fronted Geese were rarely observed at feeding grounds. In the 1980 s the numbers rapidly increased to a maximum of5000, but in the 1990s numbers dropped. In 2003 the number suddenly increased again to a maximum of12,000 and through 2008 numbers remained high. During this period in the river valley of Dwingelderstroom, north of Dwingelderveld often 3000-12,000 feeding White fronted Geese were counted, but only a minority of900 showed up at the roost Dwingelderveld. These geese most likely spent die night on peat moors north of the study area, near Assen.The same was observed for some thousands ofWhite-fronted Geese feeding north of Meppel and roosting in wetlands west of Meppel, outside the study area. Since 2001 however some 700-6700 White-fronted Geese near Vledder irregularly were found roosting at a sandpit, in marshland or in fens of the Doldersummer- en Wapserveld. Since 1970 an increasing number of species and individuals of wintering geese were observed in SW-Drenthe (Appendix 1). Increasing numbers of geese also started breeding in SW-Drenthe since 1993. In 2008 numbers of Egyptian Goose amounted to 130 pairs, Greater Canada Goose 58 pairs, Greylag Goose 50 pairs and Barnacle Goose 7 pairs. Between 1975 and 2007 the mean data of the arrival of wintering Bean- and Greater White-fronted Geese at the roost of Dwingelderveld adv anced with 44 and 78 days respectively (Fig. 3, Appendix 3). The mean data of departure in early spring however did not change significandy. Increasing numbers of wintering geese in SW-Drenthe did not change the distribution of main feeding areas, but the number of regularly used nocturnal roost increased. Increasing numbers of winteringTundra Bean Geese were also reported elsewhere in the northern parts of the Netherlands. Other studies also revealed an earlier arrival in the wintering areas. Within the Dutch context the Province of Drenthe and especially the study area plays an important role for wintering Bean Geese (Table 2). Counts at foraging sites revealed more precise data about species composition than counts at roosts. Counting foraging birds however is more time consuming and a combination of both gives the best information about use of die area by geese. It is suggested to count geese at feeding areas simultaneous at the same day, since movements can give confusions. Identifying Taiga and Tundra Bean Goose and sampling their numbers needs improving.