Veranderingen in voorkomen en gedrag van de Kok- en Zilvermeeuwen Larus ridibundus en L. argentatus van de VAM

In February-April 1998, we counted weekly the numbers of gulls feeding at the waste processing plant near Wijster and at their nearby roosting sites. Herring Gulls Larus argentatus (up to 6800 birds) and Black-headed Gulls L. ridibundus (up to 1200 birds) predominated at the Wijster plant (Table 1). Present numbers are 63% and 91%, respectively, lower than in the late 1970s, when waste was untreated dumped at an open disposal site resulting in a larger food availability for the gulls than at present. Almost all gulls feeding in Wijster roosted in two sand pits (De Mussels, Echten) at short distances from the plant and on roofs of factories in nearby Hoogeveen (Figure 1, Tables 2, 3). In the late 1970s, large numbers of gulls roosted at distances of over 20 kilometres from the VAM. Roosting in more nearby areas may be an adaptation to the relatively worse present feeding situation at the Wijster plant Roosting on roofs of buildings is a rather new phenomenon and might be the result of increasing numbers of Red Foxes Vulpes vulpes and Goshawks Accipiter gentilis.