Since its settlement as a breeding bird in the Netherlands, the Common Gull Lams canus has nested in the dune area between Camperduin en Callantsoog (Noord-Holland; figure 1). The colony at the Zwanenwater, established between 1910 and 1919, is the oldest within the region. During the 1980s, the dune area was gradually colonized by the Red Fox Vulpes vulpes. Since, numbers declined at Zwanenwater, and settlements outside the dune area became rather frequent. The most important of these new settlements were found on patches of wasteland with limited human access and inside a small reserve of shallow ponds and islets. These areas are not safe from either future human disturbance or Fox predation, and, in fact, it is concluded that at present the only safe breeding site is the fenced-in dune area of the Energy Centre of the Netherlands (ECN) company in Petten. The gulls are relatively well-tolerated here and Foxes have not yet settled within the fenced-in area, thus enabling this colony to grow from 400 pairs in 1988 to 1500 in 1992. Further growth may last for a couple of years, but future plans for the construction of more buildings within the ECN complex are considered a threat for the last thriving mainland dune colony of the Common Gull in the Netherlands.