Between 1987 and 1994, numbers of Lesser Black-backed Gulls breeding on roofs of buildings in the industrial area of Umuiden, The Netherlands, have increased. The increase was induced by the appearance of Red Foxes in the area, which systematically plundered ground nests. Only since 1991 ground nesting in one particular area (a lock complex) occurred again (figure I), presumably indicating the disappearance of foxes in this area. The fate of Lesser Black-backed Gull chicks reared on roofs is now studied by means of colour-ringing.