De Stormmeeuw Larus canus als broedvogel in de Schoorlse Duinen

This paper summarises the rise and fall of the largest-ever Dutch Common Gull colony in the dunes of Schoorl (Noord-Holland; figure 1). After the settlement in 1931 of at least 10 pairs, numbers increased rapidly. The increase within a limited protected area of the dunes between 1952 and 1969 being on average 31.2% per annum (figure 2). In the entire dune area of Schoorl a mean yearly increase of 8.9% was recorded between 1970 and 1983 (figure 3), but after a peak number of 6000 breeding pairs in 1983 and similar numbers until 1986, a sharp decrease took place to only 800 pairs in 1992. This fall coincided with the colonization of the area by the Red Fox. Activities in the colony and their seasonal timing are briefly summarised (figures 4, 5) and some behavioural changes in the adult birds (as possible consequence of the presence of the Fox) are discussed. Reproductive success during the mid-1970s was estimated at c. 0.57 fledged young per pair, in spite of quite extensive legal and illegal egg-collecting. From 1982 onwards, breeding success dropped each year, with a total failure from 1990 onwards. In 1991 many birds did not even lay eggs. Predation by Foxes of adult birds as well as eggs and chicks (at first older chicks only, in later years also younger ones) is qualitatively described. In spite of very low reproductive success or even total failure. Common Gulls kept returning in reasonable numbers to the Schoorl colony until 1991, probably because of high site fidelity. It was not until 1992 that appreciable numbers left the colony early in the season to try and settle elsewhere.