Some 500 breeding colonies are rather evenly distributed over the lower grassland areas and wetlands, with the highest numbers in the provinces of Noord- and Zuid-Holland. Urban colonies are common feature in several cities. Numbers/colony range up to 300 bp, larger numbers of over 1000 bp are historic. In 1998-2000 10,000-12,750 bp were recorded, roughly comparable to 1973-77 (8700-11,000). Besides from temporary declines after severe winters, some decrease was recorded in the northeast of the country. A firm increase and range extension was reported in the south and southwest, although most colonies here are rather small. Highest-ever number was 13,500 bp in 1995.
- SOVON Vogelonderzoek Nederland 2002. Atlas van de Nederlandse broedvogels 1998-2000. Nederlandse Fauna 5: 1-584. Nationaal Natuurhistorisch Museum Naturalis, KNNV Uitgeverij & European Invertebrate Survey-Nederland, Leiden.