The breeding population is concentrated in about 14 colonies in the Wadden Sea area, around Lake IJsselmeer and in the Delta area, with colonies of up to 230 bp. Some 13 tiny settlements are recorded elsewhere. A tenfold increase since the 1970s occurred on the Wadden Sea islands (nowadays 600 bp). Newly established colonies appeared elsewhere, notably in the Delta area. Breeding at Naardermeer, annually occupied during most of the 20th century, stopped by 1988. The obvious increase reported between 1973-77 (190-240 bp) and 1998-2000 (1008-1270) is a result of improved water quality (ban on pesticides) and better protection outside the breeding season. Some regional displacements of colonies are related to increased predation risks by red foxes.
- SOVON Vogelonderzoek Nederland 2002. Atlas van de Nederlandse broedvogels 1998-20005: 1-584. Nationaal Natuurhistorisch Museum Naturalis, KNNV Uitgeverij & European Invertebrate Survey-Nederland, Leiden.