The Black-headed Gull population in The Netherlands, as in other parts of Europe, has seriously declined since the 1980s. The decline has been noted both at coastal and mainland breeding sites. The reasons for the general decline are not known. Therefore a monitoring programme was established to measure breeding results of Black-headed Gulls in six Dutch colonies in 1997. Remarkable differences were found in hatching success and breeding success. Hatching success was high (75- 90%) in the three northern colonies situated in or near the Wadden Sea, whereas in all other colonies hatching success was extremely low (0-43%). Most eggs were lost through flooding, trampling by geese, depredation or nest desertion. Only in the colony at Griend, fledging success was high (66%) and some young fledged in Julianapolder. In all other colonies, most chicks died of starvation or depredation. In those colonies the food mainly consisted of marine prey (e.g. fish, shrimps and lugworms), whereas in the other colonies the chicks were fed with a large proportion of insects and other invertebrates.