In 1988–98, 70 volunteers and students participated in the fieldwork on Griend; we thank them all, particularly R. Hoekstra, J. van Gils, P.F. Battley, P. Wolf, A. Haberer, R. Kuiper and S. Nebel. G. Boots, R. Hoekstra, D. van Os, C. Ott, C. Raaijmakers, A. Scheele, S. Visser, T. Verbeek and J. Zuidewind helped with the samples. For logistic help we thank J. Bosch and H. Dommerholt, caretakers of Griend for Natuurmonumenten, skippers J. van Dijk and D. Kuiper of the surveillance ship Phoca, the water-police in Harlingen, the crews of the Griend and the Navicula, and D. van Holk and the late Kees de Boer of Griend's summer supply boat Griltje. D. Visser drew the figures, J. van der Meer advised on experimental design, P. de Goeij, R.H. Drent, K. Reise, R.T. Paine and L. Zwarts provided encouragement, W. de Bruin and J. Zuidewind helped with the Balgzand and Piet Scheve Plaat data, P. Tydeman helped with the Groningen data, and A. van Marrewijk processed the Hengst data. R.H. Drent, S.J. Hall, J. van der Meer, I. Johnstone, R.T. Paine, W.J. Wolff, A. Oost, C.J.M. Philippart, L. Zwarts, P. de Goeij, an anonymous referee and the editor commented on the manuscript. Since 1996, T.P. and A.D. have been supported by a PIONIER-grant to T.P. from the Netherlands Organization for Scientific Research (NWO). In addition, BirdLife/Vogelbescherming Nederland and Vereniging Natuurmonumenten, through E.R. Osieck, C.N. de Vries and T.M. van der Have, supported part of the data analysis. This is NIOZ-publication 3248.