Since the 1970s, the Dutch Common Tern breeding population has been recovering from a serious decline due to organochloride poisoning. The strictly protected bird reserve on Griend in the Dutch Wadden Sea exemplified this, growing to 3300 breeding pairs in 1994. Yet even in this period breeding success on Griend was low and Common Terns often fed their chicks poor quality prey like crabs and shrimps, indicating an insufficient availability of fish. Between 1994 and 2006, the Griend population declined again to 915 pairs. A decline was also noted in other Wadden Sea colonies and strongly contrasts with increasing trends elsewhere in Europe. Here we try to explain this decline using data on population development elsewhere in the Netherlands, ring recoveries and measurements of breeding success on Griend. Population developments elsewhere suggest that birds from Griend have initially moved to other Wadden Sea islands as well as to the mainland coast of Groningen and Friesland and, at a later stage, to new islands in lake IJsselmeer. After 1997 numbers along the mainland coast decreased, while numbers in the IJsselmeer increased. Particularly the newly created island Kreupel became a very important breeding site, hosting 4100 pairs in 2006. Recoveries of adult terns ringed on Griend show movements within the Wadden Sea region (including the mainland coast) but only to a lesser extent to the IJsselmeer (Fig. 3). Only a few birds ringed in the Wadden Sea were recovered elsewhere and only a few immigrants originated from outside the Wadden Sea (Fig. 2). Estimates of breeding success made in enclosures on Griend averaged 0.41 fledglings per nest since 1992. A simple population model indicates that poor breeding success only partly explains the observed population decline on Griend. Particularly in 1996-1998 large scale emigration from Griend must have occurred, but afterwards (1998-2007) the population decline was probably mainly caused by poor breeding success (Fig. 4). Estimates of the annual net emigration from ...