STIENEN EWM, A BRENNINKMEIJER & J VAN DER WINDEN (2009) The decline of the Common Tern Sterna hirundo in the Dutch Wadden Sea: exodus or gradual demise?. LIMOSA 82 (3): 171-186.

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 Griend correlate well with population changes elsewhere in the Wadden Sea region, but not with those elsewhere in The Netherlands. Together with the ringing data this suggests that the Wadden Sea holds a relatively closed metapopulation of Common Terns.
      The reasons for the observed population movements are various, yet it is not clear which are most important. Feeding conditions near Griend were unfavourable in a number of years. Predation by Short-eared Owl Asio flammeus seriously disturbed the colony in some years. Flooding occurred regularly, particularly in the last decade. This caused, at least in 2007, late replacement clutches of Griend pairs on Kreupel. Altogether, breeding success on Griend, one of the best protected Dutch tern colonies, was lower than in many other colonies in the Wadden Sea and elsewhere, although differences in methodology hamper this comparison (Tab. 2). We plead for better and more standardized monitoring of breeding success of Common Terns across multiple breeding colonies. This information can be used for understanding fluctuations in the breeding populations and for underlining the importance of new breeding habitats. We still have a long way to go until the Dutch Common Tern population recovers to its original size of 30 000-40 000 pairs in the 1950s.

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