Grote Sterns Sterna sandvicensis op Griend

This paper provides a description of the life cycle of the colony of Sandwich Terns Sterna sandvicensis on the uninhabited Wadden Sea island of Griend (Netherlands) from 1964 onwards. The total number of breeding pairs was at its lowest in 1965, following a population crash due to contamination of the marine environment with chlorinated hydrocarbons. Since then, the population has recovered slowly but gradually till reaching 7600 pairs in 1988 (only a mere third of the former 21.500 pairs in 1935). The first arrivals of Sandwich Terns at Griend (end of March/beginning of April) show quite a lot of annual variation. This is probably connected with weather circumstances. On the other hand, the way in which the colony is being occupied -at first by large roosting groups- seems to ensure a good synchronization of the actual breeding cycle, of which the timing does not show so much variation. Unfavourable weather circumstances seemed to affect the course of events more markedly in April, when the birds still have to produce the eggs, than later on in summer. Only small annual differences were noted in the number of young birds fledged per pair.