6. Summary

This report deals with the events recorded during seawatches along the Dutch North Sea coast in the first half of 1983. It consists of three main parts, the first of which deals specifically with rather detailed descriptions of the observed movements of Brent Goose, Velvet Scoter, Kittiwake and the larger auks (Razorbill and Guillemot). In these species accounts it is attempted tot get to some sort of explanation for and provide more background information to the observed phenomena than is usually done in similar reports. The second part is a chronological review in which the most important movements observed are presented month by month. third part eventually provides figures In which for all important species observed the average numbers per observation hour are plotted for each seven-day period. Because of the rather marked differences among the three main coastal areas in which regular observations take place (Zuid-Holland, Noord-Holland and Wadden Sea area, Camphuijsen & van Dijk 1983), for most species three different figures have been produced. Besides tables with total numbers of hours observed per 7-day period per observation post and with all species of non-passerine birds observed and their total numbers flying in each direction are presented. This year showed an exceptionally good migration of Brent Geese. During the whole month of March and the first two weeks of April an estimated total of some 65000 birds flew north past the coast of Noord-Holland. This extrapolation, based on the mean numbers per hour and the total number of hours of daylight, would correspond to some 30% of the world population of Dark-bellied Brent Geese in the winter of 1982/1983 after an exceptionally successful breeding season in 1982 (Ebbinge 1984). Similar extrapolations for the same observation post and the same period for previous years suggest that the amount of migration is directly related tot the world population which in its turn depends (strongly on the breeding success of the year before Ebbinge 1984). Along the other parts of the Dutch coast migration of Brents never shows such large numbers. It is suggested that in Zuid-Holland less birds are observed, because geese arriving from Britain would concentrate northwards along the Dutch west coast, while the individuals from northern Franca might make a short-cut over sea in their NE movement. Thus they would avoid the Belgian coast and the coast of Zuid-Holland but would hit the coast of Noord-Holland. On the North Sea coast of the Frisian isles (Wadden area) less Brent Geese are seen, because many move into the Waddenzee to feed there on the salt marsh vegetation during April and May to be able to depart in optimal fat condition to the breeding areas in the Taymir peninsula, Siberia. This particular spring, due to favorable weather conditions, the salt marshes provided more food than usual (Ebbinge 1984). This might explain why this year, unlike most other springs, hardly any birds were seen passing by in June.