Parelduikers Gavia arctica in de Nederlandse kustwateren

Black-throated Divers are obviously the less common species of the two smaller divers along the Dutch coast. However, occurrence and numbers are clouded by identification problems. By analysing 45,734 hours of seawatching along the Dutch coast in the years 1972-1989 some conclusions about occurrence and numbers could be drawn. The higher percentage of Black-throated Divers in the seventies than in the eighties (figure 1) could be explained by identification problems in the seventies. Occurrence in autumn and winter is not very clear. It is calculated that the occurrence involves a few hundred birds. However, from April till June there is a clear northward migration peak (figure 2) of birds in summer plumage along the northern Dutch coast. During this period Red-throated Divers are temporarily outnumbered by Blackthroated. These birds presumedly originate from the wintering grounds along the French north coast. Since the migration peak is not noticed in Belgium and the southern part of Holland, it is presumed that the bight in the coast will be cut off by the birds. Divers have increased along the Dutch coast during the eighties. Mean numbers of Blach-troated Divers increased on a smaller scale. So despite the general increase of divers, the Black-throated Diver remains a scarce species along the Dutch coast.