Zeevogels langs de Nederlandse kust: wanneer, welke soorten en onder welke omstandigheden?

In this paper the occurrence of so-called "true” seabirds along the Dutch North Sea coast is briefly described over the years 1972-1987 as has become apparent from the systematic seawatches carried out by the "Club van Zeetrekwaamemers" (CvZ). Generally, the vast majority of the seabirds turn up in autumn (Sep-Nov, cf. figure 1) and over 96% consists of Fulmar, Gannet, Kittiwake and auks (mostly Guillemot and some Razorbill, figure 2). These species have also turned out to be the most numerous offshore in the southern North Sea. Among the other species Manx and Sooty Shearwater are still relatively frequent. Especially in autumn a wide variety of rarer birds has been recorded, among which Great and Pomarine Skua, Leach’s Petrel, Puffin and Little Auk were the commonest. The appearance of most seabirds within sight range coincides in many species with the occurrence of onshore gales. Apart from Gannets and auks, over 70% of the individuals of the most frequent species have appeared in autumn during these conditions (figure 3). Nonetheless, the number of days with onshore gales in Sep- Nov did not show a significantly positive (nor negative) correlation with the hourly means of any of the species (table 1). The correlations found in the commonest species were even negative, while positive correlations were found in the rarer ones. Total numbers of seabirds seen per hour in autumn correlated negatively with annual Sprat catches in the North Sea and positively with Herring numbers in Dutch coastal waters. The latter correlations were particularly high in the commonest species. Summarizing it is suggested that strong winds may cause wind drift into the southern North Sea as well as increased difficulty in foraging in the more exposed waters of the north. Both effects may cause the most pelagic seabirds to seek shelter in the southern North Sea, where we witness either their drifting off SW or their return north. In the commoner species wind may cause them to leave our waters and the most important factors driving them inshore seem to be scarcity