Voorjaarstrek van Grote Sterns Sterna sandvicensis langs de Nederlandse kust: interpretatie van seizoenspatroon

This paper decribes the seasonal pattern of the spring migration of the Sandwich Tern Sterna sandvicensis along the Dutch North Sea coast. It was found that in most years a distinctive peak occurred during the last two weeks of April and the first week of May, followed by a second peak lasting till the end of June (fig. 1). The first peak was assumed to refer largely to potential breeding birds, while birds participating in the second peak would probably be arriving late and so to a large extent unsuccesful in occupying a territory on the breeding colonies. In some years, however, the second peak was both much more pronounced and more prolonged while the first peak was virtually absent (fig. 2). Both low temperatures during March and April and a high percentage of onshore winds, probably causing turbulent water with low transparencies, showed a negative influence on the numbers of terns involved in the first peak. Hourly averages of migrating birds during the first peak correlated positively but not significantly with the number of breeding pairs in Hollands most important colony at Griend (fig. 3). A calculated ”timing factor” (peak period hourly average/overall hourly average) correlated positively and significantly with the colony size (fig. 4). Weather and food conditions met during spring migration may cause serious delays in migration timing, resulting in lower numbers at important breeding colonies.