VARIATION IN DENSITY AND HEIGHT DISTRIBUTION OF NOCTURNAL MIGRATION IN THE SOUTH OF ISRAEL

ABSTRACT

The seasonal period of nocturnal migration in southern Israel is shorter than in central Israel and much shorter than in central Europe. Long-distance migrants arrive later because the average distance from the breeding grounds is larger and many of them stop to refuel before reaching the dry southern parts of the Mediterranean. Most short-distance migrants do not cross southern Israel at all, thus reducing early spring and late autumn migration. The night—to-night variation in the density of migration is less pronounced than in central Europe due to smaller variation in meteorological factors. There is a clear gap between nocturnal and diurnal migration, practically no passerine migrants continuing migration at dawn and dusk. The steeper increase in migration after dusk in the Negev Highlands compared to the Arava Valley is explained by better resting areas in the Highlands. The schedule of passage throughout the night shows a faster decline after midnight in autumn than in spring, indicating abbreviation of nocturnal flights when birds migrate towards the desert and prolongation when approaching more fertile areas. High temperatures do not seem to abbreviate nocturnal flights. The altitudinal distribution of migration when mountain ranges are crossed is influenced mainly by atmospheric conditions and less so by the relief itself. The trade wind conditions promote migration below windshear (1200–2000 m above sea level) in autumn, and flights above this limit in spring. More than 50% of the altitudinal distribution of nocturnal migration could be predicted by the change in the tailwind component between height intervals of 200 m.