Migratory birds have to cross or circumvent two vast areas, the Sahara Desert and the Mediterreanean Sea, during their migratory journey between their breeding areas in Europe and their wintering areas in Africa. A lot of day migrants such as raptors, herons and storks avoid crossing the Mediterranean Sea and concentrate at the Straits of Gibraltar and Bosporus. But most birds migrate by night and until recently relatively little was known about how and to what extent these night migrants cross the Mediterranean Sea and the Sahara.
It was supposed that they fly on a more or less broad front across the Mediterranean and the Sahara during night and day (Moreau 1972, Curry-Lindahl 1981). Early radar and visual observations gave no evidence for concentrations along the shortest routes, such as Gibraltar, Sicily or Malta (Casement 1966) and many surveillance radar studies suggested no deviations in tracks when birds flew across coastlines (Eastwood 1967). In recent years however, field observations have shown that mountain ridges can lead to local concentrations of the migratory stream (Bruderer 1996) and that coasts can influence the flight behaviour of migrants (Alerstam, 1977). It was argued that at least some of the birds maintained their temporal pattern of flying by night and resting during the day even over a large ecological barrier such as the Sahara (Biebach et al. 1986, Bairlein 1988).
Therefore, the Swiss Ornithological Institute carried out a study supported by the Swiss National Science Foundation, where bird migration across the western Mediterranean was investigated in autumn 1996 and spring 1997. The aim of this study was to find out if the the migratory stream occurs over a broad front across the Mediterranean Sea and if islands such as the Balearic Islands, in particular the coastlines, influence the migratory stream. Two tracking radars were operated, one on the southern tip of the island Mallorca, the other near Malaga at the coast on the Spanish mainland. Moon and infrared observations along the French and Spanish coast provided additional information over a larger scale.
This diploma thesis was part of the project. It focused on the temporal and spatial pattern of nocturnal bird migration over Mallorca during the spring season as recorded by radar. The thesis was divided into two themes which were analysed and discussed in detail:
1) the temporal and seasonal schedule of the migration intensities and
2) the direction, speed, and potential recruiting areas of the migrants.
The thesis can be downloaded as a pdf (1.6 MB).