Strategics of passerine migration across the Mediterranean Sea and the Sahara Desert: a radar study

Radar observations of the diurnal timing of bird migration in the Sahara De sert are presented for autumn migration. Study sites were on a transect alo ng the north-south migratory direction. Three groups of birds migrating eit her during day, evening or night in the northern part of the Western desert in Egypt were identified. The maximum of day and night groups occurred lat er the further south the study sites were. Based on the distance between si tes and the timing of peak migration, birds were flying at an estimated gro und speed of about 20 m/s. The maximum of the evening group was at about 21 :00 h at all sites. The three groups were classified according to three dif ferent strategies of migration across the Mediterranean Sea and the Sahara Desert: (1) the day group of birds performed a non-stop flight across the s ea and at least the northern part of the desert; (2) the night group perfor med an intermittent migratory strategy with stopover at the coast of Egypt to continue migration the next evening; (3) the evening group birds were al so intermittent migratory fliers, but they stopped somewhere in the desert after a continuous flight across the sea and part of the desert. About 20% of all migrants are involved in non-stop migration and 80% in intermittent migration with stopover at the coast (70%) or with stopover in the desert ( 10%). It is argued that any species of small passerine has the option to us e any of the three strategies.