Avian Ecology and Behaviour 1, 1998:50—67. Flight directions and density of nocturnal passerine migration in the northern part of the Balkan Peninsula in autumn: preliminary results Casimir V. Bolshakov, Pavel Zehtindjiev, Victor N. Bulyuk & Alexandra Sinelschikova Abstract: Bolshakov, C.V., Zehtindjiev, P., Bulyuk, V.N. & A. Sinelschikova (1998): Flight directions and density of nocturnal passerine migration in the northern part of the Balkan Peninsula in autumn: preliminary results. Avian Ecology and Behaviour 1: 50—67. First results of study of flight direction and density of nocturnal passerine migration by the moon- watch method during 22 nights in autumn in NE Bulgaria are presented. Trapping data from August - October 1977, 1979 and 1997 concerning passerine nocturnal migrants at two stopover sites is analysed. In August and September 98-100% of trapped nocturnal migrants are trans-Saharan migrants. The main direction of nocturnal passage of these birds is SSE (165°-170°) in August and SSE-S (176°-179°) in September. The seasonal shift of main flight direction of nocturnal migration from SSE-S (August and September) to SSW-SW (October) is caused by differential passage of species with different wintering areas. The average density of nocturnal passerine migration (migration traffic rate) comprised 14 490 birds · night -1 · km -1 in September and 8080 birds· night -1 · km -1 in October. On one September night an av- erage of 11 million passerines may cross the whole northern part of the Balkan Peninsula. Key words: autumn, Balkan Peninsula, flight direction, migration density, migration traffic rate, migra- tory stopover, moon-watch method, nocturnal passerine migration, NE Bulgaria, seasonal changes, trans-Saharan nocturnal migrants. Addresses: C.V.B., V.N.B., and A.S.: Biological Station "Rybachy", Zoological Institute, Russ. Acad. Sci., Rybachy, Kaliningrad Region, 238535, Russia. E-mail: Rybachy@bioryb.koenig.su P.Z.: Institute of Zoology, Bulgarian Acad. Sci., Biological Station "Kalimok", Tsar Osvoboditel 1, Sofia 1000, Bulgaria. E-mail: Zoology@bgcict.acad.bg Introduction Radar observations of nocturnal migration over the Mediterranean (Casement 1966) support the hypothesis suggested by Moreau (1961, 1972) that many passerines from the Western Palaearctic migrate over this area on a broad front towards their African winter quarters. The flight directions over the Mediterranean are SW in the western part of the re- gion and SSE in the eastern part (Casement 1966, Hilgerloh 1988). Moreover, the available ringing recoveries suggest that some passerines migrate to African winter quarters in SW di- rection, others in SE. Different populations of Palaearctic migrants may reach Africa by dif- ferent routes, from SW to SE. Migratory divides were reported from a number of central European passerine species (Bairlein 1985, Berthold 1993, Berthold et al. 1990, Mead 1983, Zink 1973-1985). Ringing recoveries and available radar observations suggest a possibility of non-uniform distribution of African nocturnal migrants over the Mediterranean, with larger concentra-