干旱区生态环境知识资源中心

Long-distance migrants have to pass enormous barriers during their seasonal movements, and this is especially true for the birds that breed in Central Palaearctic and spend the winter in Africa. These birds have to cross the highlands and deserts of Western Asia, before reaching Africa. This paper reviews our long-term study on how nocturnal passerine migrants pass these barriers. Species composition and spatial distribution of these Palaearctic-African passerine nocturnal migrants were studied in deserts and highlands of western Asia (37degrees-45degreesN and 53degrees-78degreesE) by mist-netting at stopovers at 18 sites in spring and at 21 sites in autumn. The data on directions and densities of nocturnal migration were obtained by large-scale moon-watch observations. In this area only 10-20% of passerine nocturnal migrants belong to species migrating to Africa, the others winter in Asia. Over the desert zone, transit species of passerines comprise by numbers only c. 13% of all Palaearctic-African passerine migrants. In spring, migration across the desert zone occurs in a broad front but numbers at stopovers are by a factor of 4.1 higher in the north than in the south. In autumn, numbers of Palaearctic-African migrants are 9.1 times higher in the north than in the south. In the desert zone to the west of 65degreesE the numbers of transit and mainly transit species at stopovers is higher in spring than in autumn by a factor of 3.2 and 2.7, respectively. According to moon-watch observations, the density of the nocturnal flow of Palaearctic-African migrants over the western deserts is in spring 2.6 times higher than in autumn (on the average 1150 and 450 birds.km(-1).night(-1), respectively). Captures at stopovers and the few long-distance recoveries of ringed birds suggest that in autumn most transit passerines from the forest zone of the Central and Eastern Palaearctic make a detour from the north and north-west and avoid the deserts. Such behaviour makes the migratory route for the forest species of the Central and Eastern Palaearctic longer, but allows migration over more ecologically hospitable areas. Transit species of passerines from the populations of the forest zone of the Central and Eastern Palaearctic do not cross the highlands of western Central Asia in either season.