ROUTES OF MIGRATING SOARING BIRDS

Soaring migrants travelling through Israel use three principal routes which are used in the opposite directions during the spring and autumn : (1) the Western Route lies mainly along the western edge of the cent ral mountain range, (2) the Eastern Route lies mainly along the Jordan Valley, crossing the mountain range during part of the day, continuin g southward along the Dead Sea towards the Sinai, and joining the West ern Route in autumn and (3) the Southern-flat Mountains Route, The geo morphological structure of Israel, with a central mountain range divid ing the country roughly into three landscape units, plays a central ro le in route selection, In the autumn, the Western Route migration axis is deflected at the beginning of the day from east to west for 10-25 km, depending on weather conditions and the flock's roosting locations , Between 10.00 h and 11.00 h, the daily breeze blowing from the Medit erranean Sea influences the migration axis, which is slowly deflected back to the east, A parallel deflection of the migration axis occurs i n the Eastern Route in the autumn. The route moves southwest over the eastern slopes of the central mountain range during the morning hours and over the slope, which absorbs direct radiation from the sun, creat ing good soaring conditions. Towards late afternoon, when the breeze f rom the sea starts, the axis is deflected to the east, to the Jordan V alley, In the flat Mountains, the wind flow plays a similar role, but because the topography of the southern Arava Valley causes a change in wind direction, the axis moves during the day in a north-south direct ion. In addition to the axis movement on a daily scale, a seasonal def lection of the migration axis from east to west also exists. During au tumn migration, early migrants (e.g. White Storks Ciconia ciconia) ten d to travel on an eastern route, while late migrants (e.g. White Pelic an Pelecanus onocrotalus) travel along the Mediterranean coast, This f luctuation was probably because of sub-optimal soaring conditions alon g the coastal plain during August. In September, temperature differenc es between the sea and land decrease and the influence of the marine i nversion gradually declines, until its influence disappears completely in October, A comparison of the numbers of soaring birds seen over Is rael in the autumn and spring shows significant seasonal differences i n the use of the various routes. For example, only one species, the St eppe Eagle Aquila nipalensis, flies over the flat Mountains in the aut umn, compared to more than 30 species in the spring, In the autumn, Wh ite Storks pass over only along the Jordan Valley axis, whereas in the spring, about half the migrating storks also pass over the western ed ge of the central mountain range, Honey Buzzards Pernis apivorus fly a long the Western Route in large numbers in the autumn, while concentra ting almost totally over the flat Mountains in the spring. These diffe rences are related to the global migration routes between the breeding and the wintering grounds in relation to the Red Sea, which birds avo id crossing. thus causing them to follow different routes in autumn an d spring.