THE USE OF THERMALS BY SOARING MIGRANTS

The use of thermals during the spring and autumn migration across Isra el by four species of soaring birds (White Pelican Pelecanus onocrotal us, White Stork Ciconia ciconia, Lesser Spotted Eagle Aquila pomarina and Honey Buzzard Pernis apivorus) was studied by monitoring them with a motorized glider, light aircraft and radar. This is the first study in which soaring migrants have been followed in night for any length of time and their night performance has been recorded directly, The bi rds new in an average height band between 344 and 1123 m above ground level. Altitude increased from the morning towards noon and decreased again in the afternoon. Average velocities were 29.2 km/h, 38.7 km/h, 50.9 km/h and 45.2 km/h for White Pelicans, White Storks, Lesser Spott ed Eagles and Honey Buzzards, respectively. Atmospheric conditions had a major effect on flight velocity. White Storks showed a positive cor relation between the night velocity and the height between the base an d top of the thermals. In White Pelicans, there was a correlation betw een velocity and mean height, Wing load (body mass/wing area) was posi tively related to the climbing time in thermals and negatively related to the mean height used by a species, There was also a positive, but not significant, relationship between wing load and velocity. Soaring birds appreciably extend the distance covered in migration in relation to the straight line from their breeding to wintering grounds (by 48- 91%). The increased distance, caused through circumventing sea areas, ranged between 22-34%, while the increase resulting from soaring accou nted for an additional 22-57% of the route.