The use of thermals by soaring migrants

Abstract

The use of thermals during the spring and autumn migration across Israel by four species of soaring birds (White Pelican Pelecanus onocrotalus, 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 flight for any length of time and their flight performance has been recorded directly. The birds flew 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 Spotted Eagles and Honey Buzzards, respectively. Atmospheric conditions had a major effect on flight velocity. White Storks showed a positive correlation between the flight velocity and the height between the base and top of the thermals. In White Pelicans, there was a correlation between velocity and mean height. Wing load (body mass/wing area) was positively 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 accounted for an additional 22–57% of the route.

Understanding soaring bird migration through interactions and decisions at the individual level

“…Little is known about distances at which birds can sense thermals yet we expect thermals with birds in them to be detectable at farther distances than thermals without birds. Field observations show that storks regularly fly to thermals that are several kilometres away and occasionally use visual cues to reach thermals that are even tens of kilometres away (Leshem and YomTov, 1996b). A study on foraging seabirds…”

“…Climbing efficiency accounts for the difference between thermal lift and the actual climb rate of the bird; climb rate is influenced by its specific morphology and the characteristics of the thermal. In our simulations ce is set to 0.8 to account for time lost during circling (Leshem and YomTov, 1996b). When entering a thermal, a bird is (for simplicity but without loss of generality) placed in the centre of the thermal and climbs in a straight vertical line.…”

“…The reason why we do not use total time travelled (ttt) for evaluating the model is based on the fact that the distance can be related better to data that is available from the literature (e.g. Leshem and YomTov, 1996b).…”

“…No parameter optimization is employed to tune the model so that particular results are achieved. The simulation length is eight hours, a typical duration of one day of flight for white storks (Leshem and YomTov, 1996b) times. In each simulation, the locations of thermals differ (uniform random distribution over the simulation domain), but the starting locations of the birds remain the same.…”

“…The simulation length can lay between one hour (corresponding to visiting a dozen thermals) and eight hours (which corresponds to migration over one day, Leshem and YomTov, 1996b), and the simulation time step should approximately be one minute. A time-step of one minute, in combination with realistic model parameter values (see Table 2), prevents overshooting when a bird glides towards or climbs in a thermal.…”