Flight behaviour of White Stork Ciconia ciconia on their migration over southern Israel

More than 200 flight paths of White Storks were recorded during spring and autumn 1991/92 at two sites in southern Israel by tracking radar. White Storks did not adapt flight behaviour to maximize cross-country speed as predicted by theory. Gliding airspeed was not adjusted to climb rate in thermals; cross-country speed increased simply with climb rate due to longer gliding phases. Large flocks had on average better glide ratios than single birds. Lateral wind drift was compensated only by large flocks, and the scatter of directions was reduced in flocks compared to single birds. Following the start of migration 2-3 h after sunrise, flight altitude increased until early afternoon up to 2000 m above ground. High flying birds profited from powerful thermals, while those close to the ground used all kinds of thermals. Differences in flight behaviour between spring and autumn were caused by environmental conditions. Storks adapt their flight behaviour to a lesser extent to vertical and horizontal winds than raptors, but seem to compensate this lack by profiting from migrating in flocks.

Liechti F., Ehrich D. & Bruderer B., ARDEA 84 (1): 3-13.