Predicted and observed migration speed in Lesser Spotted Eagle Aquila pomarina.

Satellite telemetry has recently been used to track the migration of eight Lesser Spotted Eagles Aquilla pomarina and one Spotted Eagle A. clanga (Meyburg et al. 1993, 1995a and b). I used aerodynamic theory to predict the overall migration speed in these eagles using either thermal soaring or flapping flight. Using Rapping eight the predicted migration speed is maximum 95 km day-1 and in thermal soaring the predicted speed varies largely depending on the assumptions on energy deposition rate and climb rate in thermals. The observed mean migration speed was 139 km day-1 for the Lesser Spotted Eagle and 150 km day-1 for the Spotted Eagle. These speeds are significantly higher than had the birds migrated by flapping flight, and so soaring flight must be involved to a large extent. There were some indications that the prediction in best agreement with the observations will be found for an energy accumulation rate Pdep = 1 x BMR and a climb rate Vc = 1-1.5 m s-1. However, at present these conclusions are provisional and the predictions may be fraud with uncertainties. I discuss which factors have a major influence on the migration speed and they include fuel accumulation rate in the field, climb rate in thermals, limitations on thermal time and energy cost of gliding flight. Future studies of migration speed using satellite telemetry should focus on these factors.

Hedenstrom A., ARDEA 85 (1): 29-36