Testing predictions from flight mechanical theory: a case study of Cory’s shearwater and Audouin’s gull

Predictions from flight mechanical theory concerning optimal flight speeds were tested in the field in two Mediterranean seabirds, the Cory’s shearwater Calonectris diomedea and the Audouin’s gull Larus audouinii. Both species were commuting off the coast of Isola di San Pietro, 6 km south-west of the coast of Sardinia. Heading and airspeed were obtained by vector calculation of flight tracks and measured wind. The Cory’s shearwater used a mixture of gliding and active flight. At low wind speeds the proportion of active flight was large but it decreased with increasing wind speed. The mean airspeed was 12.0 m s^–1, which is not significantly different from minimum power speed (V _mp) in active flight or the speed for best glide (V _bg) used in gliding flight. However, the shearwaters showed a significant response to wind increment/decrement, indicating that they were not flying at V _mp, which should be unaffected by head and tailwind. Furthermore, shearwaters can potentially reduce induced drag by the ground effect while flying close to the sea surface at weak winds, which leads to a reduction in characteristic flight speed. We suspect that the predictions for gliding flight are most valid for shearwaters at moderate to high wind speeds, when they should be maximising distance by using V _bg. Audouin’s gulls used active flight exclusively, with a mean airspeed of 11.3 m s^–1 that was significantly different from the predicted V _mp. Interestingly, though, the gulls did not show any significant wind response, indicating that they were flying close to their true V _mp when foraging along the coast. Received: 17 May 2000 / Received in revised form: 21 November 2000 / Accepted: 8 January 2001