RIBAK, G.*; WEIHS, D.; ARAD, Z.; Technion, Haifa, Israel; Technion, Haifa, Israel: Underwater maneuverability of cormorants

Great cormorants (Phalacrocorax carbo sinensis) forage by swimming underwater in search and pursuit of fish. As buoyant, foot-propelled, submerged swimmers cormorants generate hydrodynamic lift from their body and tail to overcome buoyancy during shallow dives. This implies that the birds must swim above a minimal speed underwater to generate sufficient hydrodynamic forces that will prevent them from surfacing. This minimal swimming speed limitation should affect the ability of the birds to perform sharp turns. We studied the maneuverability of cormorants in pitch, by filming birds performing maneuvers induced by an underwater (1m deep) obstacle course. The difficulty of the maneuver was progressively increased in a series of experiments by reducing the available turning radius inside the obstacle course. The kinematic analysis of the maneuvering birds suggests that minimum turning radius in pitch was 0.54 of the total body length of the birds. In all but the widest turning maneuvers the birds performed powered turns that were comprised of 3 successive paddling cycles per turn. The kinematic analysis suggests that the tail is used as a control surface to generate torque moments that initiate and stops the pitching moments of the body. This mechanism for orientation of the body is supplemented by the hydrodynamic drag from the long cylindrical shaped neck and by variation in the propulsive force of the feet between successive paddling cycles during the turn.