JOHANSSON, L. Christoffer; G�teborg University: A lift-based mechanism for foot-propelled diving in cormorants (Phalacrocoracidae)?

To examine the hydrodynamic propulsion mechanism of a diving great cormorant (Phalacrocorax carbo), the kinematics was determined by digital analysis of sequential video images of the lateral view. During the acceleration phase of this foot-propelled bird, the feet move through an arc in a plane nearly parallel to the bird’s line of motion through the water. The acceleration phase is initiated by the feet being accelerated backwards resulting in a forward directed thrust. However, the feet are only moved backwards relative to the water during a brief (if at all) period of the power stroke. Instead the feet are moved upwards (towards, but beside the tail) and forward relative to the water during the later part of the acceleration phase of the bird. The results thus suggest that the foot-propelled locomotor mechanism of cormorants is based, predominantly, on a combination of acceleration reaction forces and lift, in contrast to the drag-based locomotion assumed before. This is a different lift-based mechanism than the one previously described for diving grebes. Instead, the mechanism closely resembles the thrust producing mechanism of fast starting fish (the c-start). This kind of mechanism may be applicable to a wide range of foot-propelled animals, including other birds but also mammals like the sea otter.