Lift-based Paddling in Diving Grebe

To examine the hydrodynamic propulsion mechanism of a diving great crested grebe (Podiceps cristatus), the three-dimensional kinematics was determined by digital analysis of sequential video images of dorsal and lateral views. During the acceleration phase of this foot-propelled bird, the feet move through an arc in a plane nearly normal to the bird's line of motion through the water, i.e. the toes move dorsally and medially but not caudally relative to the water. The kinematics of the grebe's lobed feet is different from that in anseriforms, whose feet move in a plane mostly parallel to the bird's line of progress through the water. Our results suggest that the foot-propelled locomotor mechanism of grebes is based primarily on a lift-producing leg and foot stroke, in contrast to the drag-based locomotion assumed previously. We suggest that the lift-based paddling of grebes considerably increases both maximum swimming speed and energetic efficiency over drag-based propulsion. Furthermore, the results implicate a new interpretation of the functional morphology of these birds, with the toes serving as a self-stabilizing multi-slotted hydrofoil during the power phase.