Underwater paddling kinematics and hydrodynamics in a surface swimming duck versus a diving duck.

Some duck species mostly swim on the water surface while others frequently dive underwater. We compared the paddling kinematics of mandarin ducks (Axis galericulata) that feed on the surface and diving ferruginous pochards (Aythya nyroca) that feed underwater. Both species were trained to perform the same horizontal, submerged swimming at 1 m depth in a controlled set-up. Mandarins used alternate foot paddling exclusively, while pochards varied their gait between alternate foot paddling and simultaneous paddling with both feet. Unlike mandarins, pochards swam with their body tilted at an angle that was negatively correlated with the swimming speed and limited their foot motion to a smaller arc. Hydrodynamic modeling revealed that lift generated by the webbed foot provided thrust to propel both duck species forward. However, mandarins' feet generated lift-based upthrust that interfered with the need to counter their buoyancy, while pochards directed the foot lift to provide vertical downthrust against their buoyancy. The relatively subtle differences in foot motion between the two species result in a substantial hydrodynamic effect that may hint at the kinematic changes required when transitioning from surface to submerged swimming in the evolution of foot-propelled diving waterfowl.