Bill sweeping in the spoonbill, Platalea leucordia: Evidence for a hydrodynamic function

Spoonbills (Aves: Platalea spp., Ciconiiformes) characteristically feed by wading in shallow water and rhythmically sweeping their submerged and partially opened bills in an arc. This bill sweeping contrasts with the direct grasp, strike or stab employed by related species (storks, Ciconiidae; herons, Ardeidae) for similar prey (crustaceans, fish). A related feature of spoonbills is the shape of the bill: both upper and lower mandibles are dorso-ventrally flattened, the upper mandible being convex in cross section while the lower is tucked in, to result in an almost flat surface. The bill is wide throughout and broadens at the distal end. The hypothesis presented here is that spoonbills use their broad, flattened bills and lateral sweeping to shed a vortex off the tip of the bill that results in hydrodynamic suction on the bottom, which disturbs and moves prey. To achieve this the bill is used as a hydrofoil. The analysis predicts, and experimental results show that (1) in feeding, the tip of the bill is kept close to the bottom, (2) there is an inverse relationship between bill immersion depth and sweeping speed, and (3) bill sweeping over submerged prey results in the prey being lifted into the water column. This is the first reported case of an avian bill being used as a hydrofoil.