Abstract
Diving ducks use their webbed feet to provide the propulsive force that moves them underwater. To hold position near the bottom while feeding, ducks paddle constantly to resist the buoyant force of the body. Using video sequences from two orthogonal cameras we reconstructed the 3-dimensional motion of the feet through water and estimated the forces involved with a quasi-steady blade-element model. We found that during station holding, near the bottom, ducks use drag based propulsion with the webbed area of the foot moving perpendicular to the trajectory of the foot. The body was pitched at 76±3.47° below the horizon and the propulsive force was directed 26±1.9° ventral to the body so that 98% of the propulsive force in the sagittal plane of the duck worked to oppose buoyancy. The mechanical work done by moving both feet through a paddling cycle was 1.1±0.2 J which was equivalent to an energy expenditure of 3.7±0.5 W to hold position while feeding at 1.5 m depth. We conclude that in shallow water the high energetic cost of feeding in ducks is due to the need to paddle constantly against buoyancy even after reaching the bottom. The mechanical energy spent on holding position near the bottom, while feeding, is approximately 2 fold higher than previous estimates that were made for similar bottom depths but based on the presumed motion of the body instead of motion of the feet.
Highlights
Many diving ducks are bottom feeders that make shallow vertical dives from the surface to the benthos
Large air volumes trapped in the waterproofed plumage, air in the air-sac system and light skeletons may be useful adaptations for floating on the water surface but these adaptations translate into high buoyancy that is energetically costly underwater [4,5]
Most of the propulsive force generated during the power phase was directed on average 25.5u below the bird so that neglecting the lateral component of the propulsive force, 98% of the propulsive force projected on the sagittal plane was directed vertically to resist buoyancy
Summary
Many diving ducks are bottom feeders that make shallow vertical dives from the surface to the benthos. They propel their body underwater by synchronized paddling with both feet while the wings are folded next to the body (some sea-duck species, e.g. eiders and scoters, use both wings and feet to descend through the water column but once at the bottom they use feet alone for propulsion [1,2,3]). Large air volumes trapped in the waterproofed plumage, air in the air-sac system and light skeletons may be useful adaptations for floating on the water surface but these adaptations translate into high buoyancy that is energetically costly underwater [4,5]. As soon as paddling stops they rise passively to the surface [6]
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