WATERBIRDS are masters of physics. When they paddle furiously across a pond, their webbed feet don鈥檛 just push against the water. They also produce hydrodynamic lift.
The discovery explains how they can shimmy across the water so quickly, and why similar delta-shaped appendages evolved independently in mammals and fish as well.
脜ke Norberg of Gothenburg University in Sweden and Christoffer Johansson of Harvard University studied detailed videos of great cormorants paddling. They then made artificial webbed feet and measured the forces these generated when they were moved to mimic the paddling motion used by the birds.
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The first phase of each stroke generates forward thrust as the foot pushes against the drag of the water. But the team found that as the bird curls its feet and trails them inwards, they produce lift, boosting the forward thrust (Nature, vol 424, p 65).