Cormorants Are Double-Strokers

July 2, 2003 — Web-footed birds use a fancy two-stroke method of propulsion when they swim, according to a study published Thursday in Nature, the British science weekly.

Until now, birds had been thought to use just one form of drive when they swim or dive.

This is "drag-based" propulsion — in other words, power given by the backward kick of the webbed feet, which like oars push against the water's resistance.

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But there is a puzzle: how is the bird able to keep its momentum at the end of the kick?

After all, if the feet had only a simple fore-and-aft movement, the creature would lose a lot of the forward energy when it brings the foot forward again to start the next stroke. It would mean the birds would have to paddle much faster to keep moving than they actually do.

Underwater filming of the great cormorant (Phalacrocorax carbo) by a Swedish researcher has shown how birds not only keep the momentum but also add to it, using an upwards propulsion that derives from when the foot lifts at a critical moment in the kick cycle.

The footage showed that the bird's "drag" stroke starts parallel with the water's flow.

The legs are then brought inwards, across the line of the water's flow. This helps to create tiny vortices at the trailing edge of the webbed feet.

Towards the end of the stroke, when the feet are parallel with the bird's body, these vortices are sufficiently powerful to provide a forward and upward push.

The discovery suggests that the delta shape is no accident of nature but the result of thousands of years of evolutionary pressure to find an efficient form of propulsion, says author Ake Norberg of Sweden's Gothenburg University.

"The delta form occurs in the caudal fins of arrow worms, the caudal and pectoral fins of fishes, in the webbed feet of frogs and most swimming birds, the hind flippers of seals and the tail flukes of whales and dugongs," he noted.

And the delta shape is useful, too, in the air, not just water.

Experiments have found that many birds, such as the swallow, use the delta shape to give themselves lift — a tip from nature that has been emulated by designers of hydrofoils and airfoils.