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How do these birds run on water?

On a windswept lake in the Pacific Northwest, two black-and-white birds with uncanny red eyes drift, preening and bobbing, until with a splash, their bodies rise from the water—not to fly, but to run. Taking up to 20 steps a second, the two crook-necked birds race across the lake for several meters before diving into its depths. Often, they surface, find each other, and repeat the courting display, which is known as rushing.

Western and Clark’s grebes (Aechmophorus occidentalis and A. clarkii, respectively) are two of the few vertebrates that can pull off the stunt. Scientists have long puzzled over how the comparatively heavy birds defy gravity. (Basilisk lizards, the next largest water runners, weigh up to an order of magnitude less.) Now, a new study reveals how the grebes use their lobed feet to walk on water.

Sometimes referred to as “dancing birds,” grebes rush to secure a mate, often in pairs but sometimes in larger groups. They refuse to rush, however, in unromantic laboratory lighting. To study the courting birds in the wild, evolutionary biologist Glenna Clifton of Harvard University and two field assistants traveled to Upper Klamath Lake in southern Oregon, one of the most popular grebe breeding sites in the country, in May 2012. They set up two high-speed cameras about 40 meters apart on the lake’s edge and waited.

Catching rushing on film, however, took skill. The behavior itself typically lasts at most 7 seconds. The prerushing rituals, like preening and calling, that Clifton and her team learned to spot and used as alerts, usually preceded the rushing by only 3 to 8 seconds. Moving quickly and communicating by walkie-talkie, the trio had to manually position, focus, and zoom each camera on birds expected to rush—often in crowds of at least 200 other grebes.

Over a month of fieldwork, Clifton estimates that she and her assistants saw more than 1000 rushing displays. They filmed more than 100 of them, but in the end they could successfully calibrate only eight videos for 3D analysis. And, of those, only two showed the grebes’ hindlimbs in adequate detail. “It was a magical moment to see these birds in such detail and in slow motion and to really see what they’re doing,” Clifton says.

The videos revealed how the birds slap their lobed feet on the water, with all three forward-pointing toes splayed, as fast as 20 steps per second. At the end of a stride, they retract their feet by drawing them to the side—with the three front toes folded together, presumably to reduce drag under water—before swinging them around and slapping the water again. That’s a different technique than the basilisk lizard, which pulls its foot out of the water the same way it went in, the researchers report online today in The Journal of Experimental Biology.

By modifying a classic laboratory experiment used with basilisk lizards, Clifton showed that grebes can generate about 50% of the force needed to keep their heads above water by slapping. As for the other half, previous research has shown how grebes’ feet have a unique ability of generating hydrodynamic lift when diving. It’s possible that they could be using a similar stroke, which works like a propeller turning about a third of a full circle, to produce the rest of the force needed to walk on water, says evolutionary ecologist Christoffer Johansson of Lund University in Sweden, who has studied how grebes use their feet while diving and swimming and was not involved in the new work.

Still, until someone studies how the feet are used under water, no one can say for sure how the rest of the force is generated. The logistics of such research, however, are daunting. “It either requires bringing the birds into the lab and convincing them to perform the dance there,” Johansson says, “or to do high-speed filming under water in the field, which I consider next to impossible. Although I wish they would try.”

(Video credit: G. Clifton, A. Turner, M. Wysocki - 2012)