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A western grebe and a Clark's grebe rushing. Photo credit: Susan Bissell

For true ballet aficionados, a snowy corps de ballet sailing across the stage in Swan Lake is one of the pinnacles of the classical cannon. But for ballerina Glenna Clifton, there is a duet that surpasses even the finest human performances. Recalling the remarkable ‘rushing’ behaviour of courting grebes – where the birds literally run across the surface of the water – Clifton describes the gravity-defying display as a beautiful pas de deux. Yet, little was known about the forces that maintain the birds above the water as they sail gracefully over the surface. Explaining that she was already interested in bird swimming, Clifton and her thesis advisor, Andrew Biewener, decided to unite her two passions and take a closer look at the birds’ extraordinary courtship display. However, rushing is a natural behaviour that can only be seen in the wild, so instead of bringing the birds to Biewener's Harvard laboratory, Clifton relocated to the grebes’ southern Oregon mating grounds.

Setting up a pair of high-speed cameras 40 m apart on the southern edge of the Upper Klamath Lake with field assistants Autumn Turner and Matthew Wysocki, Clifton soon became accomplished at predicting when a small number of the birds were about to rush: leaving the scientists as little as 8 s to accurately train both of the cameras on the birds to capture their manoeuvres in fine detail. And the fun and games didn't end there. Clifton explains that the team also filmed a short pole mounted on a remote control boat at the location where the grebes had just been moving, which they could use as a reference to reconstruct the 3D position of the grebes from the 2D movies. Eventually, after discarding hundreds of shots because of filming issues and even more after Ty Hedrick's help with the 3D calibration, Clifton was left with 100 reliable movies. Tracking the progress of the birds’ beaks to measure their speed, Clifton could see that the grebes achieved speeds of up to 4 m s^–1, while their feet rattled along at insanely high step frequencies of up to 20 steps per second.

However, when she scrutinised the video in more detail, only two of the hard-won movies showed the movements of the birds’ feet in sufficient detail to deconstruct the dainty footwork, allowing her to see all three forward-pointing toes splayed wide as the bird slapped its foot down onto the water. And when she analysed how they withdrew their feet at the end of a stride, she was surprised to see that they rotated the lower leg and pulled the foot out to the side. Clifton also noticed that the three front toes collapsed on top of each other as the foot withdrew from the water, forming a single streamlined super-toe to reduce drag.

Having revealed the movements that contribute to keeping the running birds afloat, Clifton turned to models of grebe feet to learn more about the forces that buoy them up. Plunging foot reconstructions into a bucket of water and measuring their velocity and deceleration traces as they hit the surface, Clifton calculated that the birds could generate up to 55% of the impulse required to remain above the water when they slapped the foot down. However, as she was unable to see what is going on beneath the water, Clifton is less certain about how grebes generate the additional force that is necessary to remain above the surface, but suspects that the birds sweep their feet like oars beneath the water to generate the additional lift required to keep them afloat.

• © 2015. Published by The Company of Biologists Ltd