We may have to shop around for airline deals, but the world's most frequent fliers long ago mastered cheap travel. Now, two studies show how some birds do it: Pelicans save energy by flying in formation, while sandpipers fly more efficiently by gorging themselves before long migrations. The findings promise to improve scientists' understanding of bird flight.
Since the dawn of human aviation, biomechanics experts have theorized about bird flight. They predicted, for instance, that birds flying in a V-formation should save energy by trailing in the wake of the leader, and that the muscle power needed for flight should increase with body mass at a certain rate. But scientists lacked the means to test such ideas on free-flying birds.
Then ecologist Henri Weimerskirch of the Centre d'Etudes Biologiques de Chizé in Villiers en Bois, France, crossed paths with a film company that had trained great white pelicans to fly in formation behind motorboats and ultralight planes. Weimerskirch and his colleagues fitted these birds with heart rate monitors and filmed them. Their results confirm for the first time the century-old conjecture that flying in a V-formation saves energy: The heart rates of pelicans flying in formation are lower than those of solo birds flying under similar conditions. And they appear to save energy not because it takes less effort to flap, as theory predicted, but because lift from the wakes of other birds allows them to flap less often.
The shorebirds known as red knots haven't been trained for the silver screen, so Anders Kvist of Lund University in Sweden and his colleagues used a wind tunnel to study these sandpipers. These small birds can wolf down almost their weight in tidal invertebrates in preparation for their 4000-kilometer migrations. Kvist's team studied the birds as they zoomed full tilt through the tube for hours without making any headway. Heavier birds used less energy per unit of mass than lighter birds, a counterintuitive result that may be a clue to how and why knots make such long nonstop journeys between hemispheres, the group suggests.
The papers, which both appear in the 18 October issue of Nature, are important for their new approaches and "add considerably to what we can say about the energetics of bird flight," says biomechanics authority Jeremy Rayner of the University of Leeds, United Kingdom, in a commentary appearing with the studies. "We haven't always asked the right questions, between the aerodynamics and what the bird experiences" physiologically, he says.