Birds flying in 'V' formation save energy with flatter flaps, new research shows Lisa Lock Scientific Editor Robert Egan Senior Editor Scientists have known for years that birds like geese and ibises gain an aerodynamic advantage from flying in a "V" formation. Now, researchers from Brown University have provided new insights into the nature of that advantage. In a study published in Proceedings of the National Academy of Sciences, Brown researchers Olivia Pomerenk and Kenny Breuer developed an aerodynamic model that simulates the forces at play when one flapping bird—specifically a northern bald ibis—follows another behind and off to the side, the classic V-formation sweet spot.

The model showed that birds in that position experience an 11% reduction in the mechanical power needed for flight. Those savings are driven largely by a reduction in the vertical distance of their wing flaps. "The big change we see in this position is in the amplitude of flapping," said Pomerenk, a postdoctoral researcher in Brown's School of Engineering.

"At least for this specific bird species, we're looking at an amplitude that is something like 70% of what it would be if the bird were flying alone. That's a pretty dramatic change." Breuer, a professor of engineering and ecology, evolution and organismal biology at Brown, has long been interested in animal flight. His lab at Brown, which is equipped with a custom-made wind tunnel festooned with high-speed cameras, has made numerous discoveries about the flight dynamics of both bats and birds.

Experiments with starlings in the wind tunnel a few years ago helped confirm that the V formation helps trailing birds save energy—reducing the energetic cost of flight by 25% in wind tunnel flights. Where the savings come from The energetic benefit is likely linked in some way to wingtip vortices, tiny horizontal tornadoes that spin off the tips of each wing. The airflow from those vortices creates an area of aerodynamic "downwash" directly behind the bird and an area of "upwash" off to the side.

That upwash zone, Breuer and Pomerenk say, is the likely source of the aerodynamic advantage, but exactly how it translates into less work for a bird was a mystery. That's what this new model attempts to reveal. A major question is whether the upwash generates extra lift for the birds or extra thrust.

"If I'm a flying bird, I have two problems to solve," Pomerenk said. "I have to generate lift so I can counter gravity, and I have to generate thrust so I can fly forward. Those two problems both require energy to solve, but they might be affected differently by flying in another bird's wake." Prior research has either oversimplified the problem or overcomplicated it, Pomerenk says.

The simplest models treat birds much like fixed-wing aircraft. That eliminates the dynamics introduced by flapping, which are of obvious importance to bird flight. But the more complicated models and real-world experiments are problematic, too, often burying important dynamics in a jumble of complexity.

A model built frame by frame The researchers' new model breaks the problem down to the essentials.