Birds of a feather may flock together, but why they fly in V formations has never been known for certain.
Now, with the help of 14 northern bald ibises fitted with lightweight sensors on a 600-mile migration from Austria to Tuscany, Italy, researchers suggest the explanation is one that was long suspected but never proved: The formation helps the birds conserve energy.
Reporting in the journal Nature, the scientists write that the ibises positioned themselves in spots that were aerodynamically optimal — allowing them to take advantage of swirls of upward-moving air generated by the wings of the bird ahead. The lead bird gets no lift advantage; the ibises regularly switched leaders.
“We’ve been wondering for years whether flapping birds can save energy by following each other in the right way,” said Geoffrey Spedding, chairman of the aerospace and mechanical-engineering department at the University of Southern California, who was not involved in the study. “This work answers that question, and the answer is yes.”
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The study looked at ibises, but experts say it could apply to other birds that fly in V formations as well, like geese, ducks and pelicans.
The scientists, led by Jim Usherwood of the Royal Veterinary College in England, said a major challenge was obtaining the data. The ibises, given fetching names like Amadeus, Archimedes, Balthasar and Emma, were hatched at Zoo Vienna in March 2011 and raised as part of a conservation project aimed at reintroducing the critically endangered species to its natural range in Europe.
Some of the study’s authors served as human foster parents, taking the young birds on training flights in Salzburg, Austria. The humans rode in a paraplane, a lightweight aircraft that looks like a dune buggy attached to a parachute, and the birds followed.
“They definitely got better at flying in a V as their training flights went on,” said one of the authors, Steven J. Portugal, a postdoctoral researcher at Royal Veterinary College.
Eventually, the foster parents taught the birds their 600-mile migration route from Salzburg to Orbetello, Italy, by flying alongside them. The birds wore custom-made data loggers that allowed the researchers to track flapping, speed and direction.
Weighing less than an ounce, the devices included an accelerometer, a gyroscope, a magnetometer, a memory card, a battery, a microcontroller and a GPS unit “much better than on your iPhone,” Usherwood said. It is accurate to about 1 foot and refreshes five times per second — the resolution necessary to track the birds’ positions in relation to one another.
“Ten years ago, this wouldn’t have been possible, in terms of size and sampling frequency,” Portugal said.
The hardware and software to analyze the data sets are “the big advances, allowing us to observe behaviors of animals under natural conditions,” said Andrew A. Biewener, a biology professor and a director of Harvard’s Concord Field Station who was not involved in the study.
The researchers analyzed the birds’ positions over 7 minutes of flight and compared them with theoretical predictions generated by aerodynamic models. The upward-moving swirls of air, called tip vortices, are a byproduct of winged flight, said Kenny Breuer, a Brown University professor who, with David Willis and other colleagues, developed the predictions. As wings push air down to generate lift, other air rises to the right and left of the wings, forming the vortices.
Airplane wings also shed them; they are sometimes visible as vapor trails.
But a bird’s wake is more complicated. “The strength of those tip vortices varies throughout the phase of the wing-beat,” Breuer said. “There’s a favored position you want to fly in and a favored phase you want to flap in to take advantage of the leading bird.”
An analysis of 24,000 flaps showed that the ibises on average adjusted their position and wing phase to optimize the lift from the vortices, and readjusted their phasing when they changed positions within the V.
The new study does not say how much energy the ibises saved by these maneuvers, but small gains could be useful over long migrations, experts say.
Another open question is how the birds know to fly in these optimal spots. Usherwood said that they might have evolved “rules of thumb” for flying, or that “they have good sensors” and adjust to find spots that feel good.
“Splitting apart those possibilities would be possible with cunning experiments we have planned,” he said.
As for the ibises, they made it to Tuscany in September 2011. They are expected to spend a few years there and then, if all goes well, migrate back to Salzburg.
“This spring would be the first they would think of returning,” Portugal said. “This will be a telling year.”