Ibises flying in V formation. Photo: Darren Pateman
New York: Birds of a feather may flock together, but why they fly together in V formations has never been known for certain.
Now, with the help of 14 northern bald ibises fitted with lightweight sensors on a 1000-kilometre migration from Austria to Tuscany, researchers are suggesting that 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.)
A northern Bald Ibis in flight. Photo: AP
"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."
The current study looked at ibises, but experts say it could apply other birds that fly in V formations as well, such as 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 such as 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.
The Bald Ibises involved in the study. Photo: AP
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 1000km 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," Dr Usherwood said. It is accurate to about 30cm 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," Dr Portugal said.
The hardware and software to analyse the data sets are "the big advances, allowing us to observe behaviors of animals under natural conditions," said Andrew Biewener, a director of Harvard's Concord Field Station and a professor of organismic and evolutionary biology, who was not involved in the study.
The researchers analysed the birds' positions over 7 minutes of flight and compared those observations 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 professor in the school of engineering and a professor of ecology and evolutionary biology at Brown University, 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. Aircraft wings also shed them; they are sometimes visible as vapour trails.
But a bird's wake is more complicated than an aircraft.
"The strength of those tip vortices varies throughout the phase of the wing-beat," Professor Breuer said. "There's a favoured position you want to fly in and a favoured 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 optimise 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 manoeuvres, 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. Dr 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," Dr Portugal said. "This will be a telling year."
New York Times