The mystery of why so many birds fly in a V formation may have been solved.
(This article first appeared in BBC NEWS: SCIENCE AND ENVIRONMENT on 16 January 2014. - Ed.)
Researcher Steven Portugal explains new findings that reveal why birds fly in a V formation.
Scientists from the Royal Veterinary College fitted data loggers to a flock of rare birds that were being trained to migrate by following a microlight.
This revealed that the birds flew in the optimal position - gaining lift from the bird in front by remaining close to its wingtip.
The study, published in the journal Nature, also showed that the birds timed their wing beats.
A previous experiment in pelicans was the first real clue to the energy-saving purpose of V formations. It revealed that birds' heart rates went down when they were flying together in V.
But this latest study tracked and monitored the flight of every bird in the flock - recording its position, speed and heading as well as every wing flap.
This was possible thanks to a unique conservation project by the Waldarappteam in Austria, which has raised flocks of northern bald ibises and trained them to migrate behind a microlight.
The aim of this unusual project is to bring the northern bald ibis back to Europe; the birds were wiped out by hunting, so the team is retraining the birds to navigate a migration route that has now been lost.
Fitting tiny data loggers to these critically endangered ibises showed that the birds often changed position and altered the timing of their wing beats to give them an aerodynamic advantage.
Lead researcher Dr Steven Portugal explained: "They're seemingly very aware of where the other birds are in the flock and they put themselves in the best possible position."
This makes the most of upward-moving air generated by the bird in front.
This so-called "upwash" is created as a bird flies forward; whether it is gliding or flapping, it pushes air downward beneath its wings.
"Downwash is bad," explained Dr Portugal. "Birds don't want to be in another bird's downwash as it's pushing them down."
But as the air squeezes around the outside of the wings, it creates upwash at the wingtips.
"This can give a bit of a free ride for the bird that's following," said Dr Portugal. "So the other bird wants to put its own wingtip in the upwash from the bird in front."
The other really surprising result, the researchers said, was that the birds also "timed their wing beats perfectly to match the good air off the bird in front".
"Each bird [kept] its wingtip in the upwash throughout the flap cycle," Dr Portugal explained.
Just as the birds save energy by gaining lift from other birds, many companies that are developing unmanned aerial vehicles, or UAVs, are looking to copy the energy-efficient V formation.
"Elucidating this mechanism might go some way to helping [companies] understand how they can replicate that with their plane formation to save fuel," said Dr Portugal.
But for scientists, it is the insight into a remarkable natural phenomenon that is truly exciting.
"V formations are so beautiful," said Adrian Thomas, professor of biomechanics at Oxford University.
"We see them all the time and really want to understand and explain them, and this team has gone a long way towards doing that."
Dr Portugal added: "What these birds are able to do is amazing.
"They're able to sense what's going on from the bird in front, where this good air is coming from and how to position themselves perfectly in it.
"So from a sensory point of view, it's really incredible."
MORE ON FLAPPING AND FLYING
As a bird's wings move through the air, they are held at a slight angle, which deﬂects the air downward.
This deflection means the air flows faster over the wing than underneath, causing air pressure to build up beneath the wings, while the pressure above the wings is reduced. It is this difference in pressure that produces lift.
Flapping creates an additional forward and upward force known as thrust, which counteracts the weight and the "drag" of air resistance.
The downstroke of the flap is also called the "power stroke", as it provides the majority of the thrust. During this, the wing is angled downwards even more steeply.
You can imagine this stroke as a very brief downward dive through the air - it momentarily uses the animal's own weight in order to move forward. But because the wings continue to generate lift, the creature remains airborne.
In each upstroke, the wing is slightly folded inwards to reduce resistance.