Bird flight explained

Great white pelicans flying over the Senegal River

Scientists have given a clear demonstration of why birds fly in a "V" formation.

It is something the animals do because it reduces energy expenditure, helping large birds to migrate in groups.

Everything that has been done until now has been theoretical, based on aerodynamic models

Dr Henri Weimerskirch

A French team was given a unique opportunity to study great white pelicans that had been trained to fly behind an aircraft for a feature film.

They found that the birds' heart rate went down when they were flying together and they were able to glide more often.

The energy saving may be crucial for migrating birds, which spend thousands of kilometres on the wing.

Pelicans, like many other large birds, fly in "squadron" formation flapping in time with their leader.

Gliding not flapping

Scientists from the Centre National de la Recherche Scientifique, Villiers en Bois, France, taped heart monitors to the birds' backs.

The pelicans had been trained to follow a light aircraft and a motor boat, allowing the team to observe and study them during flight.

The researchers found that when the pelicans were flying in formation - benefiting from each other's airstreams - their heart rates were lower than when they were flying solo. They also spent more time gliding.

"They fly in formation to save energy," team leader Henri Weimerskirch told BBC News Online. "It's not because they are using the upward airstream of their neighbour, it is because they are able to glide more often."

Aerodynamic models

The aerodynamic benefit of formation flying has long been suspected. But until now, it has proved impossible to test.

"Everything that has been done until now has been theoretical, based on aerodynamic models," said Dr Weimerskirch. "It cannot take in the full complexity of the flight of the bird."

The work, published in the journal Nature, suggests that formation flight evolved because it allowed birds to reduce their energy expenditure and fly further.

This would be an advantage both for migration and in hunting for food.

Flying in a "V" also has social advantages: it allows birds to communicate with each other while on the wing.

The film featuring the pelicans, Le Peuple Migrateur, goes on general release in France in December.

Images courtesy of Dr Henri Weimerskirch.