Why do birds fly in V formations?

The latest in our 'Why is it so?' series looks at birds' uncanny aerobatic skills. James Mitchell Crow explains.

A soaring flock of ibis is one of nature’s more elegant and intriguing sights.

The poor old northern bald ibis. Close up, it is not an attractive bird. Its oversized and ungainly beak, beady yellow eyes, and the wrinkly pink skin on its feather-free head do not come together in a becoming visage. From a distance, though, things are different. A soaring flock of ibis is one of nature’s more elegant and intriguing sights. As with many other large migratory birds, flocks of ibis cross the sky in a graceful V formation.

Presumably, the birds aren’t formation flying just to look elegant. As a flock, they must be saving energy somehow. But why not fly directly in the slipstream of the bird in front, as cyclists do in the velodrome or racing cars do on the track? Why fly in a V formation? Why is it so?

For a bird – or an aircraft – energy minimisation is all about catching updrafts. When a wing slices through the sky to create lift, the air is left spinning it its wake. Directly behind the wing is a strong downwash of air. Flying there requires extra energy to maintain altitude through the downward flowing air. The slipstream is a very bad place to fly.

At the wingtip, however, the air flows differently. The spinning air rolling off the end of the wing forms a small vortex of upward-flowing air, giving followers flying there a bit of a lift.

Formation flying: Birds leave a strong downwash (red) in their wake, but a small upwash (blue) spills from their wingtip, which a following bird can ride. – F.T. Muijres & M.H.Dickinson, Nature, vol 505, p295

By riding precisely in the upwash of a lead plane, fighter jets can reduce their energy consumption by up to 18%, although this feat requires precision flying to ride the sweet spot. For birds, the challenge is even greater. The sweet spot constantly moves up and down as the lead bird flaps its wings.

To make the most of this upwash, the following bird would not only have to position itself relative to the leader, but also choreograph its flapping to keep its wing within the upward flowing air. Simply mimicking the squadron leader’s flapping doesn’t catch the air in the right way. Instead, the follower has to slightly delay its wing beats, to account for the delay in the air rolling off the leaders wings and reaching him.

Is such sophisticated flying really possible? Until recently, the only evidence was indirect: pelicans flying in V formation have a lower heart rate than those flying alone, suggesting they are doing less work to keep airborne.

To try to gather more direct evidence, a team of European researchers got up close and personal with the ungainly northern bald ibis. Once widely distributed across northern Africa and the Middle East, the species is now critically endangered. Just a few hundred adults remain in the wild. But captive breeding programs have been very successful and animals in zoos now outnumber those in the wild.

The Northern Bald Ibis is not an attractive bird but it has unlocked the secret of why large migratory birds adopt such elegant flying formations. – D. Sheldon/F1 Online/Corbis

It was captive birds, hatched at Vienna Zoo in Austria, which the team used in their research. The birds were “imprinted” on to human foster parents as soon as they hatched. The birds were taught to fly behind a powered parachute called a paraplane. The young birds had no experienced adult ibis to copy, but soon started flying in the characteristic V formation.

To gather data on how the birds fly, the team fitted 14 individuals with miniaturised GPS backpacks that log body and wing position. From the paraplane, they also filmed the birds in flight. At the end of a day’s flying, each bird’s flight data was downloaded and analysed.

Sure enough, they found that the free-flying birds really do precisely surf the upwash of the bird in front. Writing in Nature this January, the researchers confirmed that, in northern bald ibis at least, the birds accurately modulate their body position and wing motion to best catch the upward flowing air.

Problem solved – and yet there are plenty of questions still to be answered, say bird flight experts. How do the birds precisely find the sweet spot? And why for example is it only large birds such as ibis and geese that do it? Is the energy payback not the same for small migratory birds?

Perhaps some pretty little songbird will unlock these answers. But they won’t match the majesty of the migrating ibis.

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