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V is for vortex


An endangered species helps scientists to learn why migrating birds fly in a familiar formation.

The northern bald ibis (Geronticus eremita) was once such a widespread sight in the skies of north Africa that the bird was immortalized as an ancient Egyptian hieroglyph. The picture symbol denoted the word akh, which means ‘to be resplendent, to shine’. Ibis populations are less resplendent today, with just a few hundred of the wild birds remaining, mainly in Morocco. They can still shine, however; a study of 14 northern bald ibises reported this week on page 399 offers the first experimental evidence that helps to resolve one of the great questions of the natural world: why do migrating birds often fly in an elegant V formation?

The obvious answer is that it saves energy. Just as the mass ranks of a peloton in a cycle race make life easier for riders, and as tight formations can save aircraft fuel, the signature shape of a flock of ibises or geese is assumed to make flight less of a flap — at least for the bulk of the birds that follow the leader. (That is another, less obvious, theory for the V shape: that the bird at the front is the best navigator.)

Some of the most influential research studies do little more than test whether the obvious answer to a question is the correct one. When it comes to bird flight, the validity of the obvious answer has, until now, been concealed by an obvious problem. Namely, that the equipment for monitoring the flight of wild birds tends to disappear over the horizon along with the bird to which it is attached. (Sensors that are able to relay the data tend to be too heavy for birds to carry.)

This is where the endangered plight of the northern bald ibis offered an opportunity to science. Several captive-breeding programmes exist, and a big part of preparing the birds for release is to teach them their traditional migration routes. Hand-reared ibises are trained to follow conservation experts who are inside a microlight aircraft. So, crucially, when these birds set off to fly in formation, they come back.

Steven Portugal, a researcher at the Royal Veterinary College in Hatfield, UK, used the training flights of ibises raised at a zoo in Vienna to test the benefits of formation flying. His team fitted the birds with lightweight data loggers that could measure both their body position and flapping movements.

The juvenile birds took a while to get into shape; a V formation is harder to achieve and maintain than it looks, and it looks pretty difficult. (RAF pilots told to fly in a tight V shape during the Second World War spent more time watching the position of the plane in front than scanning for enemy fighters.) Still, the 14 ibises did manage it for long enough for the scientists to accurately record both the distance between each bird and the timing of the creatures’ wing flaps.

The results: when in formation, each bird was able to synchronize the flapping of its wings so that it could exploit the updraught created by the swirling vortex of air from the flapping wingtip of the bird in front. When the flock got it right, each following bird delayed its wingbeat by just enough to spread a wave of synchrony through each arm of the V. When they got it wrong and a following bird drifted directly behind the bird in front, the follower registered the problem and adjusted the timing of its flaps so that it did not become tangled in the powerful downdraught of the same vortex. For more, see the associated News & Views article on page 295. Or look up at the sky, and delight in the rare beauty of an obvious answer.

Related links

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Related links in Nature Research

Precision formation flight astounds scientists 2014-Jan-15

Bird flight: Fly with a little flap from your friends 2014-Jan-15

Upwash exploitation and downwash avoidance by flap phasing in ibis formation flight 2014-Jan-15

Nature video: Come fly with me

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V is for vortex. Nature 505, 262 (2014).

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