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Page last updated at 11:40 GMT, Thursday, 11 November 2010
How starling flocks create their aerobatic displays

By Matt Walker
Editor, Earth News

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An aerial ballet

This autumn, dark clouds will gather over the UK's skies.

Far from being foreboding, they herald the start of one of nature's great spectacles.

Over the coming weeks, millions of starlings will take to the air performing a series of breathtaking aerial ballets each evening before dusk.

Such murmurations, as they are called, are a testament to the amazing, complex behaviours that animals are capable of.

Yet scientists are only just beginning to understand how they do it.

Let the show begin

The Royal Society for the Protection of Birds (RSPB) reports that many autumn starling roosts are already forming well this year.

The society expects more of the birds to flock together over the coming weeks, with numbers swelling to 10,000 or more in some places.

European starlings (Sturnus vulgaris) create these flocks upon returning to their roosts from foraging trips up to 40km away.

For centuries they have fascinated naturalists, but amateur and professional biologists alike could not explain why the flocks occur, how they move in such seemingly perfect synchrony, wheeling and turning at sharp speed, or how each starling could avoid the hundreds of other birds performing similar aerial acrobatics around it.

Until very recently, the beautiful collective movements of starlings have defied rational explanation.

STARLING SHOW
European starling (image: David Tipling; NPL)

Ideas ranged from flocks having individual leaders that orchestrate each murmuration, to the notion that some birds simply fly too fast, striking out in a new direction before swiftly returning to the following fold.

It was even speculated, with some seriousness, that flocks of birds were somehow capable of "thought transference" or "telepathy", with each bird reading the others' minds about where a flock would twist and turn next.

Gradually, more specific studies were conducted, some for instance showing that starlings have a reaction time of under 100 milliseconds, illustrating just how quickly they can respond to another's flight pattern.

Then it became generally agreed that each starling must obey three basic rules: move in the same direction as your neighbour, remain close to them, and avoid collisions.

But until 2008, no-one had been able to collect enough empirical evidence of how starlings flock together to test such ideas.

That was until a team of researchers in Italy conducted ground-breaking studies on the starlings of Rome.

Led by Dr Michele Ballerini and Dr Andrea Cavagna, the scientists used a series of interlinked cameras to directly measure in three dimensions how individual shapes of starling flocks change over time.

Some flocks measured contained up 2,700 birds.

They found that the crucial factor determining the shape of a flock is not the distance between each bird, but the number of birds flying between individuals.

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Bill Oddie conducts a starling spectacle

As a consequence, the number of birds that influence a flock's movement remains constant, they found, even if it the actual individual birds keep changing.

These scientists also found that starling flocks are not homogeneous, as the birds pack more tightly in the flock's centre than at the edges.

Birds also keep altering their place in the flock, taking turns to be at the front, sides and back.

Fewer starlings also fly in front and behind one another.

Instead most fly alongside each other. That maybe due to the structure of the starling's eyes, which see best to the side, and have a blindspot when looking to the rear.

So starlings tends to fly alongside each other, perhaps because that is where they can best see each other.

Murmuration mystery

Why starlings flock in this way is another question.

In a review published in the journal Animal Behaviour last year, Dr Iztok Bajec of the University of Ljubljana, Slovenia and Dr Frank Heppner of the University of Rhode Island, US, describe some of the main ideas.

The hypothesis most often put forward is that starlings come together in their dusk flocks to protect themselves against predators such as hawks or peregrine falcons.

That might explain for instance why individual birds keep changing places, as they are minimising their risk of being killed.

WILL THE SHOW GO ON?

Starling numbers are a fraction of what they used to be, according to the RSPB

In recent years, the UK population has crashed by up to 70%

The decline is believed to be due to the loss of permanent pastures, the use of pesticides and a shortage of food and nesting sites

That is because the edges of a flock are most vulnerable to aerial predators, so individuals limit the time they spend there.

A study published earlier this year in the Journal of Avian Biology describes the success rate of peregrine falcons hunting European starlings.

Dr Francesca Zotta of the University of Rome and colleagues describe nine different hunting strategies used by the falcons.

Peregrines attacked whole flocks rather than lone flying starlings during 90% of their hunts.

Generally, they took starlings during one in four of their attacks, with the most fruitful approach being the so-called "surprise attack" where the falcon flies directly into the flock from a long distance out.

The researchers also found that starlings at greater risk of predation by peregrines form larger, more compact flocks, adding to the evidence that murmurations are indeed an anti-predator tactic.

Energy efficient?

Another idea is that starlings flock to save energy, as flying together is somehow more aerodynamically efficient.

But this "energy-saving principle" appears to be have been ruled out by Dr Ballerini and Dr Cavagna's studies, as the distance between birds, and therefore any aerodynamic gains that would result, appears unimportant.

Despite this recent progress, after centuries of watching murmurations, we are still yet to fully comprehend why and how they do it.

In one sense, that is not surprising.

The starlings' displays are so complicated that they are being researched by physicists, aeronautical engineers, mathematicians and computer scientists, as well as biologists.

All are keen to reveal their secrets.

But with that comes a reminder: that we don't always need to ask why.

Sometimes, maybe we should just sit back and enjoy the show.

Watch a special report on starling murmurations broadcast by BBC Autumnwatch on Thursday 18 November on BBC Two at BST 20.30.



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