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Uploaded:2014-04-28
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If you've ever seen a group of starlings in flight, you've appreciated one of nature's most hypnotic sights -- the lava-lamp-like flow of a murmuration. SciShow explains the biology and mathematics behind this beautiful phenomenon.
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Sources:
http://www.collective-behavior.com/publ/KingSumpter2012.pdf
http://www.wired.com/wiredscience/2011/11/starling-flock/
http://blog.allaboutbirds.org/2013/02/21/how-do-starling-flocks-create-those-mesmerizing-murmurations/
http://www.theatlantic.com/infocus/2014/02/the-murmurations-of-starlings/100690/
http://arxiv.org/abs/1012.2242
http://www.allaboutbirds.org/Page.aspx?pid=2588
http://www.pnas.org/content/early/2010/06/11/1005766107.abstract
http://www.wired.com/2010/06/starling-physics/
[Intro]

A bunch of goats is called a herd, whales travel in pods, baboons in troops, and owls in parliaments, but starlings may have the loveliest grouping of all:  murmurations.

Starlings are found all over the world, including in North America, where they were introduced in the late 1800s by a misguided Shakespeare fan who wanted to see every bird the Bard ever mentioned flooding about in the New World.

If you've ever been lucky enough to see a group of starlings winging across the Irish or Iowan sky, you've no doubt appreciated one of nature's most hypnotic sights, the fluid, lava lamp-like flow of a murmuration.

Lots of animals move in groups of course, mainly for defensive reasons.  Biologists sometimes call this the selfish herd, as large groups diminish the odds that any individual gets picked off by a predator.  But murmurations appear to be more complex than that.  Research has found that flocks of starlings respond faster to the presence of a predator, be it falcon or fox or farmer, the larger the group gets.  The more birds you have together, the more allies you have around you gathering intel, and the faster evasive action can be taken.  So instead of there being a single leader, or even group of leaders, the entire flock responds as one.  And when a predator attacks, murmurations often form waves that roll in the opposite direction.

Studies of starlings being hunted by peregrine falcons have shown that these undulations make the falcons much less successful in their hunts as they get confused and waste a lot of energy chasing the fluctuations.

So, how do these coordinated movements work, and why don't the birds crash into each other all the time?  I mean imagine a herd of 2,000 humans running around!  It would get messy really fast.

Turns out starlings have a system.  Video analysis of murmurations in Rome has shown that every starling in a flock interacts with a fixed number of birds directly around it, on average six or seven other birds.  This means that they're not connected to the whole flock or even all the birds within a certain distance.  Instead, they just link up with the next bird over in any direction, no matter how near or far.  And the shape of the flock, rather than the size, has the greatest effect on this number.  Seven seems to be the optimal number for tightly connected flocks that starlings are known for.

So if each bird's direction and velocity is influenced by the six or seven starlings around it, and each of those is interacting with a half dozen birds around it, well, when you scale that out, you begin to see how a murmuration can become so complexly and wonderfully dynamic.  They truly move as one.

But we still have a lot to learn about how these groups form in the first place and how they're maintained.  Mathematically speaking, a murmuration isn't a bunch of birds so much as it is a tipping point.  Scientists sometimes describe these points as phase transitions, like when liquids become gases or metals become magnetized or snowpack lets loose into an avalanche.  In this case, starlings start out as individuals, but moments after a certain number of them flock together, they're connected into a cohesive unit ready to respond to anything.

We also haven't quite figured out how the birds use their senses and other data to interact with each other so quickly and precisely.  Biologists have often used the word "mystery" to describe it, and it's a beautiful one at that.

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