A Conundrum: Dynamic Crowd Control

Here’s a little story about two guys and a true marvel of nature.

Its early afternoon in Texas and the sun is like a burning coin, sharing the sky with a smear of high wispy clouds. Two friends are making their way across a grassy knoll, talking quietly when a mesmerizing fog of black birds rises up before them and nearly blots out the sun.

Startled, Shorty screams, “Whoa. . ! Look, Jim, there must be a thousand birds—what the hell is happening?”

The huge blob shape-shifts in a swirling liquid mass, pulsating, then suddenly shrinking to a narrow band at its center, then wildly and expansively moving upward, undulating and tumbling as if it was one large animal.

“Check it out,” Jim says pointing to a smudge in the sky, “a hawk is after the starlings. Look at ‘em twirl and dive.”

“I never seen anything like this . . . wow!” Shorty yells, “how do they do that, how do they know when to turn, where to turn and all without flying into one another? What’d you say they were?”

“I’m pretty sure these are starlings, originally from Europe but they live here now too,” Jim responds. “I don’t really know how they pull off these aerial acrobatics—I just know they’re good at it and it usually gets them out of a jam.”

Shorty: “Man. . .They’re way cooler than the Blue Angels!”

After a short time the hawk gives up, banks and vanishes into the tree line. What these two guys just witnessed is called a murmuration, a phenomenon that results when hundreds—sometimes thousands—of starlings fly in swooping, intricately coordinated patterns through the sky. Perhaps you’ve seen it for yourself.  Other birds sometimes use this evasive tactic but starlings are especially known for their show.  Here’s a video that went viral last year and it’s easy to see why: murmurations are spell-binding.

But Shorty raised some interesting questions. How do they do it with such precision, and without flying into one another? Does one bird act as choreographer, calling all the shots? Turns out, flocks of birds are seldom led by a single individual. Even with flocks of geese, the movements of the flock are actually governed collectively by all flock members.

Murmuration is a curiosity that’s been the subject of several scientific studies. Back in 2010, scientists from the National Council of Research (the University of Rome) found that starling flocks exhibit a complex physical anomaly known as “scale-free correlation.” When correlation length is proportional to the system size, it is said to be scale-free. Stated simply, the group responds as one and cannot be divided into independent parts—no matter how large the flock! When one starling changes direction or speed, each of the other birds in the flock responds to the change, and they do it nearly simultaneously regardless of the number of birds.

In effect, information moves across the flock very quickly and with nearly no degradation. Researchers call this a high signal-to-noise ratio. Compare this to the old elementary school game of “pass it along,” where one person whispers a message into the next person’s ear, who repeats it to another, and so on. Bottom line? The original message loses information very quickly. As people we cannot, it seems . . .murmurate.

There’s more science to this wonder than meets the eye. Actually, these patterns exhibit traits less from biology and more from cutting-edge physics.

When a flock turns in unison, it’s a type of phase transition. It’s one of the aspects of particle physics that defines critical transformations like those we see in proteins and neurons. Much of what is going on with starlings is best understood from the literature of “criticality,” like what happens when crystals are formed or that hypercritical tipping point just before an avalanche collapses. These events are metamorphic, hanging by just the thinnest thread, ready for near instantaneous transformation—just like our flight of starlings. Unfortunately, we simply don’t know how starlings create or maintain this state of criticality.

All right, you say, but Shorty wanted to know how, exactly, the birds avoid crashing into one another during all the chaos?

Researchers, led by George Young at Princeton, recently did their own study to attempt to answer this question (the journal PLOS Computational Biology). Interestingly, what they found is that starlings in large flocks pay very close attention to the movements of their seven nearest neighbors. So one bird mimics its seven closest neighbors, and each of those neighbor’s movements affect their closest seven neighbors and so on throughout the flock. How they do this simultaneously is still not understood, but whatever it is, it’s part of their method for achieving scale-free correlation. Short conclusion: using the tools of high-powered video analysis and computational modeling—we still don’t have a clue how it’s done! For now, it remains one of those riddles of nature.

The Princeton folks also learned that the shape of the flock, rather than the size, has the largest effect on this number. How strange is it that the number seven seems to contain the magic needed for the tightly connected flocks that starlings tend to use? Why not three, or fifteen? Here’s what we do know: the implications of this phenomenon go way beyond just birds. Expect more research in the coming years.

Shorty: “Human aviators can’t do what those birds just did. Hell, there’s always somethin’ on the news about two planes running into each other—even taxiing down the runway!”

Jim: “For sure. Animals have some incredible survival tools. Take fish, for example. You’ve seen videos of schools of fish turning on a dime? Same deal. How do they do that?”

Researchers confirm that schooling of fish is a lot like murmurization. Schools turn, contract, expand—even part and come back together—all without missing a beat. Schooling of fish can be a defense against predators or even a way to increase feeding efficiency, in some cases.

We know fish rely on visual contact to help coordinate their actions but there are also a number of other physical factors involved. For example, fish have a lateral line, an organ along each side of the body that can sense subtle changes in water pressure around them. When a swim mate turns or speeds up, for example, it will be felt in the lateral line and the fish can respond accordingly. Researchers also believe fish establish their placement and direction in a school by using other senses, like hearing, sight and even their sense of smell. All of this comes together to produce a massive coordinated movement of thousands of fish turning as one—in the blink of an eye. Problem is, the how of this schooling behavior in fish is no easier to decipher than it is in starlings. We just simply don’t know how these complicated movements are carried out.

“Hey Jim, I just had another thought. What about herding animals,” asks Shorty, “you know, like stampeding herds of bison or African wildebeests? Ain’t that about the same thing? I mean, do you think the same stuff’s happening, something that makes ‘em all move together and stay together like that?”

“Never thought about it before . . . very interesting question,” Jim said. “Now that you mention it, it does seem to happen a lot in nature. What about migrating elk, Monarch butterflies, bats, insects or any other animal that moves together as a group?”

So, this is how Jim and Shorty stumbled upon a conundrum. While herding and migration go hand-in-hand, they are not the same thing as murmurization—or are they? There are many reasons why animals migrate, but herding is done for protection using the power of large numbers. When an animal on the outside of a herd is threatened, it immediately moves toward the center, away from the danger—isn’t that essentially what murmurization is?

As a stripe of incandescent sky burns beneath the cloud base on the northwestern horizon, the two friends pause, turn and look at each other. Shorty is the first to speak:

When I was a kid I used to wonder about what made bears hibernate, caterpillars turn into butterflies and coyotes howl. As I got older, I learned the facts about all those things. It just seems to me like there are rules that nature rolls by.”

Seems Shorty has collided with a larger concept. Are there common rules underlying how shoals swim in synchronicity, flocks fly in formation and herds hurry in harmony?

Shorty’s face looked as if he was threading a needle, “In this case, we just don’t understand what those rules are.”

A muscle jumped in Jim’s jaw. “Maybe” Jim replied, “It’s also possible there’s nothing to understand, but much to appreciate and enjoy . . . perhaps there’s a divine hand in it.”

After a brief silence, the companions walked away and disappeared like quiet smoke.

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