How Just One Bird Can Urge an Entire Flock to Change Directions | Science | Smithsonian
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A dense flock of starlings in the sky above Rome. (© Manuel Presti/Science Photo Library/Corbis)

How Just One Bird Can Urge an Entire Flock to Change Directions

The equations that describe these movements are equivalent to those that govern waves

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The starlings show up over Rome around dusk, heading for their roosts after a day of feeding in the countryside. In flocks of several hundred to several thousand, they form sinuous streams, whirling cylinders, cones or ribbons spread across the sky like giant flags. Wheeling and dipping together, they reminded Andrea Cavagna, a physicist at the National Research Council of Italy, of atoms falling into place in a superfluid state of matter called a Bose-Einstein condensate. Out of curiosity, Cavagna deployed a camera to record the flights. As a particle physicist, he says, “it was refreshing to work with something you can actually see.” But keeping track of a thousand birds turned out to be much more complicated than a billion billion atoms.

Cavagna was hardly the first scientist to be intrigued by these acrobatics—known, in a rare instance of technical language coinciding with poetry, as “murmurations.” Other animals that travel in groups—schooling fish, most obviously—show the same uncanny ability to move in apparent unison away from a predator or toward a food source. One 20th-century ornithologist seriously proposed that they coordinated their movements by telepathy. That possibility hasn’t found much support in biology. The other explanation is that a signal to change direction originates with one or a few individuals, probably on the periphery (the ones most likely to see a threat), and travels as a wave front across the flock, like a ripple spreading across a pond from a dropped pebble. It is just an artifact of human vision that we can’t see it happen in real time. But high-speed cameras can capture it, and computers can model the behavior.

It is the nature of waves that they can travel through a medium faster than the medium itself: The sound of a bicycle bell reaches your ears at a speed much greater than the bicycle itself, or than any wind that has ever blown on earth. Prince­ton biologist Iain Couzin and MIT oceanographer Nicholas Makris have shown that in the presence of a predator, or a potential food source, or an opportunity to spawn, a wave of movement crosses a school of fish five to ten times faster than any one of them can swim—“incredibly well orchestrated,” says Couzin, “like a ballet.” The fish they’ve studied exhibit a threshold response, changing course only when a sufficiently large fraction of their visible neighbors have.

As for starlings, Cavagna and his collaborators have shown recently that each keeps track of the six or seven closest starlings, adjusting its flight to stay in synchrony. In a new paper, they show how a signal originating with a single individual can cross a hundred-yard-wide flock in a fraction of a second, with virtually no distortion or diminution. The equations that describe this are those that govern waves—rather than, say, the diffusion of a gas or liquid. In the broadest sense, the same laws that photons obey are in play when a flock of starlings encounters a peregrine falcon.

Cavagna is agnostic, for now, about the evolution of such a remarkable ability, although he assumes its purpose is to defend against predators, who prefer to attack lone individuals. “I want to know how the birds do it,” he says, “not why.”

Wave phenomena show up in many biological systems. Couzin has found them in the nests of certain ant species, which exhibit a wavelike pattern of arousal and quiescence. Every 20 minutes or so a burst of activity begins near the middle of a nest and spreads outward by physical contact between individuals. He draws an analogy to brain waves, speculating that both evolved to conserve energy. Activity—whether carrying a leaf or consolidating a memory—is metabolically expensive and cannot be sustained indefinitely, so ants, or neurons, rest until they receive their cue. In its never-ending search for the most efficient solution, evolution discovers, over and over, a fundamental structure found throughout the physical universe.

As Makris observes, human beings sometimes show the same behavior. Consider “The Wave,” when a critical mass of fans at a stadium stand and raise their arms; the movement travels through the arena at a rate the Hungarian physicist Tamas Vicsek calculated at 40 feet per second.

But we don’t spend much of our time seated in tiered rows, and human society doesn’t much resemble the regular array of a school of herring. We are bombarded on all sides by information, and are driven by motives far more complex than escaping a tuna. If people could be led as easily as starlings, advertising would be a science, not an art. Waves degrade and dissipate in a noisy or disordered medium—which, it turns out, is us.

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