The secret was not revealed through a birder’s binoculars, but in math. For more than a decade, physicists and computer animators have been trying to simulate flocking behavior. Among the mysteries that may help with human crowd control: Why don’t fast-moving birds crash into each other? If one birdbrain tries to go astray, why don’t others follow? This month, a paper in the journal Physical Review E provides another clue to the mystery by thinking in terms of fluid dynamics–the behavior of gases and liquids. Four years of number-crunching by two American physicists showed that birds flock in the same way that fluids flow, with a propensity to absorb ““errors,’’ like a rogue sea gull that wants to go north for the winter. This breakthrough builds on work pioneered by software designer Craig Reynolds, whose 1986 program Boids simulated real-life flock patterns. His technique was to ““tell’’ a group of figures to move in the same direction, each maintaining a certain distance from the other. Boids quickly went Hollywood, inspiring the computer-generated bat hordes in ““Batman Returns’’ and the stampeding wildebeests in ““The Lion King.''
Boids was fine for the movies. But the ideas behind it didn’t explain why, when one member of a flock breaks formation, the flock as a whole keeps going, unperturbed and on course. Shouldn’t the whole thing fall apart? In 1993, Tamas Vicsek, a physicist at Eotvos University in Budapest, tried a different tack. He noticed that rotating colonies of bacteria–a flock of another kind–lined up like atoms in a magnet. Atoms in a bar of magnetic iron have a remarkable way of self-correcting when some get out of line.
But researchers still had a problem. Magnets are three-dimensional, with plenty of atoms on all sides to straighten out an error. But flocks can self-correct in a two-dimensional ““V’’–a more intricate problem. The new Physical Review E paper found a fresh analogy in the notion of fluid dynamics. University of Oregon physicist John Toner and his coauthor, Yuhai Tu of the IBM Watson Research Lab, assembled a model that showed a key difference between birds in flocks and the atoms in a magnet was that flocks move (graph). The new theory doesn’t try to explain how birds pick directions–Florida not Fargo–or even how the flock stays together. ““We leave that up to zoologists,’’ says Toner. But scientists hope that what they’re learning about birds and fluid dynamics will someday help them design better public spaces, such as train stations built to allow commuters streaming in from different directions to flow together smoothly–with a minimum of shoving. ““Thousands of people leaving a stadium is similar to fluid poured from a jar,’’ says Vicsek. Now if they could just get them to stop doing the wave.
Flocks maintain organized groups, even when individual birds make misjudgments.
1 Erring bird: When one member of the flock swerves, the neighboring birds will attempt to follow. The effect of the error begins to spread to the group.
2 Accommodating neighbors: Each neighboring bird goes off-course less than the one it’s shadowing. The error spreads sideways across the formation.
3 Back on track: The error diffuses among more and more birds, until it vanishes. The formation as a whole keeps heading in the original direction that it was flying in
