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## Misjudgment in a Bird Flock

Physicists have been studying the remarkable process of how a flock of birds moves flawlessly as an organized group, even if the individual birds make frequent misjudgments. If a bird in a flock makes an error in the direction it should travel, it will tend to swerve side-to-side rapidly. You might think this error in judgment would overwhelm the other birds, causing the flock to become disoriented and fly apart very quickly. But this process actually helps keep these misjudgments under control, by quickly spreading the error among many birds so that it becomes very diluted. At one instant the erring bird might be on the left side of the flock (1), and the error affects its neighbors that are on the left side. But the next instant, the bird may be in the middle of the flock (2) -(3), and the error influences its new neighbors over there. A moment later, and the bird is on the right hand side, and it spreads to its neighbors there. As a result, the error is quickly shared among many birds and it is diluted before it has a real chance to affect the direction of a flock. At the same time the bird looks to its neighbors, which are travelling in the correct direction, and has an opportunity to straighten itself out.

When a bird misjudges the direction it should travel in a flock, it will tend to veer side-to-side in the flock much more often than advancing forward or falling behind. To understand why this happens, consider the following analogy. Imagine an inattentive driver on the highway. He starts off by moving 65 miles per hour in his lane. But he starts talking on his cell phone and veers off at a 10 degree angle. When he does this, he will still be traveling 64 miles per hour along his lane, but 11 miles per hour side to side! So he makes a big change in his side-to-side speed (over 11 mph), but the change in his forward speed is 1 mph! In short, his side-to-side motion changes drastically, while his motion in the forward direction stays about the same.

This research is reported by John Toner and Yuhai Tu in the October 1998 issue of Physical Review E. (Figures by Malcolm Tarlton, AIP.)