A Flock of Birds Flows Like a Liquid and Shatters Like a Solid

In how they move; their physical state is more accurately described as "birds."

Image: Les Chatfield/Flickr

The question of "what would happen if a flock of birds flew right into a wall" has vexed physicists for who-knows-how-long (seriously, who knows?), but thanks to the wonder of computer models and some Yale physicists, we now know: Even though flocks in motion flow like beer, when they hit that wall they shatter like the bottle. Flocks of birds, turns out, don't exhibit surface tension.

This all, I admit, sounds nuts. But the dynamics of a flock are a ripe field of research that spans mathematics, zoology, and people who just have a heart for the swarm. We're not sure how the birds or fish or bugs in question know what everyone else in their group is doing; we're not really sure how the flocks work ourselves.

All metaphors used to explain swarming seem to fail. A group of researchers found that the way flocks of starlings react to changes in individual flock member's orientation is, mathematically, similar to the way that electrons spin off of particles and align with each other, as a metal is becoming magnetized. The change ripples across, starting with the first seven birds around the one that's going its own way. This same team, two years earlier, found that when one bird changes velocity, however, the whole flock is impacted, irrespective of distance from the slowing bird. This is called scale-free correlation, and is akin to the motion of avalanches. As Wired's Brandon Keim put it: “The birds’ aerial formations don’t just transcend biology, but span multiple physical phenomena.”

Which brings us back to the liquid/solid question, posed in the new study published in the American Physical Society 's journal, Physical Review E. These physicists, having observed the fluid-like motion of the flock, wanted to know if the flock also exhibited something like surface tension, wherein the individuals in the outer layer behave differently than those of the inner.

So they used a computer simulation to fire a flock of virtual birds, who were programmed to key off the direction and speed of their neighbors, right into the side of a wall. Just take a moment to consider how impossible and grim this it would be to model this without computers.

Anyway, so the virtual birds smacked into the virtual wall. “Initially, the impacting flock flattened into a pancake, compressing against the wall. Individuals at the front rebounded off the wall, disrupting the oncoming flock. This process continued until most of the particles were moving away from the wall and the original flock had broken into distinct smaller clusters,” the paper states. The clusters in a liquid would be of a uniform size, but the, uh, virtual bird clusters were more like a solid shattering.

In this round of “what is a flock like,” it would appear that there isn't anything like surface tension holding the flock together. Obviously more modeling is in order—what if a bunch of wildebeests hit a wall, for instance?—but this study is another incremental achievement in the examination of one nature's most beautiful and still-confounding phenomenon.