How bird flocks are like liquid helium
By Marcus Woo
27 July 2014 1:00 pm
A flock of starlings flies as one, a spectacular display in which each bird flits about as if in a well-choreographed dance. Everyone seems to know exactly when and where to turn. Now, for the first time, researchers have measured how that knowledge moves through the flock—a behavior that mirrors certain quantum phenomena of liquid helium.
“This is one of the first studies that gets to the details of how groups move in unison,” says David Sumpter of Uppsala University in Sweden, who was not part of the study.
The remarkable accord with which starling flocks fly has long puzzled researchers and bird watchers alike. In the 1930s, the ornithologist Edmund Selous even suggested that the birds cooperate via telepathy. Researchers have since turned to more scientifically sound ideas, using mathematical models.
In the 1990s, physicist Tamás Vicsek of Eötvös Loránd University in Budapest came up with one of the more successful models, which is based on the principle that each bird flies in the same direction as its neighbors. If a bird angles right, the ones next to it will turn to stay aligned. Although this model reproduces many features well—how a flock swiftly aligns itself from a random arrangement, for example—a team of researchers from Italy and Argentina has now discovered that it doesn’t accurately describe in detail how flocks turn.
In their new study, the team, led by physicists Andrea Cavagna and Asja Jelic of the Institute for Complex Systems in Rome, used high-speed cameras to film starlings—which are common in Rome and form spectacular flocks—flying near a local train station. Using tracking software on the recorded video, the team could pinpoint when and where individuals decide to turn, information that enabled them to follow how the decision sweeps through the flock. The tracking data showed that the message to turn started from a handful of birds and swept through the flock at a constant speed between 20 and 40 meters per second. That means that for a group of 400 birds, it takes just a little more than a half-second for the whole flock to turn.
“It’s a real tour de force of measurement,” says Sriram Ramaswamy of the Tata Institute of Fundamental Research’s Centre for Interdisciplinary Sciences in Hyderabad, India, who wasn’t part of the research.
The fact that the information telling each bird to turn moves at a constant speed contradicts the Vicsek model, Cavagna says. That model predicts that the information dissipates, he explains. If it were correct, not all the birds would get the message to turn in time, and the flock wouldn’t be able to fly as one.
The team proposes that instead of copying the direction in which a neighbor flies, a bird copies how sharply a neighbor turns. The researchers derived a mathematical description of how a turn moves through the flock. They assumed each bird had a property called spin, similar to the spins of elementary particles in physics. By matching one another’s spin, the birds conserved the total spin of the flock. As a result of that conservation, the equations showed that the information telling birds to change direction travels through the flock at a constant speed—exactly as the researchers observed. It’s this constant speed that enables everyone to turn in near-unison, the team reports online today in Nature Physics.
The new model also predicts that information travels faster if the flock is well aligned—something else the team observed, Cavagna says. Other models don’t predict or explain that relationship. “This could be the evolutionary drive to have an ordered flock,” he says, because the birds would be able to maneuver more rapidly and elude potential predators, among other things.
Interestingly, Cavagna adds, the new model is mathematically identical to the equations that describe superfluid helium. When helium is cooled close to absolute zero, it becomes a liquid with no viscosity at all, as dictated by the laws of quantum physics. Every atom in the superfluid is in the same quantum state, exhibiting a cohesion that’s mathematically similar to a starling flock.
The similarities are an example of how deep principles in physics and math apply to many physical systems, Cavagna says. Indeed, the theory could apply to other types of group behavior, such as fish schools or assemblages of moving cells, Sumpter says.
Other models, such as the Vicsek model or others that treat the flock as a sort of fluid, probably still describe flock behavior over longer time and length scales, Ramaswamy says. But it’s notable that the new model, which is still based on relatively simple principles, can accurately reproduce behavior at shorter scales. “I think that’s cool,” he says. “That’s an achievement, really.”
Sumpter agrees. “It’s kind of reassuring we don’t need to think about the telepathic explanation,” he says.