Explaining Bird Flocks
Winging at speeds of up to 40 miles per hour, an entire flock of birds can make hairpin turns in an instant. How do they do it? A group of investigators is closer than ever to finding out.
The result has been an infusion of quantifiable observation into a field long rife with speculation. By zooming in on the three-dimensional reconstructions, the researchers can begin to understand the spatial relationships individual starlings within it have with one another. They've found that however dense a flock appears from the outside, its members are not evenly distributed like points on a grid. Rather, each member has a good deal of space behind and in front. Like drivers on a freeway, starlings don't appear to mind having neighbors nearby on their sides--or above and below, for that matter--as long as they have open space ahead.
That makes sense, since the presence of a clear path in the direction of travel minimizes the likelihood of collisions should the birds need to shift their course abruptly, as is likely when a falcon attacks. But what's really nifty about this spatial asymmetry is that the researchers have been able to use it to calculate the number of neighbors to which each starling pays close attention--a quantified elaboration of Potts's chorus line idea. By looking at correlations between the movements of neighboring starlings, they can show that each bird always pays attention to the same number of neighbors, whether they're closer or farther away.
How many neighbors is that? Six or seven, says Cavagna, who points out that starlings in flocks can almost always see many more nearby birds--but the number may be closely tied to birds' cognitive ability. Laboratory tests have shown that pigeons are readily able to discriminate between up to six different objects, but not more. That seems to be enough. Focusing on more than one or two neighbors enables a starling to maneuver quickly when needed. But by limiting to six or seven the number of neighbors it pays attention to, it may avoid cluttering its brain with less reliable, or simply overwhelming, information from birds farther away.
Whether watching those neighbors is all they do, though, is not yet known. Several StarFLAG collaborators at the University of Groningen, in the Netherlands, have been using these closely watched flocks to calibrate computer simulations more sophisticated than any others used before to analyze flock behavior. They're trying to refine the models created by the physicists to more accurately reflect the real conditions starlings face, such as gravity and turbulent air. The researchers are also trying to understand how starlings in flight communicate; though everyone agrees that they use sight to navigate in close quarters, that may not be all they use.
"I think it's acoustic and visual," says Carere, "but the exact way it works no one knows." He suggests that a starling may even use the tactile sense of onrushing air from close neighbors to help guide its direction. Clearly, there's a lot still to be learned from these most mundane of birds.
Frank Heppner is confident that researchers will soon be able to explain many such mysteries, even as he continues to question some of the most basic assumptions about flocking behavior. He wonders, for example, why the Roman starlings so spectacularly maneuver above their roosting sites for many minutes before settling down. If they really wanted to avoid falcons, he asks, wouldn't they disappear into the trees more quickly? "What they do is not predator avoidance," he says. "It's inviting predators."
He speculates that there may be some fundamental math-based behavior going on--the kind of thing that physicists call an "emergent property," in which the whole is much greater than the sum of its parts. Starlings may do what they do simply because their individual programming makes complex behaviors, like flocks, inevitable. Birders, of all people, ought to understand that, since they know how simple biological rules like a basic human interest in brightly colored, moving objects can lead to unpredictable and apparently irrational behaviors--such as jetting off to Brownsville to spot a golden-crowned warbler.
"It may be that these types of behaviors are like a mathematical by-product of the rules the birds follow," Heppner says. "It is entirely possible that you get unpredictable behavior out of predictable rules." Perhaps Rome's starlings will yet shed some light on collective decision making by people.
Some scientists affiliated with the StarFLAG project are examining how voters affect one another's choices, and whether decisions on where to locate new bank branches constitute a possible example of flocking behavior.
Such practical applications of understanding flock behaviors might be worth as much to some people as knowing the intentions of the gods. Yet they're probably less valuable than an acknowledgement of how people have already affected flocks. Starlings did not winter in Rome in such numbers in years past, but climate change, combined with other factors, has made the city more comfortable for them. Flocks of many shorebirds are diminishing as their habitats and foods are altered. And it is due to us, of course, that no one can anymore enjoy the sight of one of the greatest of flocking species: the passenger pigeon.
The most quintessentially human behavior flocks reveal, though, may turn out to be the quest to both understand and enjoy them. People want to know how the world at large operates, but they also want to simply appreciate it. Those flashing dunlins, and those starlings whirling like swift black smoke, will remain a compelling sight no matter what the computer models postulate. At least in part, they'll continue, as Richard Wilbur wrote, "refusing to be caught . . . in the nets and cages of my thought."
This story originally ran in the March-April 2009 issue as "Flight Plan."