Going with the Crowd
Why do birds travel en masse? Even with the help of computers and high math, scientists looking for answers are still up in the air
As the sun sets on the New Jersey Turnpike, the homecoming traffic at Exit 6 clogs to a familiar standstill. Suddenly, from a field adjacent to the freeway, a huge flock of European starlings takes to the air. Commuters, including sharp-eyed University of Rhode Island zoologist Frank Heppner, watch in awe as the birds--hundreds, perhaps thousands of them--perform a series of synchronized aerobatic maneuvers. They turn this way and that, wheeling, cavorting, rising, swooping.
They do it all so sharply, so quickly and with such exquisitely timed togetherness that it seems the starlings must be in instantaneous communication with each other. For the casual commuter, it's one fine air show. But for scientists like Heppner, the birds' concerted gyrations are the fleeting pieces of an enormous puzzle. Each swoop, each turn is another clue that helps bring him closer to unlocking the secret of flocking behavior, one of nature's oldest and most confounding mysteries.
From the deft maneuvers of starlings to the symmetrical V-shaped formations of Canada geese and the seemingly chaotic "gang flight" of cedar waxwings, the coordinated movement of birds on the wing has delighted human spectators for thousands of years. Beyond its purely aesthetic value, though, flock flight raises a host of fascinating and difficult questions.
Why, for instance, does one species fly in a symmetrical formation while another congregates in an aerial ball? And what about the organization of the flock--do birds in the air line up in the same social positions as they might on the ground, then follow the orders of some "leader"? If not, what is the "glue" that holds flocks together?
Scientists have been pondering these questions since the days of Pliny the Elder, the early Roman naturalist who gave us the first recorded observations of geese and starling flocks in flight. For Pliny, and for the 20 centuries of researchers who followed him, the most persistent question has been the most fundamental: Why do birds fly in flocks at all? Or, in more modern evolutionary terms, what is the adaptive value of coordinated group flight? What advantage does it confer on the individual birds, and how does it help them survive?
There are a number of possible answers, depending, to some degree, on the species and the geometry of its group flight. One of the oldest answers, posited by Pliny himself, applies to large, relatively ponderous birds like geese, which tend to fly in a V, and cormorants, which fly single-file to feeding grounds. (Smaller flocking birds--starlings, waxwings, warblers and the like--are more apt to fly in massed, tightly bunched formations.)
According to the theory, called the "aerodynamic" or "racetrack" hypothesis, birds flying single-file carve the air in such a way as to create a partial vacuum, or slipstream, which makes the going easier for the others. (Any driver who has cruised in the slipstream behind an 18-wheeler is familiar with this effect.)
Another theory suggests that birds flying in a staggered formation can save energy by placing their wingtips in the rising whirlwind of air streaming off the wings of their neighbors. About 20 years ago, researchers P.B.S. Lissaman and Carl Schollenberger at the California Institute of Technology calculated that flying in a staggered V could gain birds an energy savings of as much as 70 percent.
Even for birds that don't fly single-file or in V's, group flight apparently can confer -an aerodynamic advantage. For example, individual birds within migrating flocks of western gulls have been seen following one another into updrafts, where they can get more lift and make better headway. This allows the gulls to do what University of California at Davis biologist William Hamilton calls "cheap flying." Instead of using up energy by flapping their wings, "the birds rise and coast, rise and coast," he explains. In following one another into updrafts, he says, the gulls are essentially exchanging information--"telling" each other where the cheap flying is.
As some scientists see it, the flock possibly serves as a sort of "information center" for its members. All kinds of vital information can be exchanged through the collective experience of the individuals. For one thing, the flock can operate as a navigational aid on-the-wing, a landmark-finder that guides the way toward a destination. In this way, "if one goose or cormorant sees something familiar," says Hamilton, "it may take the lead and shift its orientation." Thus alerted and guided, the rest of the flock follows the temporary leader and stays on course.
This information center may also help the birds locate food. In the now-dwindling colonies of tricolored blackbirds at California's central valley, for example, "you'd often see long, linear flocks of birds, tens of thousands of them, in a tornadolike swirl, extending like a tentacle from the roost to the foraging area," says Hamilton. Such behavior, he adds, provides "strong evidence that being in the flock gives the individual birds information as to where the feeding ground is."
Information-sharing and aerodynamic energy conservation are two of the more popular theories scientists have put forth to explain the tendency of birds to fly in flocks. Perhaps the most widely accepted hypothesis, though, is that the flock affords protection against predators.
"Many flocking birds are weak fliers," explains Harry Power, a biologist at Rutgers University. "They're not strong enough as individuals to escape from, say, a hawk. Mountain bluebirds, on the other hand, don't forage in flocks during the nesting season, but they're strong enough as solitary fliers to outmaneuver hawks. In fact, I've watched mountain bluebirds fight off hawks attacking their young."
Safety is sometimes a matter of probability, as William Hamilton points out. "A flock in the air is certainly less vulnerable to predation than individuals flying alone," he says. "It's a simple piece of math: If there are a thousand of you and the merlin's going to get one, then, if you're not weak, your individual odds are a thousand-to-one. If there are only ten of you, then obviously the risk to each individual is much greater."
The predator-control idea has been supported by countless observations. British ethologist Nikolaas Tinbergen, for example, once watched a flock of starlings close ranks and "ball up" in the air when attacked by a peregrine falcon. This balling-up makes it much more difficult for the falcon to cull a single starling for its dinner. In addition, the falcon isn't likely to risk injury by trying to dive through the bunched flock.
Tinbergen is not the only scientist to see flock predator-control in action. On a fall day in 1957, biologist A.J. Meyerriecks watched a Cooper's hawk attack a flock of about 25 cedar waxwings near Southboro, Massachusetts. In ten minutes the hawk made five distinct sorties at the waxwings, which were flying in a chaotic, strung-out pattern. Every time the hawk attacked, the waxwings bunched together and the hawk veered off. Eventually, the frustrated--and dinnerless--predator gave up the chase and flew away.
Building on these observations, biologist Barbara Kus spent three winters in the late 1970s and early 1980s watching flocks of sandpipers along the shores of northern California's Bolinas Lagoon. During that time she saw the sandpipers attacked by merlins as many as 689 times. Gathering her data carefully, she found that the sandpipers were most likely to be attacked when flying alone, in very small flocks 12 to 25 birds) or in very large flocks (more than 500 birds).
Why the difference? Kus, now at San Diego State University, thinks that as long as the flocks stay below the 500-bird threshold, the sandpipers' intricate aerial maneuvers--darting, twisting and turning in virtual unison--successfully confuse the hungry merlins. But in larger flocks, she speculates, the sandpipers themselves are more likely to become confused and thereby vulnerable to attack.
In any case, Kus' work lends strong support to the idea that flying in flocks serves as a means of warding off predators. Still, while a great many scientists are satisfied with the idea, there are others who think that predator-control cannot be the whole story. One such skeptic is Frank Heppner, who has been pondering the mysteries of flocking behavior ever since he first spotted clouds of starlings along the New Jersey Turnpike more than 15 years ago.
"Watch a flock of starlings when it returns to the roost at dusk," Heppner says. "They spend a half hour or more just wheeling and turning in the air before they land. If flocking is to avoid predation, then this behavior doesn't make sense, because the longer they stay in the air, the more they're exposing themselves to predators."
How to explain this apparently gratuitous, even risky, behavior among starlings? Is the flock more or less mindlessly following the maneuvers of a designated "leader"? To find the answers, Heppner and graduate student Harold Pomeroy made painstaking observations of pigeons in flight during the early 1980s. They discovered that no one bird remains at the head of a flying flock for long. Pigeons that are out in front of the group going into a turn may even end up at the back when the turn is completed. "In other words," Heppner concludes, "there's no leader."
But if the other birds were not following a leader, what explained the drill-team precision of their group flight? Was their amazing synchronicity governed by some sort of collective "flock mind," a virtually instantaneous mental network? The answer was years in coming, and it came from a wholly unexpected source.
In the mid-1980s, Heppner happened to buy a small personal computer. It came with a demonstration program called "The Game of Life," which sets up a pattern of dots, then, as the players make moves, sets the dots in motion. As the dots travel, they form geometric configurations that stay coherent briefly, then break up to form other configurations, occasionally sending out "colonies" of dots along the way. "I looked at some of the movements of those colonies," Heppner recalls, "and I said: 'That's a flock of birds.'"
Heppner knew that the movements in the game were governed by a set of mathematical rules devised by the program's inventors. He began to wonder if a similar body of rules might underlie the movements of real bird flocks. "Maybe a bird flock is just a mathematical manifestation of some very basic rules that the individuals follow," he mused. "And maybe those rules have to do with the individuals responding to what their neighbors are doing."
To put his theory to the test, Heppner got in touch with Ulf Grenander, a mathematician at nearby Brown University. Grenander was intrigued and the two began to collaborate. Drawing from his zoologist's knowledge of bird behavior, Heppner suggested some possible behavioral "rules" that might have a role in determining the movements of the flocks.
First of all, he reasoned, birds have a certain amount of attraction to one another; that is, when in motion they want to stay relatively close together. On the other hand, they need to maintain enough distance to avoid colliding in midair. And to use its energy most efficiently, each bird might also have a desire to maintain a certain velocity. Heppner also suggested that individual birds have a certain amount of attraction to the flock's roost.
Finally, he determined, each bird in flight is influenced by a number of variables, such as wind, rainfall and the appearance of a predator. He learned later just how vital these "random factors" were to the success of his experiment.
With the help of Brown graduate student Daniel Potter, Grenander translated Heppner's rules into a mathematical language. The scientists then used the resulting equations to write a computer program that, they hoped, would simulate a flock of birds in flight.
The early results were disappointing: The dots "flew" around the computer screen in an every-bird-for-himself jumble, forming no coherent, organized flight pattern. But when the scientists adjusted the program to account for the random factors, they hit pay dirt. From the earlier chaos, a flocklike order began to emerge. "I played with the system," Heppner says, "adjusting values of each of the rules, and, sure enough, I eventually found that I could get something that looked very much like a flock of birds."
The question, of course, is just how realistic all of this is. As Heppner puts it, "How do we know these rules are actually being followed by real birds? It could be just mathematical fiction"--intriguing but impossible to prove.
Still, Heppner believes these simulations may effectively put to rest the idea of the flock mind. "I think our work shows that you don't need any such notion to explain how bird flocks do what they do," he says. Rather, it may be that the movements of the flock emerge from the behavior of the individuals within it.
In Heppner's view, this piece of "biomath" goes a long way toward explaining the puzzling twilight aerobatics of starling flocks. "Our simulations may be telling us that this behavior may have nothing to do with avoiding predators," he says. "It may simply be a byproduct of organized flight. The birds turn and wheel because they're organized--'programmed' to be attracted to their neighbors."
Not all biologists find Heppner's explanation satisfying. "Starlings are nasty birds," says Rutgers' Harry Power. "It may be that during those twilight flights the individual birds are showing how acrobatic they are. Maybe the birds that don't keep up get picked on later."
As to the broader question--why do birds fly in flocks in the first place? --Power isn't satisfied with a single answer, whether it be aerodynamics, information-sharing or predator-control. "Flying in flocks probably serves a multitude of functions and causes," he says. "At this point, we simply don't know them all."
Massachusetts writer Bill Lawren took in the starlings' air show during a recent traffic jam on the New Jersey Turnpike.