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.
A dark flock of dunlins sprints straight over a marsh--until a merlin appears and they all veer at the same moment, flashing their bright white underparts and rearranging their group into an hourglass shape with shocking swiftness. A distant murmuration of starlings--and yes, that really is the marvelous term for a group of these often-maligned birds--10,000 or more, rolls "like a drunken fingerprint across the sky," as the poet Richard Wilbur wrote, smudging the dusk horizon with the quickness of a pulsating jellyfish.
Since primeval times people have looked at masses of birds moving as one and wondered how they do it. The ancient Romans had their explanation: Gods, they believed, hinted at their intentions in the way birds flew. Scientists of the early 20th century, perhaps almost as credulous, groped for such mysterious and even mystical concepts as "natural telepathy" or a "group soul." "It is transfused thought, thought transference--collective thinking practically. What else can it be?" mused one British naturalist, rather plaintively, in 1931.
Many birds flock, of course. But only a relative handful really fly together, creating what University of Rhode Island biologist Frank Heppner, in the 1970s, proposed calling "flight flocks": namely, highly organized lines or clusters. Pelicans, geese, and other waterfowl form lines and Vs, presumably to take advantage of aerodynamic factors that save energy. But the most impressive flockers are arguably those that form large, irregularly shaped masses, such as starlings, shorebirds, and blackbirds. They often fly at speeds of 40 miles or more per hour, and in a dense group the space between them may be only a bit more than their body length. Yet they can make astonishingly sharp turns that appear, to the unaided eye, to be conducted entirely in unison. Imagine doing unrehearsed evasive maneuvers in concert with all the other fast-moving drivers around you on an expressway, and you get an idea of the difficulty involved.
No wonder observers have been left groping for an explanation. When Heppner, now semi-retired, began studying pigeon flocks more than 30 years ago, he suggested that they communicate through some sort of neurologically based "biological radio."
"The fact that we weren't hooted out of town is an indication of how desperate we were to explain this stuff," he says now.
Today, though, technological innovations, from high-speed photography to computer simulations, have enabled biologists to view and analyze bird flocks as never before. So has a new wave of interest from other scientists, including mathematicians, physicists, even economists. As a result, researchers are closer than ever to really getting inside the mind of the flock.
"There's a lot we don't know now," says Heppner, "but I think we're actually going to know how and why birds fly in organized groups within five years."
On one level it has long been obvious what's going on when animals synchronize their movements--be they ducks, wildebeest, herring, or social insects. More eyes and ears mean increased opportunities to find food and improved chances of detecting a predator in time.
It's when a predator lunges, though, that being in a crowd really pays off. Numerous studies have shown that individuals that travel in groups are almost always more vulnerable when they stray off by themselves. That's due in no small part to the bewildering things that an assemblage can do. By turning rapidly or simply tilting a bit on their axis, dunlins are able to shift the appearance of their plumage from dark (their upperparts) to light (their underparts), creating a swift flashing effect that might startle or confuse predators. Studies have shown that merlins hunting shorebirds are in fact most successful when they're pursuing individuals. Falcons do go after tightly packed crowds of dunlins and other shorebirds, but those hunts are most likely to succeed when the attack causes a solo bird to stray. Safety in numbers, in other words: Birds that stay together tend to survive together.
"Being single is always more risky," says Claudio Carere, an Italian ornithologist who is involved in a collaborative study of flocking starlings in Rome.
The British evolutionary biologist William Hamilton, in 1971, coined the term "selfish herd" to describe this phenomenon. Each member of a flock, he wrote, acts out of simple self-interest. When a predator approaches a flock, all the individuals in the group move toward the safest place--namely, the middle of the group--in order to reduce the chances of being captured. Observations of juvenile shorebirds have hinted that it may take them a while to get the hang of this, because they learn to form cohesive congregations only over time. As they do, natural selection dictates that the birds least able to hang with the group are most likely to be caught by predators.
Self-interest by itself may explain many of the observed dynamics of flock motion, such as density. But it can't explain how the birds get the information they need to move in synchrony and avoid a predator. There's no way every member of the group can see a fast-flying falcon at the same time. How, then, can they possibly know what direction to move in to avoid it?