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.
One clue came from studies of fish. Many schooling species maneuver as intricately as the most cohesive bird flocks--and they're much easier to study, because they can be watched and photographed from above in open tanks. In the 1960s a Russian biologist, Dmitrii Radakov, tested schools and found that they can successfully avoid predators, as a whole, if each fish simply coordinates its movements with those of its neighbors. Even if only a handful of individuals know where a predator is coming from, he wrote, they can guide a huge school by initiating a turn that their neighbors emulate--and their neighbors' neighbors, and so on. Unlike linear flocks of geese, which do have a clear leader, clusters are democratic. They function from the grassroots; any member can initiate a movement that others will follow.
Refining Radakov's theory had to wait until the 1980s, when computer programmers began to create models that show how simulated animal groups can respond to the movements of individuals within them. It turns out that only three simple rules suffice to form tightly cohesive groups. Each animal needs to avoid colliding with its immediate neighbors, to be generally attracted to others of its kind, and to move in the same direction as the rest of the group. Plug those three characteristics into a computer model, and you can create "virtual swarms" of any sorts of creatures you like. They change density, alter their shape, and turn on a dime--just as real-world birds do. The makers of movies, from The Lion King to Finding Nemo, have used similar software to depict realistic-looking movements in large groups--whether stampeding wildebeest or drifting jellyfish.
The real world, though, doesn't run like software. One problem with the basic model is that it doesn't adequately explain how bird flocks can react as quickly as they do. That's something Wayne Potts realized as a graduate student in the late 1970s. Now a biologist at the University of Utah, Potts ended up studying dunlins on Puget Sound. By making movies of their flocks and analyzing, frame by frame, how each individual bird moved, he was able to show that a turn ripples through a flock just as a cheerleading wave passes through sports fans at a stadium. He explained the finding with the name of his theory: the "chorus line hypothesis." An individual dancer who waits for her immediate neighbor to move before initiating her kick will be too slow; similarly, a dunlin watches a number of birds around it, not just its nearest neighbors, for cues. This finding put to rest the old telepathy idea.
"The wave was propagating through the flock at least three times faster than could be explained if they were just watching their immediate neighbors," says Potts. "But there was probably nothing extrasensory going on."
Every year flocks of many thousands of starlings winter at large roosts in Rome. Smearing the dimming sky each afternoon, just before dusk, they fly in from the rural olive groves where they feed--faithful commuters in reverse, as Rachel Carson once wrote about birds' predictable habits. Thousands coalesce and form dense spheres, ellipses, columns, and undulating lines, sequentially changing the shape of their flocks within moments. They exasperate many residents, who tire of the droppings they leave behind. Others love their elaborate displays.
"As they approach the roosts, the starlings are regularly attacked by falcons and display amazing flocking behaviors," says Carere. "They compact and decompact, split and merge, form 'terror waves' "--pulses that move away from an approaching falcon in a split second. "This is something that by sight is fantastic, like Indian smoke signals."
In the coastal wetlands of southwestern Denmark, where some starling flocks in spring can number more than a million, locals term their late-afternoon displays "black sun" because they literally darken the sky. But the starlings in Rome are particularly convenient to study because one of their principal roosts is in a park between the city's central railroad station and one of the branches of the Roman National Museum.
Researchers from a collaborative, pan-European project named StarFLAG logged a lot of hours on the roof of the museum's historic Palazzo Massimo in two recent winters, aiming a pair of aligned cameras at flocks of many thousands of starlings performing aerobatic displays. Some researchers had previously used high-speed stereoscopic photography to analyze the structure of the whole, but they were able to do so only with relatively small groups. Once a flock exceeded 20 to 30 birds, its structure became impossible to tease apart. "You have to say who is who in the images from the different cameras, which look very different from one another," says Andrea Cavagna, an Italian physicist working with StarFLAG. "This is very difficult to do by eye, and totally impossible for a thousand birds."
By using software borrowed from the field of statistical mechanics, which explains properties of materials by examining their molecular structure, Cavagna and other physicists have now been able to match up to 2,600 starlings in different photographs with one another. That allows them to map the three-dimensional structure of flocks much more precisely than has ever been possible before. Onscreen, they can take what appears to the human eye as a solid, rounded mass of birds and learn whether it is in fact a ball or rather some other more complicated shape, such as a pancake, a column, or an open cup. They can view it from any angle, and watch it alter shape at 10 frames per second.