Stanley Tate on Birds
A Murmurartion of Starlings
Earlier this year an e-mail from a friend linking me to a short online video titled “Dylan Winter and the Starling Murmurations” landed in my inbox. Unsure what to expect, I clicked on the link.
For the first few seconds the video showed only a man walking along a field road at sunset. As he walks, he talks about photographing wildlife all over the world. Then he describes a flock of thousands of starlings that cover the sky before going to roost near his home in England. As he continues to talk the camera adjusts its focus and you realize that the sky is full of birds silhouetted in impossibly coordinated patterns from one horizon to the other.
For the next several minutes the birds perform an intricate aerial display. Their wings murmur and whoosh as they make low passes over him. Then they climb and form dark masses that bunch and flatten, divide, swoop, loop, and pass low over a field again and again. Suddenly they go to ground and it’s over. Starlings habitually gather at twilight, sometimes by the hundreds of thousands in late summer; and just before they settle into bed the birds collectively patrol the airspace above their sleeping quarters, sometimes for half an hour or more.
Why starlings do this is a good question. Burning extra energy before drifting off? Guiding incoming stragglers? Keeping an eye out for predators? Who knows, but the spectacle is as beautiful as it is mysterious. They resemble iron filings suspended in the sky and guided by an invisible magnet. How can they stay in such a rapidly changing close formation without crashing into one another?
Scientists call this collective behavior self-organizing, which makes starlings interesting because things in our universe tend toward disorder, not order.
Let us digress. When Serena Williams cracked a backhand down the line to open the third set of her quarter final match against Jennifer Capriati in the 2004 U. S. Open a line judge ruled the ball good. But the chair umpire thought it had landed out and overruled the call. Williams threw a tantrum and lost the match. Slow motion replays later showed that the line judge was correct and that at least two other balls had been incorrectly called against her in the same set. The tournament apologized and yanked the umpire, too late to stop a firestorm of protests from indignant players and fans. The masses demanded the use of instant replays.
The tennis association had been testing a system called Hawk-Eye and within a year of Serena’s controversial loss, the instant replay was approved for professional tournaments. Hawk-Eye takes images from ten different cameras around a tennis court and fuses them into one to predict where the ball will land relative to a line on the court.
At about the time Hawk-Eye’s unblinking lens began to gaze down from the stands at tennis tournaments, a group of Italian physicists and statisticians were peering into the sky over Rome, trying to make sense of the enormous flocks of starlings that appeared as if by magic every evening. They knew that computers had been used to simulate flocks of bats and penguins for the movies. But until the incident at the 2004 U.S. Open there had been no way to collect data that described actual flocks of birds. What would happen, they wondered, if they combined the Hawk-Eye principle and the concepts used to produce bats and penguins for a movie?
The Italian scientists set up cameras overlooking the Baths of Diocletian where a large flock of starlings was known to roost and using the same stereo-photography methods as Hawk-Eye, snapped pictures whenever a group of starlings flew through the space in view of the cameras. They spent a year collecting images and then two more years developing an algorithm that could analyze the photos and match the birds. When complete, their program could process flocks of up to 8,000 birds with 90 percent accuracy. When they fed the program with images from their camera setup the team could see starling flocks as no one had ever done before.
Starling flocks, it turns out, are thinner than expected – more like a pancake than a football. The pancake slides around in various directions, shifting its appearance, but it generally stays parallel to the ground and maintains a constant proportional shape, no matter the size of the flock. The density of the flock is higher toward the edges – starlings are more tightly packed at the fringes than at the center. And starling flocks don’t have leaders. When a flock turns, birds fly on equal-radius paths; in other words, they each turn on the same curve at the same speed. In a column of marching soldiers, those on the outside of a turn must march faster to maintain their positions. Starlings don’t compensate like soldiers do, birds at the front of the flock end up on the right side after a left turn, those on the right side end up at the back and those at the back end up on the left side – no bird stays in the front position all the time and each bird spends the same amount of time on an edge.
Finally the scientists compared behavior of individual starlings within the flock and found that to avoid collisions they stay at least a wing’s length away from one another and seldom stray enough from one another to break up the flock. Starlings align themselves by using their nearest seven neighbors to decide which direction to fly no matter how far away they are. When a certain number of nearest neighbors are used instead of those within a certain distance flocks become less likely to break up and can expand and contract more easily in response to predators and other surprises.
The researchers assumed that even though each starling could probably see more than a dozen members of its flock around it, the bird’s brain power is limited to processing seven at a time. They also concluded that physics provided better ways to describe starlings’ flock behavior than biology. In particular the Heisenberg equations describing magnets provided the best explanation of how they perform coordinated flight.
Do equations really explain how starlings perform coordinated flight? Does physics underlie one of the most apparently spontaneous displays of life on earth? Or does the beauty of a cartwheeling flock of starlings originate with the birds themselves rather than from a universal law?
What do you think? Go to youtube.com/embed/88/UVJpQGi88 and decide.