On its own, a single starling doesn't elicit much fuss. It's a tennis-ball-sized bird, glossy black in winter, purplish or green in summer, and in autumn, sometimes speckled with white spots. But when starlings congregate in flocks of hundreds of thousands over open fields (something scientists call a "murmuration") they pitch and arc and rush at one another in a bizarre choreography that's puzzled naturalists for hundreds of years. A whole catalog of YouTube videos has documented the black shapes in flight, a movement that looks like the birds are attached to a giant rhythmic gymnast's invisible ribbon.
How they do it is an ongoing investigation. But one theoretical physicist may have pinned down one of the most accurate explanations yet, now published in the Proceedings of the National Academy of Sciences. In doing so, British PhD candidate Daniel Pearce was able to create a computer simulation of arrows that mimicked the birds' movement. Then, to test his theory, he set up a camera on the beach of a burnt-down pier and popular starling hangout in Brighton and captured murmurations at dawn and dusk.
The way Pearce explains it, the starlings are governed by three basic rules. First, each starling does whatever its neighbor is doing: If starling A bears right, so will starling B. But the second rule dictates that a starling must maintain a certain pattern of light and dark in its field of vision. If you think of a murmuration as a giant cloud of starlings, each bird won't want to move to the densest, darkest part--otherwise, looking out from within the cloud would become impossible. Similarly, birds won't want to go to the lightest bits, characterized by sky, or they'll detach from the flock entirely.
Instead, the birds are drawn to the most "complex" patterns, Pearce explains--the most disordered patches of light and dark in the whole structure. Somehow, with each bird drawn to entropy, the entire flock moves in a semi-cohesive shape. The third rule is that the birds can make mistakes.
"I just started by guessing--I'll just have a couple of goes, see what I come up with," Pearce tells me over Skype. That's when he discovered that light-dark patterns might inform the birds' movements. "If you're part of a flock of 300,000 birds, the guys at the far end of the flock are just silhouettes, so what the bird sees is just going to be a pattern of silhouettes," he says.
After returning to the University of Warwick to analyze the footage, Pearce found that the ratio of light-to-dark spaces on the camera could be predicted by his computer simulation. A successful model that predicts starling movements could have a range of applications, he adds--from air traffic control to construction to murmuration bird poop clean-up. The model could be used outside wildlife biology, too. Drone robotics might take note of how the birds are able to coordinate based on visual cues.
Yet, there's still one mystery about murmuration movement that remains. We know more about how the birds create the mass shapes, but why?
"In terms of why, there are a lot of theories. Finding food, finding mates, avoiding predators. Stuff like this, I assume. It could be a merely social thing," Pearce says.
Pearce is a theoretical physicist, after all--the "why" isn't exactly his domain. But there's clearly something about it that tickles him. "I don't know the why, to be honest," he says as he clasps his hands and smiles, staring at the ceiling.