The word for a flock of starlings on the move is murmuration. And saying murmuration is almost as wonderful as watching one. Their startling changes—thousands of birds on the wing making sudden banks—have inspired researchers interested in how large groups of individuals synchronize their movement.
A flock of 400 birds can turn in just half a second. At least some of the twists and turns are inspired by predator avoidance. A hawk or falcon swooping in for the kill can trigger a rippling black band of movement through the flock.
Now, a group led by Charlotte Hemelrijk, of the University of Groningen in the Netherlands, has described what causes those dark bands.
Agitation or shimmering waves form when individuals in a flock of birds, swarm of insects or school of fish quickly repeat the fear reaction or escape maneuver of a close neighbor. They can be seen as spirals, concentric rings or moving lines. In giant honeybees, this happens when each insect lifts its abdomen, while in anchovies, individuals temporarily move closer together or roll sideways.
Hemelrijk and her colleagues found that in starlings, the rippling bands start at one end of the flock and move to the other—away from the predator. The movement generally proceeds at a speed of 13.4 meters per second. But exactly what specific movements were causing the dark bands was a mystery. The birds were flying too far away to tell. A computation model proved that the waves weren’t birds moving closer together for a split second, but changing orientation of the bird bodies.
"On the ground, someone watching will, for a split second, see the largest wing area once a bird has rolled 90 degrees sideways," says Hemelrijk in the statement. "This temporary increase of dark surface of the wing causes us to see a black band continuously moving away from the predator when the birds repeat each others' zig maneuver." They described their findings in the journal Behavioral Ecology and Sociobiology.
Bit by bit scientists are figuring out how to mathematically describe the shapes these flocks take in the sky, the movements needed to create such patterns and how to model them. But we still don’t know why a starling murmuration can be so mesmerizing.