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| Earth on Dec 27, 2012

How do flocking birds move in unison?

How do flocks of birds swoop and turn in unison? The leading theory – called the chorus line hypothesis – goes back to 1984.

We’ve all seen flocks of birds wheeling and swooping in unison, as if choreographed. How do they do this? Zoologists say they aren’t simply following a leader, or their neighbors. If they were, the reaction time of each bird would need to be very fast – faster than birds actually do react, according to scientists who have studied the reaction times of individual birds in laboratory settings. The classic research on how flocking birds move in unison comes from zoologist Wayne Potts, who published in the journal Nature in 1984. His work showed that bird in flocks don’t just follow a leader, or their neighbors. Instead, they anticipate sudden changes in the flock’s direction of motion.

EarthSky Facebook friend Guy Livesay posted this photo in late December 2012. It’s Red-winged Blackbirds, over Mattamuskeet Lake in Hyde County, North Carolina. Thank you Guy! View larger.

And he said, once a change in direction begins in the flock, it then “spreads through the flock in a wave.” The propagation of this manoeuvre wave, as he called it, begins relatively slowly but can reach speeds three times faster than would be possible if birds were simply reacting to their immediate neighbors. Potts called this ability among flocking birds the chorus line hypothesis. That is, he said, birds are like dancers who see an approaching leg kick when it’s still down the line, and anticipate what to do. He said:

These propagation speeds appear to be achieved in much the same way as they are in a human chorus line: individuals observe the approaching manoeuvre wave and time their own execution to coincide with its arrival.

Potts used high-speed film – and a frame-by-frame analysis – of flocks of red-backed sandpipers (Calidris alpina) to conduct his study. He found that the flock typically responded only to birds that banked into the flock, rather than away from it. That makes sense, since flocking among birds serves the purpose of protecting birds from predators (although there are other purposes as well; for example, when one bird finds food, others in a flock eat, too). Individual birds, those who are separated from the flock, are more likely to be picked off by predators.

Wild life cameraman and travel journalist Dylan Winter filmed the video below and uploaded it to YouTube on November 13, 2010.

Red-winged blackbirds at sunset via Wikipedia

Bottom line: According to Wayne Potts, a zoologist who published in the journal Nature in 1984, birds in flocks are able to change direction quickly not just because they are following a leader, or their neighbors, but because they see a movement far down the line and anticipate what to do next. Potts called this the chorus-line hypothesis for bird movement.