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Observation shows that there are no leaders (at least not for more than a few seconds at a time), since different birds will be at the front of the flock every time it changes direction. Research by Wayne Potts, published in the journal Nature in 1984, helped explain how flock movements are initiated and coordinated. Potts, through a frame-by-frame analysis of high-speed film of sandpiper flocks, found that any individual can initiate a flock movement, which then propagates through the flock in a wave radiating out from the initiation site. These "maneuver waves" could move in any direction through the flock, including from back to front. However, the flock usually only responded to birds that banked into the flock, rather than away from it. Since birds turning away from the flock run the risk of being separated from it and getting picked off by the predator, others will not follow them. Besides its obvious benefits for individuals, this rule helps prevent indecision by the flock and permits it to respond rapidly to attack.
Once one of these waves began, Potts found that it spread through the flock far more rapidly than could be explained by the reaction times of individual birds. A bird's mean startle reaction time to a light flash as measured in the laboratory was 38 milliseconds, but maneuver waves spread through the flock between birds at a mean speed of less than 15 milliseconds. However, the first birds to respond to an initiator took 67 milliseconds to react. Potts proposed that birds farther away from the initiation site were able to see the wave approaching them, and could "get set" to respond before it actually reached them. He dubbed this the "chorus line hypothesis," in analogy to Rockettes at Radio City Music Hall who can see and anticipate an approaching high leg kick when it is still well down the line. Films of human chorus lines show that rehearsed maneuvers, initiated without warning, propagate down the line at less than 108 milliseconds, almost twice as fast as the human visual reaction time of 194 milliseconds.