Are they following a leader? No, the reaction time would simply be too short. The best explanation is what scientists call a “maneuver wave.” Wayne Potts, a zoologist who published in the journal Nature in 1984, says birds are able to change direction quickly not just because they are following a leader, but because they see a movement far down the line and anticipate what to do next. Potts called this the chorus-line hypothesis, “These propagation speeds appear to be achieved in much the same way as they are in a human chorus line: individuals observe the approaching maneuver wave and time their own execution to coincide with its arrival.” The evidence for this came from studying high-speed film. He also noticed the flock typically responded only to birds that banked into it, which makes sense, since flocks protect birds from being picked off by predators.
To read more, go to http://earthsky.org/earth/how-do-flocking-birds-move-in-unison? . The photo, “Red-winged blackbirds over Mattamuskeet Lake in Hyde County, North Carolina, from EarthSky Facebook friend Guy Livesay,” came from that site.