I just ran across this 2009 article from Audubon Magazine about research into how birds can fly in flocks comprising thousands of members, at speeds of 40 miles per hour, without colliding.
Theories in the 1960s, author Peter Friederici explains, posited that the tight maneuvers of flocks of birds were similar to the movements found in schools of fish — and that each individual need only mimic the movements of its closest neighbors.
But in the 1970s, graduate student Wayne Potts discovered a flaw in the theory: it didn’t explain how flocks can execute maneuvers so quickly. Friederici writes:
By making movies of their flocks and analyzing, frame by frame, how each individual bird moved, he was able to show that a turn ripples through a flock just as a cheerleading wave passes through sports fans at a stadium. He explained the finding with the name of his theory: the “chorus line hypothesis.” An individual dancer who waits for her immediate neighbor to move before initiating her kick will be too slow; similarly, a dunlin watches a number of birds around it, not just its nearest neighbors, for cues. […]
“The wave was propagating through the flock at least three times faster than could be explained if they were just watching their immediate neighbors,” says Potts.
Some of the latest research into flock behavior was taking place in 2009 at the Roman National Museum in Rome, Italy, where a roost of starlings gathers each winter. Scientists are now using software designed for statistical mechanics to create three-dimensional maps of flocks in flight.