Here’s one recent study: Propagating waves in starling, Sturnus vulgaris, flocks under predation (Procaccini et al). You probably can’t access the full article, but what they did was videotape and analyze large flocks of starlings at two urban winter roosting sites in Italy over two years. There was no need to release falcons, because hawks and falcons regularly prey on the flocks. In total, 81 episodes of flocks under attack by falcons were recorded in HD and analyzed in slow motion.
The data are very detailed and complex, but here’s a paragraph summarizing their findings:
“Wave events (a train of several observable pulses of optical intensity that propagates along a given direction across the flock) are a straightforward example of emerging complexity: a simple functional response (escaping from predators), when affecting even just a relatively small portion of a cohesive flock of thousands of individuals, can produce very complex patterns. Probably, in birds, wave events occur only when the perturbation is so strong that individuals almost come into contact, which is probably the reason why they form only in the presence of a threat. Wave events possess a measurable antipredator function: it is likely that rapid variations, in local density and in both predator–prey and prey–prey distances, perturb the visual perception of falcons. As suggested for similar phenomena (e.g. Trafalgar wave) in giant honeybees (Kastberger et al., 2008 and Schmelzer and Kastberger, 2009), dunlins (Buchanan et al. 1988) and marine insects (Treherne & Foster 1981), we found that, also in starlings, waves can hamper predation success.”