Propagating waves in starling, Sturnus vulgaris, flocks under predation

The formation of waves is a vivid example of collective behaviour occurring in insects, birds, fish and mammals, which has been interpreted as an antipredator response. In birds a quantitative characterization of this phenomenon, involving thousands of individuals, is missing and its link with predation remains elusive. We studied waves in flocks of starlings, a highly gregarious species, by both direct observation and quantitative computer vision analysis of HD video recordings, under predation by peregrine falcons, Falco peregrinus. We found that waves originated from the position of the attacking predator and always propagated away from it. We measured their frequency and velocities, the latter often being larger than the velocity of the flock. A high positive correlation was found between the formation of waves and reduced predation success. We suggest that the tendency of a prey to escape, when initiated even by a few individuals in a cohesive group, elicits self-organized density waves. Such evident fluctuations in the local structure of the flocks are efficient in confusing predators.

► We quantified occurrence and velocity of waves in starling flocks.
► Waves occur under direct threat by falcons and always propagate away from the predator.
► Reduced predation success by falcon is positively correlated with wave formation.
► Velocity of a single wave pulse reaches up to 25 m/s and is higher than flock velocity.
► Frequency ranges between 0.7 and 2 pulses/s.