Propagating waves in starling, Sturnus vulgaris, flocks under predation

Abstract

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. (C) 2011 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved

Damping of waves of agitation in starling flocks

“…This extends our former results when using StarDisplay for studying what underlies agitation waves (Hemelrijk et al 2015). Here, we suggested that waves either reflected a change of density of individuals in the flock (Axelsen et al 2001;Procaccini et al 2011) or their orientation (Buchanan et al 1988;Potts 1984) or both (Hemelrijk et al 2015). We showed that only when the wave through the flock comprised a zigzag escape maneuver, our visual system observed indeed a dark band moving along the flock, not when the escape maneuver involved individuals fleeing fast forward into the flock.…”

Damping of waves of agitation in starling flocks

“…This extends our former results when using StarDisplay for studying what underlies agitation waves (Hemelrijk et al 2015). Here, we suggested that waves either reflected a change of density of individuals in the flock (Axelsen et al 2001;Procaccini et al 2011) or their orientation (Buchanan et al 1988;Potts 1984) or both (Hemelrijk et al 2015). We showed that only when the wave through the flock comprised a zigzag escape maneuver, our visual system observed indeed a dark band moving along the flock, not when the escape maneuver involved individuals fleeing fast forward into the flock.…”

The confusion effect when attacking simulated three-dimensional starling flocks

“…In line with predictions from the confusion effect, participant error in attempting to capture a starling in a flock increased with flock size. This lends evidence to suggestions that the tendency to form large aggregations over wintering grounds in starlings has an anti-predatory function [19,20,25]. Our experiment has shown that as flock size increases, predators may find it more difficult to catch a given individual.…”

Minority-Triggered Reorientations Yield Macroscopic Cascades and Enhanced Responsiveness in Swarms

“…These features closely parallel empirical observations in bird flocks [5,10], fish schools [7,13], sheep [11], and even collective cell migration [1,3,26,27]. Notably, our minority interaction offers a mechanistic explanation for the remarkably rapid information transfer observed during collective predator evasion [6,12,13]. When a small subset of individuals detects a threat, their deviating behavior can trigger an information cascade that propagates throughout the group at speeds far beyond the diffusion of direct individual interactions [12].…”

Damping of waves of agitation in starling flocks

“…This extends our former results when using StarDisplay for studying what underlies agitation waves (Hemelrijk et al 2015). Here, we suggested that waves either reflected a change of density of individuals in the flock (Axelsen et al 2001;Procaccini et al 2011) or their orientation (Buchanan et al 1988;Potts 1984) or both (Hemelrijk et al 2015). We showed that only when the wave through the flock comprised a zigzag escape maneuver, our visual system observed indeed a dark band moving along the flock, not when the escape maneuver involved individuals fleeing fast forward into the flock.…”

The confusion effect when attacking simulated three-dimensional starling flocks

“…In line with predictions from the confusion effect, participant error in attempting to capture a starling in a flock increased with flock size. This lends evidence to suggestions that the tendency to form large aggregations over wintering grounds in starlings has an anti-predatory function [19,20,25]. Our experiment has shown that as flock size increases, predators may find it more difficult to catch a given individual.…”

Minority-Triggered Reorientations Yield Macroscopic Cascades and Enhanced Responsiveness in Swarms

“…These features closely parallel empirical observations in bird flocks [5,10], fish schools [7,13], sheep [11], and even collective cell migration [1,3,26,27]. Notably, our minority interaction offers a mechanistic explanation for the remarkably rapid information transfer observed during collective predator evasion [6,12,13]. When a small subset of individuals detects a threat, their deviating behavior can trigger an information cascade that propagates throughout the group at speeds far beyond the diffusion of direct individual interactions [12].…”

Damping of waves of agitation in starling flocks

“…This extends our former results when using StarDisplay for studying what underlies agitation waves (Hemelrijk et al 2015). Here, we suggested that waves either reflected a change of density of individuals in the flock (Axelsen et al 2001;Procaccini et al 2011) or their orientation (Buchanan et al 1988;Potts 1984) or both (Hemelrijk et al 2015). We showed that only when the wave through the flock comprised a zigzag escape maneuver, our visual system observed indeed a dark band moving along the flock, not when the escape maneuver involved individuals fleeing fast forward into the flock.…”

The confusion effect when attacking simulated three-dimensional starling flocks

“…In line with predictions from the confusion effect, participant error in attempting to capture a starling in a flock increased with flock size. This lends evidence to suggestions that the tendency to form large aggregations over wintering grounds in starlings has an anti-predatory function [19,20,25]. Our experiment has shown that as flock size increases, predators may find it more difficult to catch a given individual.…”

Minority-Triggered Reorientations Yield Macroscopic Cascades and Enhanced Responsiveness in Swarms

“…These features closely parallel empirical observations in bird flocks [5,10], fish schools [7,13], sheep [11], and even collective cell migration [1,3,26,27]. Notably, our minority interaction offers a mechanistic explanation for the remarkably rapid information transfer observed during collective predator evasion [6,12,13]. When a small subset of individuals detects a threat, their deviating behavior can trigger an information cascade that propagates throughout the group at speeds far beyond the diffusion of direct individual interactions [12].…”