Abstract

When a predator attacks a flock of starlings (Sturnus vulgaris), involving thousands of individuals, a typical collective escape response is the so-called agitation wave, consisting of one or more dark bands (pulses) propagating through the flock and moving away from the predator (usually a Peregrine falcon, Falco peregrinus). The mechanism underlying this collective behavior remains debated. A theoretical study has suggested that the individual motion underlying a pulse could be a skitter (in the form of a zigzag), that is copied by nearby neighbors, and causes us to temporarily observe a larger surface of the wing because the bird is banking during turning while zigzagging. It is not known, however, whether pulses during a wave event weaken over time. This is of interest, because whereas during the usual turning by an undisturbed flock the motion is copied completely without weakening, we may expect that pulses dampen during a wave event because individuals that are further away from a predator react less because of reduced fear. In the present paper, we show in empirical data that pulses during a wave event weaken over time. Our computational model, StarDisplay, reveals that this is most likely a consequence of a reduction of the maximum banking angle during the zigzag escape maneuver rather than by a reduced tendency to copy this maneuver with time. The response seems adaptive because of lowered danger at a larger distance to the location of attack.Significance statementHuge flocks of starlings display amazing patterns of collective escape when attacked by an avian predator, such as a Peregrine falcon. One of them is the “agitation wave” in which dark bands move away from the predator. Dark bands arise probably from the temporarily larger wing area, which is observed when birds perform a skitter escape motion (zigzag) while temporarily banking sideward. Whereas during regular flock turns birds copy each other’s motion completely, it is unknown whether this happens during agitation waves, because individuals further away from the attack may be less frightened. Studying this both empirically at the group level only and in a computational model at both the level of the individual and the group, we show that pulses of waves fade out with time and that this is probably due to a reduced maximum banking angle during the zigzag maneuver rather than a lower tendency of copying. This seems an adaptive response.

Highlights

  • The study of collective motion of social animals, including humans, is an interdisciplinary field at the frontiers of science, which has greatly benefitted from combining empirical and modeling approaches (Camazine et al 2003; Grimm et al 2005; Sumpter et al 2012; Ouellette 2019)

  • We investigated whether pulses of agitation waves in empirical data of starling flocks under predation threat collected in Rome, Italy (Procaccini et al 2011) fade out and compared this with pulses, which were generated in a computational model of starling flocks, StarDisplay (Hemelrijk et al 2015)

  • StarDisplay, we represent the weakening of pulses in two ways by decreasing (a) the tendency to copy over time, and (b) the maximum angle of tilting during a skitter-like escape maneuver

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Summary

Introduction

The study of collective motion of social animals, including humans, is an interdisciplinary field at the frontiers of science, which has greatly benefitted from combining empirical and modeling approaches (Camazine et al 2003; Grimm et al 2005; Sumpter et al 2012; Ouellette 2019). Whether during an agitation wave individuals copy and repeat escape maneuvers identically or weaken the maneuver over time, so that the pulse would fade out, remains unknown This question is of interest, because in our recent analysis of reactions of a flock to a predator, we quantitatively showed that the patterns of collective response of starlings are fine tuned to the characteristics of hunting by a falcon (such as its speed and direction from which it attacks; Storms et al 2019). Weakening of a wave over time may be adaptive and reducing fear and responsiveness to a predator when it is further away is in line with theoretical predictions (Broom and Ruxton 2005)

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