Emergence of collective changes in travel direction of starling flocks from individual birds' fluctuations

One of the most impressive features of moving animal groups is their ability to perform sudden coherent changes in travel direction. While this collective decision can be a response to an external alarm cue, directional switching can also emerge from the intrinsic fluctuations in individual behaviour. However, the cause and the mechanism by which such collective changes of direction occur are not fully understood yet. Here, we present an experimental study of spontaneous collective turns in natural flocks of starlings. We employ a recently developed tracking algorithm to reconstruct three-dimensional trajectories of each individual bird in the flock for the whole duration of a turning event. Our approach enables us to analyse changes in the individual behaviour of every group member and reveal the emergent dynamics of turning. We show that spontaneous turns start from individuals located at the elongated tips of the flocks, and then propagate through the group. We find that birds on the tips deviate from the mean direction of motion much more frequently than other individuals, indicating that persistent localized fluctuations are the crucial ingredient for triggering a collective directional change. Finally, we quantitatively verify that birds follow equal-radius paths during turning, the effects of which are a change of the flock's orientation and a redistribution of individual locations in the group.

[1]  F. Heppner,et al.  Structure of Turning in Airborne Rock Dove (Columba livia) Flocks , 1992 .

[2]  G. Beauchamp,et al.  An Experimental Analysis of Spatial Position Effects on Foraging and Vigilance in Brown‐Headed Cowbird flocks , 2008 .

[3]  Iain D. Couzin,et al.  Collective States, Multistability and Transitional Behavior in Schooling Fish , 2013, PLoS Comput. Biol..

[4]  Vicsek,et al.  Novel type of phase transition in a system of self-driven particles. , 1995, Physical review letters.

[5]  J. Toner,et al.  Flocks, herds, and schools: A quantitative theory of flocking , 1998, cond-mat/9804180.

[6]  C. Hemelrijk,et al.  Self-organised complex aerial displays of thousands of starlings: a model , 2009, 0908.2677.

[7]  W. Hamilton.  Geometry for the selfish herd. , 1971, Journal of theoretical biology.

[8]  I. Couzin,et al.  Collective memory and spatial sorting in animal groups. , 2002, Journal of theoretical biology.

[9]  Thierry Mora,et al.  Flocking and Turning: a New Model for Self-organized Collective Motion , 2014, ArXiv.

[10]  W. Potts.  The chorus-line hypothesis of manoeuvre coordination in avian flocks , 1984, Nature.

[11]  S. L. Lima.  Collective detection of predatory attack by birds in the absence of alarm signals , 1994 .

[12]  G. Parisi,et al.  Empirical investigation of starling flocks: a benchmark study in collective animal behaviour , 2008, Animal Behaviour.

[13]  I. Aoki.  A simulation study on the schooling mechanism in fish. , 1982 .

[14]  Andrea Cavagna,et al.  Information transfer and behavioural inertia in starling flocks , 2013, Nature Physics.

[15]  Andrea Cavagna,et al.  Tracking in three dimensions via recursive multi-path branching , 2013 .

[16]  I. Couzin,et al.  Self-Organization and Collective Behavior in Vertebrates , 2003 .

[17]  Joseph J. Hale,et al.  From Disorder to Order in Marching Locusts , 2006, Science.

[18]  I. R. Inglis,et al.  Vigilance and Flock Size in Brent Geese: The Edge Effect , 1981 .

[19]  S. M. Evans,et al.  Influence of position in the flock and flock size on vigilance in the starling, Sturnus vulgaris , 1980, Animal Behaviour.

[20]  Andrea Cavagna,et al.  GReTA-A Novel Global and Recursive Tracking Algorithm in Three Dimensions , 2013, IEEE Transactions on Pattern Analysis and Machine Intelligence.

[21]  G. Ruxton,et al.  Evidence for a rule governing the avoidance of superfluous escape flights , 2000, Proceedings of the Royal Society of London. Series B: Biological Sciences.

[22]  W. Bialek,et al.  Statistical mechanics for natural flocks of birds , 2011, Proceedings of the National Academy of Sciences.

[23]  Charlotte K. Hemelrijk,et al.  Some Causes of the Variable Shape of Flocks of Birds , 2011, PloS one.

[24]  J. M. Black,et al.  Foraging dynamics in goose flocks: the cost of living on the edge , 1992, Animal Behaviour.

[25]  D. Sumpter.  The principles of collective animal behaviour , 2006, Philosophical Transactions of the Royal Society B: Biological Sciences.

[26]  G D Ruxton,et al.  Modelling antipredator vigilance and flight response in group foragers when warning signals are ambiguous. , 2001, Journal of theoretical biology.

[27]  Jens Krause,et al.  Mortality risk of spatial positions in animal groups: The danger of being in the front , 1997 .

[28]  F. Heppner,et al.  Laboratory determination of startle reaction time of the starling (Sturnus vulgaris) , 1977, Animal Behaviour.

[29]  A. Cavagna,et al.  Diffusion of individual birds in starling flocks , 2012, Proceedings of the Royal Society B: Biological Sciences.

[30]  J. Lazarus.  Vigilance, flock size and domain of danger size in the White-fronted Goose , 1978 .

[31]  H. Chaté,et al.  Collective motion of self-propelled particles interacting without cohesion. , 2007, Physical review. E, Statistical, nonlinear, and soft matter physics.

[32]  S. L. Lima.  Ecological and evolutionary perspectives on escape from predatory attack: a survey of North American birds , 1993 .

[33]  Giorgio Parisi,et al.  The STARFLAG handbook on collective animal behaviour: 1. Empirical methods , 2008, Animal Behaviour.