Emergence of collective changes in travel direction of starling flocks from individual birds' fluctuations |
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Authors: | Alessandro Attanasi Andrea Cavagna Lorenzo Del Castello Irene Giardina Asja Jelic Stefania Melillo Leonardo Parisi Oliver Pohl Edward Shen Massimiliano Viale |
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Affiliation: | 1Istituto Sistemi Complessi, Consiglio Nazionale delle Ricerche, UOS Sapienza, 00185 Rome, Italy;2Dipartimento di Fisica, Università Sapienza, 00185 Rome, Italy;3Initiative for the Theoretical Sciences, The Graduate Center, City University of New York, 10016 New York, NY, USA;4Dipartimento di Informatica, Università Sapienza, 00198 Rome, Italy |
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Abstract: | 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. |
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Keywords: | collective behaviour animal movement self-organization decision-making |
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