Collective decision-making in biological systems requires all individuals in the group to go through a behavioural change of state. During this transition fast and robust transfer of information is essential to prevent cohesion loss. The mechanism by which natural groups achieve such robustness, however, is not clear. Here we present an experimental study of starling flocks performing collective turns. We find that information about direction changes propagates across the flock with a linear dispersion law and negligible attenuation, hence minimizing group decoherence. These results contrast starkly with present models of collective motion, which predict diffusive transport of information. Building on spontaneous symmetry breaking and conservation-law arguments, we formulate a theory that correctly reproduces linear and undamped propagation. Essential to this framework is the inclusion of the birds’ behavioural inertia. The theory not only explains the data, but also predicts that information transfer must be faster the stronger the group’s orientational order, a prediction accurately verified by the data. Our results suggest that swift decision-making may be the adaptive drive for the strong behavioural polarization observed in many living groups.
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We thank J.G. Lorenzana for bringing Model F to our attention. We thank E. Cappelluti, C. Castellani, G.A. Cavagna, M. Cencini, F. Cecconi, F. Ginelli, S. Ramaswamy and J. Toner for discussions, and P. Calabrese and D. Levine for reading the manuscript. We also acknowledge the advice of C. Lucibello on tracking and the help of E. Silvestri on segmentation and on testing our tracking algorithm against synthetic data. This work was supported by grants IIT–Seed Artswarm, ERC–StG no. 257126 and US-AFOSR FA95501010250 (through the University of Maryland).
The authors declare no competing financial interests.
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Attanasi, A., Cavagna, A., Del Castello, L. et al. Information transfer and behavioural inertia in starling flocks. Nature Phys 10, 691–696 (2014) doi:10.1038/nphys3035
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