title: Information transfer and behavioural inertia in starling flocks.
abstract: 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, though, 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 current models of collective motion, which predict diffusive transport of information. Building on spontaneous symmetry breaking and conservation laws arguments, we formulate a new theory that correctly reproduces linear and undamped propagation. Essential to the new framework is the inclusion of the birds' behavioural inertia. The new 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.
authors.affiliation: Istituto Sistemi Complessi, Consiglio Nazionale delle Ricerche, UOS Sapienza, 00185 Rome, Italy ; Dipartimento di Fisica, Universit a Sapienza, 00185 Rome, Italy.
authors.forename: Alessandro
authors.initials: A
authors.lastname: Attanasi
authors.affiliation: Istituto Sistemi Complessi, Consiglio Nazionale delle Ricerche, UOS Sapienza, 00185 Rome, Italy ; Dipartimento di Fisica, Universit a Sapienza, 00185 Rome, Italy ; Initiative for the Theoretical Sciences, The Graduate Center, City University of New York, 10016 New York, USA.
authors.forename: Andrea
authors.initials: A
authors.lastname: Cavagna
authors.affiliation: Istituto Sistemi Complessi, Consiglio Nazionale delle Ricerche, UOS Sapienza, 00185 Rome, Italy ; Dipartimento di Fisica, Universit a Sapienza, 00185 Rome, Italy.
authors.forename: Lorenzo
authors.initials: L
authors.lastname: Del Castello
authors.affiliation: Istituto Sistemi Complessi, Consiglio Nazionale delle Ricerche, UOS Sapienza, 00185 Rome, Italy ; Dipartimento di Fisica, Universit a Sapienza, 00185 Rome, Italy ; Initiative for the Theoretical Sciences, The Graduate Center, City University of New York, 10016 New York, USA.
authors.forename: Irene
authors.initials: I
authors.lastname: Giardina
authors.affiliation: Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas (INIFTA) and Departamento de Física, Facultad de Ciencias Exactas, Universidad Nacional de La Plata, c.c. 16, suc. 4, 1900 La Plata, Argentina CONICET La Plata, Consejo Nacional de Investigaciones Científicas y Técnicas, Argentina.
authors.forename: Tomas S
authors.initials: TS
authors.lastname: Grigera
authors.affiliation: Istituto Sistemi Complessi, Consiglio Nazionale delle Ricerche, UOS Sapienza, 00185 Rome, Italy ; Dipartimento di Fisica, Universit a Sapienza, 00185 Rome, Italy.
authors.forename: Asja
authors.initials: A
authors.lastname: Jelić
authors.affiliation: Istituto Sistemi Complessi, Consiglio Nazionale delle Ricerche, UOS Sapienza, 00185 Rome, Italy ; Dipartimento di Fisica, Universit a Sapienza, 00185 Rome, Italy.
authors.forename: Stefania
authors.initials: S
authors.lastname: Melillo
authors.affiliation: Istituto Sistemi Complessi, Consiglio Nazionale delle Ricerche, UOS Sapienza, 00185 Rome, Italy ; Dipartimento di Informatica, Universitá Sapienza, 00198 Rome, Italy.
authors.forename: Leonardo
authors.initials: L
authors.lastname: Parisi
authors.affiliation: Istituto Sistemi Complessi, Consiglio Nazionale delle Ricerche, UOS Sapienza, 00185 Rome, Italy ; Dipartimento di Fisica, Universit a Sapienza, 00185 Rome, Italy.
authors.forename: Oliver
authors.initials: O
authors.lastname: Pohl
authors.affiliation: Istituto Sistemi Complessi, Consiglio Nazionale delle Ricerche, UOS Sapienza, 00185 Rome, Italy ; Dipartimento di Fisica, Universit a Sapienza, 00185 Rome, Italy.
authors.forename: Edward
authors.initials: E
authors.lastname: Shen
authors.affiliation: Istituto Sistemi Complessi, Consiglio Nazionale delle Ricerche, UOS Sapienza, 00185 Rome, Italy ; Dipartimento di Fisica, Universit a Sapienza, 00185 Rome, Italy.
authors.forename: Massimiliano
authors.initials: M
authors.lastname: Viale