Andrea Cavagna, University of Rome I
Collective decision-making in animal behaviour is regulated by how efficiently the information to change state propagates throughout the group. In this talk I will present an experimental study of starlings flocks undergoing collective turns. By reconstructing the dynamical trajectories of individual birds we analyze how the turn spreads across the flock. We find sound-like propagation, namely a linear dispersion relation, with virtually no damping. This result contrasts with well-established theories of collective animal behaviour based on alignment, which predict a much slower, diffusive spread of directional information. We suggest that the correct theory for propagation of orientation in flocks is equivalent to that of second sound propagation in a superfluid. To support our claim we measure the velocity of propagation of directional information during the turn and find it to be larger the stronger the flock’s orientational order, in quantitative agreement with second sound equations in superfluid systems.