Turning flocks of starlings are a paradigm for a synchronized, rapid change of direction in moving animal groups. The efficiency of the information transport during such a collective change of state is the key factor to prevent cohesion loss and preserve robustness. However, the precise mechanism by which natural groups achieve such efficiency is currently not fully understood. I will present an experimental and theoretical study of starlings flocks undergoing collective turns in which we analyze how the turning decision spreads across the flock [1]. We find sound-like propagation with no damping of information. This is in contrast with standard theories of collective animal behavior based on alignment, which predict a much slower, diffusive spread of information. We propose a novel theory for propagation of orientation in flocks whose key ingredient is the existence of a conserved spin current generated by the gauge symmetry of the system. The theory falls in the same dynamical universality class of superfluid transport in liquid helium, naturally explaining the dissipationless propagating mode observed in turning flocks. Superfluidity also provides a quantitative prediction for the speed of propagation of the information, according to which transport must be swifter the stronger the group's orientational order. This is confirmed by the experimental data. The link between strong order and efficient decision-making required by superfluidity may be the adaptive drive for the high degree of behavioral polarization observed in many living groups.
[1] arXiv:1303.7097, arXiv:1305.1495