  Collective decision-making in biological systems requires all individuals in
the group to go through a behavioural change of state. During this transition,
the efficiency of information transport is a key factor to prevent cohesion
loss and preserve robustness. The precise mechanism by which natural groups
achieve such efficiency, though, is currently not fully understood. Here, we
present an experimental study of starling flocks performing collective turns in
the field. We find that the information to change direction propagates across
the flock linearly in time with negligible attenuation, hence keeping group
decoherence to a minimum. This result contrasts with current theories of
collective motion, which predict a slower and dissipative transport of
directional information. We propose a novel theory whose cornerstone is the
existence of a conserved spin current generated by the gauge symmetry of the
system. The theory turns out to be mathematically identical to that of
superfluid transport in liquid helium and it explains the dissipationless
propagating mode observed in turning flocks. Superfluidity also provides a
quantitative expression for the speed of propagation of the information,
according to which transport must be swifter the stronger the group's
orientational order. This prediction is verified by the data. We argue that the
link between strong order and efficient decision-making required by
superfluidity may be the adaptive drive for the high degree of behavioural
polarization observed in many living groups. The mathematical equivalence
between superfluid liquids and turning flocks is a compelling demonstration of
the far-reaching consequences of symmetry and conservation laws across
different natural systems.
