From starling flocks to theory: some new results in flocking

Flocking, the collective motion displayed by large groups of birds, is one of the most spectacular examples of emergent collective behavior in nature. Statistical physicists have tackled the problem via minimal models of self propelled particles and hydrodynamic continuum theories. Such studies showed that flocking can be interpreted as a spontaneous symmetry breaking phenomeon occurring in a “moving ferromagnetic spin system”, a sort of non-equilibrium counterpart of the well known Heisenberg model. The basic ingredients of this description - self propulsion, lack of Galileian invariance and of momentum conservation, local ferromagnetic interactions - define an extremely rich universality class, able to describe systems as diverse as vertebrate herds bacteria colonies, driven granular matter, grasshopper swarms and active macromolecules in living cells. Flocking proper, however, remains a prominent instance of collective animal motion. In this talk I will discuss some recent theoretical advancements which have been inspired and motivated by recent experimental findings in starling flocks. In particular, I will discuss i) topological, metric free interactions and ii) finite perturbations and strong velocity correlations in finite flocks.