Phys. Rev. Lett. 119, 178003 (2017)

As if the beauty of the landscape weren't enough, flocks of birds turn sunsets above Mediterranean cities into breath-taking spectacles. Ever shrinking and expanding, flocks appear in urban skies as dark and unpredictable clouds — fascinating physicists attempting to understand this elegant display of collective dynamics. In an effort to describe such behaviour, Michael Sinhuber and Nicholas Ouellette have now shown that flocks might be more similar to clouds than one would imagine.

The researchers studied swarms of Chironomus riparius midges and, by reconstructing the three-dimensional trajectories of individual midges, they looked for truly collective features. Inspired by thermodynamic concepts that bear no meaning at the individual level, Sinhuber and Ouellette demonstrated that swarms could be described as two thermodynamic-like states at equilibrium: a condensed phase surrounded by a cloud of vapour. The unpredictable movements of individual midges proved the collective nature of this description, as they could freely move between the two phases without altering their global macroscopic properties. Hopefully, without stripping sunsets of their beauty, this might be a way to apply active-matter theories to animal behaviour.