Emergence of spontaneous collective turns in natural flocks of starlings

One of the most striking features of moving animal groups is their ability to perform sudden coherent changes in the travel direction. While this collective change of state can be a response to an external alarm cue, directional switching can also emerge from the intrinsic fluctuations in individual behaviour. To understand the cause and the mechanism by which such collective changes of direction occur, I will present an experimental and theoretical study of spontaneous rapid collective turns in natural flocks of starlings. Using the individual trajectories of all birds in the flock, we analyse the changes in the individual behaviour and reveal the emergent dynamics of turning. We show that spontaneous turns start from the individuals located at the elongated tips of the flocks, and then propagate across the whole group. We find that birds on the tips deviate from the mean direction much more persistently than other individuals, indicating that persistent localized fluctuations are a trigger for collective directional switching. Moreover, our analysis reveals two crucial ingredients leading to spontaneous collective changes of state: the non-symmetric nature of interaction between individuals and the presence of heterogeneities in the topology of the network.