  One of the most impressive features of moving animal groups is their ability
to perform sudden coherent changes in travel direction. While this collective
decision can be a response to an external perturbation, such as the presence of
a predator, recent studies show that such directional switching can also emerge
from the intrinsic fluctuations in the individual behaviour. However, the cause
and the mechanism by which such collective changes of direction occur are not
fully understood yet. Here, we present an experimental study of spontaneous
collective turns in natural flocks of starlings. We employ a recently developed
tracking algorithm to reconstruct three-dimensional trajectories of each
individual bird in the flock for the whole duration of a turning event. Our
approach enables us to analyze changes in the individual behavior of every
group member and reveal the emergent dynamics of turning. We show that
spontaneous turns start from individuals located at the elongated edges of the
flocks, and then propagate through the group. We find that birds on the edges
deviate from the mean direction of motion much more frequently than other
individuals, indicating that persistent localized fluctuations are the crucial
ingredient for triggering a collective directional change. Finally, we
quantitatively show that birds follow equal radius paths during turning
allowing the flock to change orientation and redistribute risky locations among
group members. The whole process of turning is a remarkable example of how a
self-organized system can sustain collective changes and reorganize, while
retaining coherence.
