Emergence of collective changes in travel direction of starling flocks from individual birds fluctuations

TL;DR

This study investigates how spontaneous collective turns in starling flocks originate from individual fluctuations, revealing edge birds as initiators and showing the flock's ability to reorganize while maintaining coherence.

Contribution

The paper provides the first detailed analysis of spontaneous collective turns in starling flocks using 3D tracking, highlighting the role of edge individuals and localized fluctuations.

Findings

Turns originate from edge birds deviating from the group

Fluctuations at the edges trigger the collective turn

Birds follow equal radius paths during turns

Abstract

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.