Moving animal groups are a paradigmatic example of collective behaviour in social species. Finally, we quantitatively verify that birds follow equal-radius paths during turning, the effects of which are a change of the flock's orientation and a redistribution of individual locations in the group. We find that birds on the tips 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. We show that spontaneous turns start from individuals located at the elongated tips of the flocks, and then propagate through the group.
![starling flocks flying starling flocks flying](https://i.pinimg.com/originals/85/7b/ec/857bec866271993b0c7592542112b2c6.jpg)
Our approach enables us to analyse changes in the individual behaviour of every group member and reveal the emergent dynamics of turning. 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. Here, we present an experimental study of spontaneous collective turns in natural flocks of starlings. However, the cause and the mechanism by which such collective changes of direction occur are not fully understood yet. While this collective decision can be a response to an external alarm cue, directional switching can also emerge from the intrinsic fluctuations in individual behaviour. The paper is available online from the Proceedings of the National Academy of Sciences (PNAS).One of the most impressive features of moving animal groups is their ability to perform sudden coherent changes in travel direction. The starlings’ behavior is an example of self-organization, and the collective response to events such as attack by predators gives them a distinct advantage. Such a group would move in the same direction and would appear strongly ordered, but there would be no passing of information between individuals and so behavioral fluctuations are independent, with changes in direction of an animal other than the leader having little effect on other members of the group. The behavior of the flock of starlings is different to the behavior of a group following a leader. Dynamical evolutions of a starling flock. Physicist Irene Giardina, one of the study’s co-authors, said the results suggest starling flocks behave as critical systems “poised to respond maximally to environmental perturbations,” but she said more analysis would be needed to prove this definitively. An example cited by the researchers is ferromagnetism, in which particles in a magnet display perfect interconnection at a specific “critical” temperature. The synchronization of speeds and orientation is found in critical systems. How the birds achieve this feat is a mystery. What was most surprising to the scientists was the near-instantaneous signal processing speed. In every case if any one bird turned and changed speed, so did every other bird. The flocks ranged in size from 122 birds to 4,268, but the size of the flock made no difference to the way they moved. Large domains of strongly correlated birds are clearly visible. (B) This is the 2D projection of the individual velocity fluctuations in the same flock at the same instant of time as in A (vectors scaled for clarity).
![starling flocks flying starling flocks flying](https://thumbs.dreamstime.com/z/large-flock-starlings-birds-rapidly-waving-their-wings-flying-against-blue-summer-sky-155090042.jpg)
(A) The 2D projection of the velocities of the individual birds within a starling flock. The researchers used stereometric digital photogrammetry and computer vision techniques to reconstruct the 3D positions and 3D velocities of the individual birds in the flock from the consecutive shots of the flock, which were taken at 10 frames per second. The displays appeared like an aerial dance in which the flocks moved and swirled together. The flocks of birds spent their days in the countryside and returned to the city before sunset, performing aerial displays before they settled for the night.
![starling flocks flying starling flocks flying](https://thumbs.dreamstime.com/b/flock-starlings-flying-over-tree-sunset-large-silhouette-63380631.jpg)
The researchers studied flocks of starlings over a major winter roosting site in Rome, measuring the 3D positions and velocities of individual birds. When a starling flock acts as if it were a single entity it is acting as a critical system and is also optimizing its collective response to challenges such as a predator attack, according to theoretical physicist Giorgio Parisi and colleagues from the University of Rome. The phenomenon adopted by the flock of starlings ( Sturnus vulgaris) is called “scale-free behavioral correlation,” which occurs outside of biology in events such as an avalanche or the formation of crystals, both of which are critical systems in which nearly instantaneous transformation can occur.