What these data show is that several traits of collective motion are well reproduced by relatively simple models based on local interaction rules. More recently, though, a new generation of experimental studies, both in two and in three dimensions, have been performed, establishing the basis for an empirically validated understanding of the interaction rules in collective animal behaviour. For a long time, owing to the technical difficulties in reconstructing individual motion in large groups, data have been scarce. Ī central question in collective animal behaviour is to understand what are the interaction rules through which global coordination emerges. There is no leader in the group, and the collective movement is a unique consequence of local interactions between individuals. At dusk, huge flocks move above the roost, exhibiting beautiful collective patterns. Self-organization and the spontaneous emergence of order in biological systems does not come much more spectacular than in large flocks of starlings ( Sturnus vulgaris). On the contrary, our findings suggest that a more complex dynamical process occurs at the border of the flock. We also measure the amount of neighbours reshuffling and find that neighbours change in time exclusively as a consequence of the random fluctuations in the individual motion, so that no specific mechanism to keep one's neighbours seems to be enforced. Moreover, this behaviour is strongly anisotropic with respect to the direction of motion of the flock. We find that birds move faster than Brownian walkers both with respect to the centre of mass of the flock, and with respect to each other. Here, we assess the relevance of this mechanism in large flocks of starlings ( Sturnus vulgaris). The possibility to exchange neighbours strongly enhances the stability of global ordering and the way information is propagated through the group.

When compared with other cases of collective ordering, a crucial feature of animal groups is that the interaction network is not fixed in time, as each individual moves and continuously changes its neighbours. Each individual has a tendency to align its flight direction with those of neighbours, and such a simple form of interaction produces a state of collective motion of the group. Flocking is a paradigmatic example of collective animal behaviour, where global order emerges out of self-organization.

Four flocks