Collective behaviors of swarms and flocks
Keywords: Engineering.

Throughout nature, collective animal groups operate without central control and display group-level properties and functionality that are distinct from the linear sum of those of the individuals. Collective groups display a remarkable ability to achieve their functionality robustly. Thus, many seek to adapt the collective behaviors of animal groups to building bio-inspired, decentralized engineering systems. This requires both understanding the functions of the group and ascertaining the local interaction rules between individual members, which remains a challenge. Here, we address some of the open questions of collective behavior by studying two archetypes of collective groups: the disordered insect swarms and the ordered bird flocks. We present results from empirical data of swarms and flocks that elucidate the structure and precise properties of collective groups. We first study the way that individuals in mating swarms of the non-biting midge \textit{Chironomus riparius} sample the space available to them. Here, we categorize different regions of the swarm by using Voronoi tessellation to delineate the empty space within the volume occupied by the swarm. We found that although swarms are disordered and lack apparent structure, the collective motions of swarms are not random, as individual insects in the swarms preferentially sample the edges compared to the internal regions. However, midges that primarily reside in the interior or the edges of the swarm do not exhibit statistically distinct kinematics. This suggests that we need a different way to assess the fitness of individuals. Nonetheless, our results reveal that rather than being an arbitrary aggregation of insects, swarms possess persistent internal structure. We then conducted field experiments to perturb the transit flocks of jackdaws to probe the internal structure of bird flocks. We launched a projectile at transit flocks of jackdaws, applying both an auditory stimulus from the sound of the launching and a visual stimulus of the projectile to the flocks. We determined the reaction time of each bird from the time of the projectile firing by analyzing the change in its trajectory curvature. We observed evidence for information propagation through the flock, as the time to react varied approximately linearly with the distance to the first reacting bird, and calculated the speed of information propagation for each flock. We found that rather than responding individually to the source of perturbation, birds in the flock react to each other. This suggests that birds rely on each other when responding to a perceived threat and that enhanced perception and protection from predators may be one of the functions of transit flocks. To further explore the functions of flocks, we applied the thermodynamics framework that was previously developed for insect swarms to describe the macroscopic properties of bird flocks. Following similar methodologies, we arrived at an equation of state for bird flocks using empirical data of unperturbed transit flocks of jackdaws. We also fitted the van der Waals equation of state, an extension of the equation of state for ideal gas that considers the effects of particle interactions, to the flock data. Our preliminary results show the similarities and differences between the equation of state for bird flocks and that for the insect swarms.