Interaction Ruling Animal Collective Behaviour Depends on Topological rather than Metric Distance: Evidence from a Field Study

TL;DR

This study reveals that bird flocking behavior is governed by topological rather than metric interactions, with each bird interacting with a fixed number of neighbors, which enhances flock cohesion especially during external disturbances.

Contribution

The paper provides empirical evidence that collective animal behavior depends on topological interactions, challenging existing metric-based models and highlighting their importance for flock cohesion.

Findings

Birds interact with a fixed number of neighbors (6-7).

Interaction depends on topological rather than metric distance.

Topological interaction improves flock cohesion during perturbations.

Abstract

Numerical models indicate that collective animal behaviour may emerge from simple local rules of interaction among the individuals. However, very little is known about the nature of such interaction, so that models and theories mostly rely on aprioristic assumptions. By reconstructing the three-dimensional position of individual birds in airborne flocks of few thousands members, we prove that the interaction does not depend on the metric distance, as most current models and theories assume, but rather on the topological distance. In fact, we discover that each bird interacts on average with a fixed number of neighbours (six-seven), rather than with all neighbours within a fixed metric distance. We argue that a topological interaction is indispensable to maintain flock's cohesion against the large density changes caused by external perturbations, typically predation. We support this hypothesis by numerical simulations, showing that a topological interaction grants significantly higher cohesion of the aggregation compared to a standard metric one.