Spin-Waves without Spin-Waves: A Case for Soliton Propagation in Starling Flocks

TL;DR

This paper investigates the propagation of collective turns in starling flocks, revealing that they exhibit both underdamped and overdamped wave behaviors, which can be explained by incorporating a nonlinear Fermi-Pasta-Ulam-Tsingou term into the model.

Contribution

The study introduces a novel theoretical framework that reconciles experimental observations of wave propagation in starling flocks by adding a nonlinear term to existing models.

Findings

Underdamped traveling waves coexist with overdamped correlations in starling flocks.

High-resolution experiments support the presence of both wave types.

Adding a Fermi-Pasta-Ulam-Tsingou term explains the coexistence of wave behaviors.

Abstract

Collective turns in starling flocks propagate linearly with negligible attenuation, indicating the existence of an underdamped sector in the dispersion relation. Beside granting linear propagation of the phase perturbations, the real part of the frequency should also yield a spin-wave form of the unperturbed correlation function. However, new high-resolution experiments on real flocks show that underdamped traveling waves coexist with an overdamped Lorentzian correlation. Theory and experiments are reconciled once we add to the dynamics a Fermi-Pasta-Ulam-Tsingou term.