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

Andrea Cavagna1,2,3, Guido Cimino2,1,*, Javier Cristín4, Matteo Fiorini2,1, Irene Giardina2,1,3, Angelo Giustiniani2,1, Tomás S. Grigera5,6,7,1, Stefania Melillo1,8, Roberto A. Palombella8,1 et al.

Leonardo Parisi1,8, Antonio Ponno9, Mattia Scandolo10, and Zachary S. Stamler8,1

  • *Contact author: gu.cimino@uniroma1.it
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Phys. Rev. Lett. 137, 098301 – Published 27 August, 2026

DOI: https://doi.org/10.1103/sppm-qsnb

Abstract

Collective turns in starling flocks propagate linearly with negligible attenuation, indicating the existence of an underdamped sector in the dispersion relation. According to linear response theory, 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.

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Phys. Rev. Lett. 137, 098301– Published 27 August, 2026
  • Received 28 August 2025
  • Revised 28 January 2026
  • Accepted 3 June 2026
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