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Costs and benefits of social relationships in the collective motion of bird flocks

Abstract

Current understanding of collective behaviour in nature is based largely on models that assume that identical agents obey the same interaction rules, but in reality interactions may be influenced by social relationships among group members. Here, we show that social relationships transform local interactions and collective dynamics. We tracked individuals’ three-dimensional trajectories within flocks of jackdaws, a species that forms lifelong pair-bonds. Reflecting this social system, we find that flocks contain internal sub-structure, with discrete pairs of individuals tied together by spring-like effective forces. Within flocks, paired birds interacted with fewer neighbours than unpaired birds and flapped their wings more slowly, which may result in energy savings. However, flocks with more paired birds had shorter correlation lengths, which is likely to inhibit efficient information transfer through the flock. Similar changes to group properties emerge naturally from a generic self-propelled particle model. These results reveal a critical tension between individual- and group-level benefits during collective behaviour in species with differentiated social relationships, and have major evolutionary and cognitive implications.

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Data availability

Supplementary Figs. 112 and Supplementary Tables 13 are available in the Supplementary Information. Raw images captured by one of the four cameras and the reconstructed birds’ 3D movement trajectories are provided in Supplementary Videos 16. Plain text files, each including bird ID number, position, time, velocity, acceleration and wingbeat frequency at every time step, are provided in Supplementary Data 17. A plain text file that includes mean wingbeat frequency, flight speed and local density (approximated by the number of neighbours within a distance of 5 m from the focal bird) for paired and unpaired birds in six flocks, as well as for birds flying alone, is provided in Supplementary Data 8. All data required to reproduce the results in this study are included in Supplementary Data 18. Supplementary Data and Supplementary Videos are available at https://figshare.com/s/c55eb82bab800571d25d.

Additional information

Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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Acknowledgements

This work was supported by a Human Frontier Science Program grant (No. RG0049/2017) to A.T., N.T.O. and R.T.V. We are grateful to P. Dunstan, R. Stone and the Gluyas family for permission to work on their land, and to V. Lee, B. Hooper, A. Hall, P. Petts, C. Peterson and J. Westley for their assistance in the field.

Author information

H.L., N.T.O., A.T. and R.T.V. conceived the ideas. H.L. and N.T.O. designed the methodology. G.M. and A.T. collected the data. H.L., K.V. and N.T.O. analysed the data. G.M., H.L. and A.T. performed statistical analysis. All authors led the writing of the manuscript. All authors contributed critically to the drafts and gave final approval for publication.

Competing interests

The authors declare no competing interests.

Correspondence to Alex Thornton or Nicholas T. Ouellette.

Supplementary information

  1. Supplementary Information

    Supplementary Figs. 1–12, Supplementary Tables 1–3.

  2. Reporting Summary

  3. Supplementary Video 1

    Original images captured by one of the four cameras and the reconstructed birds’ 3D movement trajectories for flock No. 1.

  4. Supplementary Video 2

    Original images captured by one of the four cameras and the reconstructed birds’ 3D movement trajectories for flock No. 2.

  5. Supplementary Video 3

    Original images captured by one of the four cameras and the reconstructed birds’ 3D movement trajectories for flock No. 3.

  6. Supplementary Video 4

    Original images captured by one of the four cameras and the reconstructed birds’ 3D movement trajectories for flock No. 4.

  7. Supplementary Video 5

    Original images captured by one of the four cameras and the reconstructed birds’ 3D movement trajectories for flock No. 5.

  8. Supplementary Video 6

    Original images captured by one of the four cameras and the reconstructed birds’ 3D movement trajectories for flock No. 6.

  9. Supplementary Data 1

    Bird ID number, position, time, velocity, acceleration and wingbeat frequency at every time step for flock No. 1.

  10. Supplementary Data 2

    Bird ID number, position, time, velocity, acceleration and wingbeat frequency at every time step for flock No. 2.

  11. Supplementary Data 3

    Bird ID number, position, time, velocity, acceleration and wingbeat frequency at every time step for flock No. 3.

  12. Supplementary Data 4

    Bird ID number, position, time, velocity, acceleration and wingbeat frequency at every time step for flock No. 4.

  13. Supplementary Data 5

    Bird ID number, position, time, velocity, acceleration and wingbeat frequency at every time step for flock No. 5.

  14. Supplementary Data 6

    Bird ID number, position, time, velocity, acceleration and wingbeat frequency at every time step for flock No. 6.

  15. Supplementary Data 7

    Bird ID number, position, time, velocity, acceleration and wingbeat frequency at every time step for 305 isolated pairs (birds with id 1 & 2, 3 & 4, 5 & 6 and so on are from a single pair).

  16. Supplementary Data 8

    Text file that includes mean wingbeat frequency, flight speed and local density (approximated by the number of neighbours within a distance of 5 m of the focal bird) for paired and unpaired birds in six flocks, as well as for birds flying alone.

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Fig. 1: Flock morphology and evidence of pairing.
Fig. 2: Pairing causes variations in local interaction.
Fig. 3: Effect of pairing on the power consumption of individuals.
Fig. 4: Pairing reduces group correlation length.