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

• Published in: Nature ecology & evolution Vol. 3; no. 6; pp. 943 - 948
• Main Authors: Ling, Hangjian, Mclvor, Guillam E., van der Vaart, Kasper, Vaughan, Richard T., Thornton, Alex, Ouellette, Nicholas T.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 01.06.2019; Nature Publishing Group
• Subjects: 631/158/856; 631/601/1332; 631/601/18; 704/158/2451; Biological and Physical Anthropology; Biomedical and Life Sciences; Birds; Cognitive ability; Ecology; Energy conservation; Evolutionary Biology; Information transfer; Life Sciences; Paleontology; Wings; Zoology
• ISSN: 2397-334X; 2397-334X
• DOI: 10.1038/s41559-019-0891-5

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. Interactions between members of a group may be influenced by their social relationships. Here, the authors track individuals’ trajectories within flocks of jackdaws and show that their social relationships transform local interactions and collective dynamics.