Abstract
Collective behaviour is typically thought to arise from individuals following fixed interaction rules. The possibility that interaction rules may change under different circumstances has thus only rarely been investigated. Here we show that local interactions in flocks of wild jackdaws (Corvus monedula) vary drastically in different contexts, leading to distinct group-level properties. Jackdaws interact with a fixed number of neighbours (topological interactions) when traveling to roosts, but coordinate with neighbours based on spatial distance (metric interactions) during collective anti-predator mobbing events. Consequently, mobbing flocks exhibit a dramatic transition from disordered aggregations to ordered motion as group density increases, unlike transit flocks where order is independent of density. The relationship between group density and group order during this transition agrees well with a generic self-propelled particle model. Our results demonstrate plasticity in local interaction rules and have implications for both natural and artificial collective systems.
Highlights
Collective behaviour is typically thought to arise from individuals following fixed interaction rules
By measuring the three-dimensional (3D) movements of flocking jackdaws (Corvus monedula), we demonstrate that the local interaction rules and group-level properties can change in different contexts
We find that the local interactions in the two types of flocks vary drastically: jackdaws use topological interactions in transit flocks, but metric interactions in mobbing flocks
Summary
Collective behaviour is typically thought to arise from individuals following fixed interaction rules. Some support comes from laboratory experiments[13,15] showing that fish in small groups may modulate rule parameters, such as the size of their repulsion zone[13] or the tendency to initiate or follow movements[15] based on food availability and predation risk It remains unknown whether animals can switch between fundamentally different types of rules[1], such as between metric[3] and topological interactions[7] (interacting with all neighbours within a fixed distance or with a fixed number of individuals regardless of physical distance, respectively). Models[7] find that topologically interacting groups should always display order regardless of density, enhancing group cohesion and response to external perturbations such as predators[28] It remains unknown, whether interaction type is species-specific or whether a single species may switch between metric and topological interactions in different ecological contexts to optimize group function. By measuring the three-dimensional (3D) movements of flocking jackdaws (Corvus monedula), we demonstrate that the local interaction rules and group-level properties can change in different contexts
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