Absence of “selfish herd” dynamics in bird flocks under threat

• Published in: Current biology Vol. 31; no. 14; pp. 3192 - 3198.e7
• Main Authors: Sankey, Daniel W.E., Storms, Rolf F., Musters, Robert J., Russell, Timothy W., Hemelrijk, Charlotte K., Portugal, Steven J.
• Format: Journal Article
• Language: English
• Published: England Elsevier Inc 26-07-2021
• Subjects: animal-robot interactions; Animals; collective decision making; collective motion; Columbidae; Falconiformes; flight dynamics; Flight, Animal; Movement; movement ecology; mutualism; Predatory Behavior; Social Behavior; collective decision making; movement ecology; social behavior; collective motion; flight dynamics; animal-robot interactions; mutualism
• ISSN: 0960-9822; 1879-0445
• DOI: 10.1016/j.cub.2021.05.009

The “selfish herd” hypothesis1 provides a potential mechanism to explain a ubiquitous phenomenon in nature: that of non-kin aggregations. Individuals in selfish herds are thought to benefit by reducing their own risk at the expense of conspecifics by attracting toward their neighbors’ positions1,2 or central locations in the aggregation.3–5 Alternatively, increased alignment with their neighbors’ orientation could reduce the chance of predation through information sharing6–8 or collective escape.6 Using both small and large flocks of homing pigeons (Columba livia; n = 8–10 or n = 27–34 individuals) tagged with 5-Hz GPS loggers and a GPS-tagged, remote-controlled model peregrine falcon (Falco peregrinus), we tested whether individuals increase their use of attraction over alignment when under perceived threat. We conducted n = 27 flights in treatment conditions, chased by the robotic “predator,” and n = 16 flights in control conditions (not chased). Despite responding strongly to the RobotFalcon—by turning away from its flight direction—individuals in treatment flocks demonstrated no increased attraction compared with control flocks, and this result held across both flock sizes. We suggest that mutualistic alignment is more advantageous than selfish attraction in groups with a high coincidence of individual and collective interests (adaptive hypothesis). However, we also explore alternative explanations, such as high cognitive demand under threat and collision avoidance (mechanistic hypotheses). We conclude that selfish herd may not be an appropriate paradigm for understanding the function of highly synchronous collective motion, as observed in bird flocks and perhaps also fish shoals and highly aligned mammal aggregations, such as moving herds.
•Pigeons responded to a robotic falcon by turning away from its direction of flight•Centroid attraction was not favored in the chased group over control conditions•Fission or fusion responses were dependent on pigeons’ proximity to the robotic falcon
It is commonly thought that animals under threat crowd toward others in a selfish battle for the center. However, if instead of crowding the individuals in the group form into alignment, they may escape the predator as a group instead. Sankey et al. find a striking alignment response in pigeons chased by a remote-controlled robotic falcon.