Aerodynamic mechanisms behind energy efficiency in migratory bird formations

To this day, the reasons for the formation flight of migratory birds still generate lively controversies. Although group flying offers numerous advantages in predation prevention, orientation, and communication, aspects like aerodynamic interaction between birds remain poorly understood due to their complexity. In this study, we investigate the wake vortex dynamics of two migratory birds based on their relative positions within the group. We assess the influence of wingbeat synchronization on aerodynamic forces using a computational code based on the finite volume method. Modeling 3D vortex structures in the wake of Canadian geese flying at 1000 and 13.9 m/s reveals a complex vortex with an undulating structure influenced by wingbeat frequency. The study demonstrates that the trailing bird improves its aerodynamic efficiency by 32% when positioning its wingtip in the upwash region of the leading bird's wingtip vortex. Furthermore, wingbeat synchronization between the leading and trailing birds is confirmed as crucial for energy gain. These findings extrapolated to larger bird groups, provide valuable insights into the organization and optimal positioning of migratory birds in V formation.