Flying in a flock comes at a cost in pigeons

Flying birds often form flocks, with social(1), navigational(2) and antipredator(3) implications. Further, flying in a flock can result in aerodynamic benefits, thus reducing power requirements(4), as demonstrated by a reduction in heart rate and wingbeat frequency in pelicans flying in a V-formation(5). But how general is an aerodynamic power reduction due to group-flight? V-formation flocks are limited to moderately steady flight in relatively large birds, and may represent a special case. What are the aerodynamic consequences of flying in the more usual 'cluster'(6,7) flock? Here we use data from innovative back-mounted Global Positioning System (GPS) and 6-degrees-of-freedom inertial sensors to show that pigeons (1) maintain powered, banked turns like aircraft, imposing dorsal accelerations of up to 2g, effectively doubling body weight and quadrupling induced power requirements; (2) increase flap frequency with increases in all conventional aerodynamic power requirements; and (3) increase flap frequency when flying near, particularly behind, other birds. Therefore, unlike V-formation pelicans, pigeons do not gain an aerodynamic advantage from flying in a flock. Indeed, the increased flap frequency, whether due to direct aerodynamic interactions or requirements for increased stability or control, suggests a considerable energetic cost to flight in a tight cluster flock.