Flying in a flock comes at a cost in pigeons

Abstract

Flying birds often form flocks, with social, navigational and anti-predator implications. Further, flying in a flock can result in aerodynamic benefits, thus reducing power requirements, as demonstrated by a reduction in heart rate and wingbeat frequency in pelicans flying in a V-formation. 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' 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.

Figure 1

Flap-number histogram contour plots for flap-averaged dorsal acceleration (a), and net pitch (b) and yaw (c) angular displacements (averaged over 5 flaps). Turning with optimal banking requires an increase in dorsal acceleration, and pitch and yaw displacement every flap (insets). Black lines show the predicted (from GPS alone) against observed (IMU alone) relationships for birds, assuming they bank optimally during turns.

Figure 2

The influence attributable to airspeed (a,e), induced power (b,f), climbing power (c,g) and accelerating power (d,h) on flap frequency (a-d) and dorsal amplitude (e-h) over each flap (18 pigeons, 171,209 flaps). Red curves show 3^rd order polynomial fits; points show the values once the influence of all other factors have been removed. Each point represents the average of 100 flaps, binned along the x-axis. Dashed red lines show +/− 99.99% confidence intervals. Blue curves (a,e) show the relationship predicted if the effect of both airspeed and induced power (which includes airspeed as a term) are combined; in effect, the relationship that would be observed for steady, straight, level flight.

Figure 3

The relationship between Flock Factor – the proportion of hemisphere-view covered by other pigeons for every flap, illustrated graphically with six neighbouring birds (c) – and flap frequency (a) or dorsal displacement amplitude (b). The vertical grey dashed line indicates the mean Flock Factor (1.7%), the underlying dotted lines the 3^rd order polynomial fits used in the statistical separation of factors, and error bars show s.e.m. Flight in a cluster flock, particularly when flying behind other birds, is associated with an increase in flap frequency and decrease in dorsal amplitude.