Flapping Frequency, Amplitude, and Organization: Associations Between Accelerometer Data and Flight Energy Costs During Solo and Small Group Flight of European Starlings

Prior work from our research group demonstrated metabolic energy savings for European starlings following behind a leader in a small-group wind tunnel flight context. However, the published analysis focused on metabolic measurements plus the relative flight position in the group and provided little insight into how flight behavior changed from the solo to group context, or behaviors associated with energy cost savings in group flight. Here, we use accelerometer data to fill this gap, highlighting how overall flight behavior and the drivers of energy costs shift between solo and group flight. Data were collected from ten birds flying solo and as groups of two or three in the Brown University animal flight wind tunnel. The metabolic energy expenditure of one bird in each flight set was measured through the elimination of 13C given as an injection of 13C NaBi before flight. Position in the wind tunnel was continuously monitored using four cameras, and a 3-axis accelerometer mounted to the back of each bird provided high-frequency measurement of flight movements. In solo flight, more continuous low-effort flapping reduced energy costs compared to bursts of high-effort flapping. Group flight increased the fraction of flight time spent flapping, but also broke the link between continuous flapping and low energy cost. Instead, low-cost group fliers used a slow and organized burst-coast rhythm, while high-cost group fliers had irregular short periods of flapping and coasting. Finally, for group flight followers, we found that their energy costs were more related to the properties of their leader than their own actions. Specifically, the degree of organization in the flapping pattern of the upstream bird had a stronger association with the follower energy costs than any metric recorded directly from the follower.