Flying birds often form flocks, with social1, navigational2 and anti-predator3 implications. Further, flying in a flock can result in aerodynamic benefits, thus reducing power requirements4, as demonstrated by a reduction in heart rate and wingbeat frequency in pelicans flying in a V-formation5. 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.
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We would like to thank T. Hubel, H. Chapman, V. Unt and T. Demes for practical assistance, and The Wellcome Trust (J.R.U.), The Royal Society (A.M.W.), BBSRC (K.R.) and EPSRC for funding.
The authors declare no competing financial interests.
The file contains Supplementary Methods and Supplementary Tables 1-2. (PDF 263 kb)
The movie shows up to 18 pigeons flying in a flock measured with back-mounted GPS and Inertial Measurement Units in June 2010. GPS measurements and logging are activated in response flapping, as identified from the dorsal accelerometer signal. This enables sufficient battery life for continuous field deployment of over two days. The tails show the paths taken in the previous 10 seconds; tail width is proportional to height; tail colours distinguish individual pigeons. Head colours represent flapping frequency above (red) or below (blue) average for each pigeon. Playback rate triple actual rate. (MOV 26519 kb)
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Usherwood, J., Stavrou, M., Lowe, J. et al. Flying in a flock comes at a cost in pigeons. Nature 474, 494–497 (2011). https://doi.org/10.1038/nature10164
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