Flying in a flock comes at a cost in pigeons

Journal name:
Nature
Volume:
474,
Pages:
494–497
Date published:
DOI:
doi:10.1038/nature10164
Received
Accepted
Published online

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.

At a glance

Figures

  1. Flap-number histogram contour plots.
    Figure 1: Flap-number histogram contour plots.

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

  2. Influence of speed, and power and flock factors on flap frequency and dorsal amplitude.
    Figure 2: Influence of speed, and power and flock factors on flap frequency and dorsal amplitude.

    ah, The influence attributable to airspeed (a, e), induced power (b, f), climbing power (c, g) and accelerating power (d, h) on flap frequency (ad) and dorsal amplitude (eh) over each flap (18 pigeons, 171,209 flaps). Red curves show third-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.

  3. Relationship between flock factor and flap frequency or dorsal displacement.
    Figure 3: Relationship between flock factor and flap frequency or dorsal displacement.

    ac, The relationship between flock factor—the proportion of hemisphere-view covered by other pigeons for every flap, illustrated graphically with six neighbouring birds in c—and flap frequency (a) or dorsal displacement amplitude (b). The vertical grey dashed line indicates the mean flock factor (FF = 1.7%), the underlying dotted lines show the third-order polynomial fits used in the statistical separation of factors, and the 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.

References

  1. Nagy, M., Ákos, Z., Biro, D. & Vicsek, T. Hierarchical group dynamics in pigeon flocks. Nature 464, 890893 (2010)
  2. Dell’Ariccia, G., Dell’Omo, G., Wolfer, D. P. & Lipp, H.-P. Flock flying improves pigeons’ homing: GPS track analysis of individual flyers versus small groups. Anim. Behav. 76, 11651172 (2008)
  3. Tinbergen, N. The Study of Instinct (Clarendon Press, 1951)
  4. Lissaman, P. B. S. & Schollenberger, C. A. Formation flight of birds. Science 168, 10031005 (1970)
  5. Weimerskirch, H., Martin, J., Clerquin, Y., Alexandre, P. & Jiraskova, S. Energy saving in flight formation. Nature 413, 697698 (2001)
  6. Higdon, J. J. L. & Corrsin, S. Induced drag of a bird flock. Am. Nat. 112, 727744 (1978)
  7. Heppner, F. H. Avian flight formations. Bird-Banding 45, 160169 (1974)
  8. Greene, P. R. & McMahon, T. A. Running in circles. Physiologist 22, S35S36 (1979)
  9. Usherwood, J. R. & Wilson, A. M. Accounting for elite indoor 200m sprint results. Biol. Lett. 2, 4750 (2006)
  10. Usherwood, J. R. & Wilson, A. M. No force limit on greyhound sprint speed. Nature 438, 753754 (2005)
  11. Pennycuick, C. J. Bird Flight Performance: A Practical Calculation Manual (Oxford Univ. Press, 1989)
  12. Pennycuick, C. J., Klaasen, M., Kvist, A. & Lindström, Å. Wingbeat frequency and the body drag anomaly: wind-tunnel observations on a thrush nightingale (Luscinia luscinia) and a teal (Anas crecca). J. Exp. Biol. 199, 27572765 (1996)
  13. Bilo, D., Lauck, A. & Nachtigall, W. in Biona-Report 3 (ed. Nachtigall, W.) 87108 (Gustav Fischer, 1984)
  14. Hedrick, T. L., Usherwood, J. R. & Biewener, A. A. Wing inertia and whole-body acceleration: an analysis of instantaneous aerodynamic force production in cockatiels (Nymphicus hollandicus) flying across a range of speeds. J. Exp. Biol. 207, 16891702 (2004)
  15. Tobalske, B. W., Hedrick, T. L., Dial, K. P. & Biewener, A. A. Comparative power curves in bird flight. Nature 421, 363366 (2003)
  16. Schmidt-Wellenburg, C. A., Biebach, H., Daan, S. & Visser, G. H. Energy expenditure and wing beat frequency in relationship to body mass in free flying barn swallows (Hirundo rustica). J. Comp. Physiol. B 177, 327337 (2007)
  17. Rayner, J. M. V. A vortex theory of animal flight. Part 1. The vortex wake of a hovering animal. J. Fluid Mech. 91, 697730 (1979)
  18. Ellington, C. P. The aerodynamics of hovering insect flight. V. A vortex theory. Phil. Trans. R. Soc. Lond. B 305, 115144 (1984)
  19. Spedding, G. R. & McArthur, J. Span efficiencies of wings at low Reynolds numbers. J. Aircr. 47, 120128 (2010)
  20. Lilienthal, O. Birdflight as the Basis of Aviation (Markowski International, 2001) [transl.].
  21. Usherwood, J. R. Inertia may limit efficiency of slow flapping flight, but mayflies show a strategy for reducing the power requirements of loiter. Bioinspir. Biomim. 4, 015003 (2009)
  22. Pomeroy, H. & Heppner, F. Structure of turning in airborne rock dove (Columba livia) flocks. Auk 109, 256267 (1992)
  23. Hedrick, T. L., Cheng, B. & Deng, X. Wingbeat time and the scaling of passive rotational damping in flapping flight. Science 324, 252255 (2009)

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Author information

Affiliations

  1. Structure and Motion Laboratory, The Royal Veterinary College, University of London, North Mymms, Hatfield AL9 7TA, UK

    • James R. Usherwood,
    • Marinos Stavrou,
    • John C. Lowe,
    • Kyle Roskilly &
    • Alan M. Wilson

Contributions

J.R.U. and A.M.W. conceived and designed the project. J.R.U. analysed the data, and wrote the paper with input from all other authors. M.S. trained the pigeons and helped perform the experiments. J.C.L., K.R. and A.M.W. developed and built the equipment.

Competing financial interests

The authors declare no competing financial interests.

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Supplementary information

PDF files

  1. Supplementary Information (263K)

    The file contains Supplementary Methods and Supplementary Tables 1-2.

Movies

  1. Supplementary Movie 1 (25.8M)

    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.

Comments

  1. Report this comment #26205

    jack kornblatt said:

    The article by Usherwood et al and the accompanying News and Views by Spedding is fascinating and, at first reading, reinforces the prejudices many of us have towards pidgeons, viz, Pidgeons are the stupidest of all animals. Having said that, it is necessary to put things into perspective. Thermodynamics teaches us one thing. It is necessary to view an ensemble of molecules from the point of view of the system. While flying in flocks may not really correspond to an ensemble of molecules and while it may be costly in terms of the energetics of flight, it must have some "redeeming value" that the pidgeons recognize.

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