Upwash exploitation and downwash avoidance by flap phasing in ibis formation flight

Journal name:
Nature
Volume:
505,
Pages:
399–402
Date published:
DOI:
doi:10.1038/nature12939
Received
Accepted
Published online

Many species travel in highly organized groups1, 2, 3. The most quoted function of these configurations is to reduce energy expenditure and enhance locomotor performance of individuals in the assemblage4, 5, 6, 7, 8, 9, 10, 11. The distinctive V formation of bird flocks has long intrigued researchers and continues to attract both scientific and popular attention4, 7, 9, 10, 11, 12, 13, 14. The well-held belief is that such aggregations give an energetic benefit for those birds that are flying behind and to one side of another bird through using the regions of upwash generated by the wings of the preceding bird4, 7, 9, 10, 11, although a definitive account of the aerodynamic implications of these formations has remained elusive. Here we show that individuals of northern bald ibises (Geronticus eremita) flying in a V flock position themselves in aerodynamically optimum positions, in that they agree with theoretical aerodynamic predictions. Furthermore, we demonstrate that birds show wingtip path coherence when flying in V positions, flapping spatially in phase and thus enabling upwash capture to be maximized throughout the entire flap cycle. In contrast, when birds fly immediately behind another bird—in a streamwise position—there is no wingtip path coherence; the wing-beats are in spatial anti-phase. This could potentially reduce the adverse effects of downwash for the following bird. These aerodynamic accomplishments were previously not thought possible for birds because of the complex flight dynamics and sensory feedback that would be required to perform such a feat12, 14. We conclude that the intricate mechanisms involved in V formation flight indicate awareness of the spatial wake structures of nearby flock-mates, and remarkable ability either to sense or predict it. We suggest that birds in V formation have phasing strategies to cope with the dynamic wakes produced by flapping wings.

At a glance

Figures

  1. V formation flight in migrating ibises.
    Figure 1: V formation flight in migrating ibises.

    a, Northern bald ibises (G. eremita) flying in V formation during a human-led migratory flight (photograph M. Unsöld). b, Three-dimensional location histogram of the 7min flight section, showing position of individual ibises (n = 14) in the V formation, with respect to flock centroid, measured by a 5Hz GPS data logger. The colour scale refers to the duration (in seconds) a bird was present in each 0.25m×0.25m grid. A plot detailing the formation shape for the duration of the entire flight can be found in Supplementary Fig. 7. c, Histogram of number of flaps (colour coded) recorded in each 0.25m×0.25m region between all birds and all other birds. Most flaps occurred at an angle of approximately 45° to the bird ahead (or behind). Transects denoted by dashed lines, directly behind or along the most populated V favoured position (just inboard of wingtip to wingtip), are the same as those detailed in Fig. 3. d, Histogram detailing the total number of flaps recorded between each bird–bird pair, with respect to position of the following bird. The shaded area (ii–i) denotes the limits of optimal relative positioning, based on fixed-wing aerodynamics.

  2. Histograms demonstrating the positional infidelity for each northern bald ibis in the V formation during the migratory flight.
    Figure 2: Histograms demonstrating the positional infidelity for each northern bald ibis in the V formation during the migratory flight.

    The grey shaded V shape behind each individual histogram (n = 14) denotes the structure for all individuals in the flock (see Fig. 1b). The colour code refers to the duration (in seconds) a bird was present in each 0.25m × 0.25m grid. Although individual birds showed some bias towards the front, back, left or right regions of the V formation, these positions were not maintained rigidly.

  3. Geometric and aerodynamic implications of observed spatial phase relationships for ibises flying in a V formation.
    Figure 3: Geometric and aerodynamic implications of observed spatial phase relationships for ibises flying in a V formation.

    Temporal phase increases as a function of position behind more advanced birds (median±95% confidence intervals of phase for each mean bird–bird interaction in a region). When positioned close to a wavelength in line with the V favoured position (ac), wingtip paths approximately match: observed temporal phases agree with those predicted from the significant spatial phase relationship (thick black lines, ±95% confidence intervals) at the most populated 1m×1m region, using the mean wavelength measured for each position. When positioned directly in line (df), following birds flap in spatial antiphase, maximally separating wingtip paths. In this case the model line is derived from the median spatial phase for all bird–bird interactions up to 4m directly behind. Induced flow velocities (blue arrows, c, f), caused by the trailing wingtip vortices of the bird ahead (vortex cores denoted by grey circles), are modelled as infinitely long, parallel vortex filaments. Birds flying in typical V formation keep their wings close to the region of maximal induced upwash (c) throughout the flap cycle. Birds flying directly behind flap in spatial antiphase, potentially reducing the adverse effects of downwash (f), both in terms of magnitude and direction. For scale, the downwash directly between the vortices would be (−)0.3ms−1, between trailing vortices behind a bird of mass 1.3kg, span 1.2m at a speed of 15ms−1 (no account is taken of flapping, viscosity or wake contraction). Alternative representations of a and d as Cartesian plots can be found in Supplementary Fig. 3, and Supplementary Fig. 4 details the extended data array shown beyond the presented model line.

Videos

  1. A section of the ibis flight
    Video 1: A section of the ibis flight
    An animated movie showing a section of the ibis flight, taken from the 5 Hz GPS logger data. Each individual bird is identified by a number displayed on the tip of the left wing.
  2. Ibis flying behind the paraplane
    Video 2: Ibis flying behind the paraplane
    A short video clip of the ibis flying behind the paraplane during a training flight.

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Author information

Affiliations

  1. Structure & Motion Laboratory, the Royal Veterinary College, University of London, Hatfield, Hertfordshire AL9 7TA, UK

    • Steven J. Portugal,
    • Tatjana Y. Hubel,
    • Stephen Hailes,
    • Alan M. Wilson &
    • James R. Usherwood
  2. Waldrappteam, Schulgasse 28, 6162 Mutters, Austria

    • Johannes Fritz,
    • Stefanie Heese,
    • Daniela Trobe &
    • Bernhard Voelkl
  3. Institute for Theoretical Biology, Humboldt University at Berlin, Invalidenstrasse 43, 10115 Berlin, Germany

    • Bernhard Voelkl
  4. Department of Computer Science, University College London, Gower Street, London WC1E 6BT, UK

    • Stephen Hailes
  5. Present address: Edward Grey Institute, Department of Zoology, University of Oxford, Oxford OX1 3PS, UK.

    • Bernhard Voelkl

Contributions

S.J.P., S.Ha., A.M.W. and J.R.U. developed the concept of the paper. J.F., S.He. and D.T. reared and trained the birds. S.J.P., S.He., D.T., B.V. and J.F. collected the field data. S.J.P., T.Y.H. and J.R.U. undertook the data processing and analyses; J.R.U. performed the circular statistics. S.J.P., T.Y.H, A.M.W. and J.R.U wrote the manuscript, with input from all authors.

Competing financial interests

The authors declare no competing financial interests.

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Supplementary information

Video

  1. Video 1: A section of the ibis flight (9.6 MB, Download)
    An animated movie showing a section of the ibis flight, taken from the 5 Hz GPS logger data. Each individual bird is identified by a number displayed on the tip of the left wing.
  2. Video 2: Ibis flying behind the paraplane (4.66 MB, Download)
    A short video clip of the ibis flying behind the paraplane during a training flight.

PDF files

  1. Supplementary Information (3.9 MB)

    This file contains Supplementary Figures 1-8 and Supplementary Table 1.

Zip files

  1. Supplementary Data 1 (504 KB)

    This zipped file contains a Google EarthTM (Landsat, KML file) image displaying the full flight of the ibis flock, recorded via the 5 Hz GPS data logger.

Text files

  1. Supplementary Table 2 (17 KB)

    This file contains full raw data set for phasing analysis (see Methods), covering both the spanwise and streamwise positions.

Additional data