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

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Abstract

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.

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Figure 1: V formation flight in migrating ibises.
Figure 2: Histograms demonstrating the positional infidelity for each northern bald ibis in the V formation during the migratory flight.
Figure 3: Geometric and aerodynamic implications of observed spatial phase relationships for ibises flying in a V formation.

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Acknowledgements

The Waldrappteam assisted with data collection and provided logistical support (J.F., B.V.). We thank members of the Structure & Motion Laboratory for discussions and assistance, particularly J. Lowe, K. Roskilly, A. Spence and S. Amos, and C. White and R. Bomphrey for reading an earlier draft of the paper. Funding was provided by an Engineering and Physical Sciences Research Council grant to A.M.W., J.R.U. and S.Ha. (EP/H013016/1), a Biotechnology and Biological Sciences Research Council grant to A.M.W. (BB/J018007/1) and a Wellcome Trust Fellowship (095061/Z/10/Z) to J.R.U.

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Authors and Affiliations

Authors

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.

Corresponding author

Correspondence to Steven J. Portugal.

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The authors declare no competing financial interests.

Related audio

Supplementary information

Supplementary Information

This file contains Supplementary Figures 1-8 and Supplementary Table 1. (PDF 4060 kb)

Supplementary Data 1

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. (ZIP 504 kb)

Supplementary Table 2

This file contains full raw data set for phasing analysis (see Methods), covering both the spanwise and streamwise positions. (TXT 17 kb)

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. (MOV 9838 kb)

Ibis flying behind the paraplane

A short video clip of the ibis flying behind the paraplane during a training flight. (MP4 4772 kb)

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Portugal, S., Hubel, T., Fritz, J. et al. Upwash exploitation and downwash avoidance by flap phasing in ibis formation flight. Nature 505, 399–402 (2014). https://doi.org/10.1038/nature12939

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