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How swifts control their glide performance with morphing wings

Abstract

Gliding birds continually change the shape and size of their wings1,2,3,4,5,6, presumably to exploit the profound effect of wing morphology on aerodynamic performance7,8,9. That birds should adjust wing sweep to suit glide speed has been predicted qualitatively by analytical glide models2,10, which extrapolated the wing’s performance envelope from aerodynamic theory. Here we describe the aerodynamic and structural performance of actual swift wings, as measured in a wind tunnel, and on this basis build a semi-empirical glide model. By measuring inside and outside swifts’ behavioural envelope, we show that choosing the most suitable sweep can halve sink speed or triple turning rate. Extended wings are superior for slow glides and turns; swept wings are superior for fast glides and turns. This superiority is due to better aerodynamic performance—with the exception of fast turns. Swept wings are less effective at generating lift while turning at high speeds, but can bear the extreme loads. Finally, our glide model predicts that cost-effective gliding occurs at speeds of 8–10 m s-1, whereas agility-related figures of merit peak at 15–25 m s-1. In fact, swifts spend the night (‘roost’) in flight at 8–10 m s-1 (ref. 11), thus our model can explain this choice for a resting behaviour11,12. Morphing not only adjusts birds’ wing performance to the task at hand, but could also control the flight of future aircraft7.

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Figure 1: Equilibrium gliding along a helical path.
Figure 2: Morphing swift wings can generate higher lift and lower drag than wings with a fixed geometry.
Figure 3: Morphing improves glide performance of swifts.
Figure 4: Morphing maintains wing structural integrity at high glide speeds.
Figure 5: Swifts roost at glide speeds that minimize energy expenditure.

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Acknowledgements

Swifts were supplied by Vogelopvang Woudenberg, Fugelpits Moddergat, Fugelhelling Ureterp, Vogelopvang De Strandloper Bergen, Vogelasiel De Wulp Den Haag, Vogelasiel Haarlem, Vogelasiel Naarden and Vogelopvang Someren. Swift photographs were provided by J.-F. Cornuet (front-view, Fig. 1a) and L.G.M. Schols (side-view Fig. 1a; Fig. 1b). N.G. Verhagen, J. Bäckman and J.H. Becking helped with background research. E.W. Karruppannan, L.J.G.M. Bongers, L. Molenwijk, L.M.M. Boermans, S. Bernardy and H. Schipper helped with the experimental set-up. F.T. Muijres and R. Petie helped with the experiments. T.P. Weber and S.M. Deban critically read the manuscript. O. Berg improved many versions of the manuscript. U.K.M. is funded by NWO, and A.H. by Carl Trygger’s Foundation.

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Correspondence to D. Lentink.

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This file contains Supplementary Figures 1-2 with Legends, Supplementary Table 1, Supplementary Equations , full protocol and additional references. (PDF 2016 kb)

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Lentink, D., Müller, U., Stamhuis, E. et al. How swifts control their glide performance with morphing wings. Nature 446, 1082–1085 (2007). https://doi.org/10.1038/nature05733

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