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A fundamental avian wing-stroke provides a new perspective on the evolution of flight

Abstract

The evolution of avian flight remains one of biology’s major controversies, with a long history of functional interpretations of fossil forms given as evidence for either an arboreal or cursorial origin of flight. Despite repeated emphasis on the ‘wing-stroke’ as a necessary avenue of investigation for addressing the evolution of flight1,2,3,4, no empirical data exist on wing-stroke dynamics in an experimental evolutionary context. Here we present the first comparison of wing-stroke kinematics of the primary locomotor modes (descending flight and incline flap-running) that lead to level-flapping flight in juvenile ground birds throughout development (Fig. 1). We offer results that are contrary both to popular perception and inferences from other studies5,6,7. Starting shortly after hatching and continuing through adulthood, ground birds use a wing-stroke confined to a narrow range of less than 20°, when referenced to gravity, that directs aerodynamic forces about 40° above horizontal, permitting a 180° range in the direction of travel. Based on our results, we put forth an ontogenetic-transitional wing hypothesis that posits that the incremental adaptive stages leading to the evolution of avian flight correspond behaviourally and morphologically to transitional stages observed in ontogenetic forms.

Our data suggest a default or basal wing-stroke is used by young and adults and may exist in all birds (Supplementary Videos). The fundamental wing-stroke described herein is used days after hatching and during all ages and over multiple behaviours (that is, flap-running, descending and level flight) and is the foundation of our new ontogenetic-transitional wing hypothesis. At hatching, chicks can ascend inclines as steep as 60° by crawling on all four limbs. From day 8 through adulthood, birds use a consistently orientated stroke-plane angle over all substrate inclines during wing-assisted incline running (red arcs) as well as during descending and level flight (blue arcs). Estimated force orientations from this conserved wing-stroke are limited to a narrow wedge (see Fig. 3b).

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Figure 2: Average stroke-plane angle among locomotor styles in three frames of reference.
Figure 3: Comparison of wing-stroke plane angle, estimated force orientation and angle of attack among locomotor styles.

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Acknowledgements

We thank the following for their suggestions and comments: A. Biewener, M. Bundle, R. Callaway, H. Davis, S. Gatesy, D. Irschick, F. Jenkins, Jr, J. Maron, T. Martin, K. Padian and B. Tobalske.

Author Contributions K.P.D. provided the conceptual foundation, funding and facilities. K.P.D. and B.E.J. wrote the manuscript. B.E.J. and P.S. performed most data acquisition and analyses.

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Correspondence to Kenneth P. Dial.

Supplementary information

Supplementary Information 1

The file contains Supplementary Figures S1-S2, Supplementary Methods and Legends to Supplementary Videos 1-4. (PDF 935 kb)

Supplementary Information 2

This file contains Supplementary Video 1. (MOV 2755 kb)

Supplementary Information 3

This file contains part one of the Supplementary Video 2. (MOV 10077 kb)

Supplementary Information 4

This file contains part two of the Supplementary Video 2. (MOV 6616 kb)

Supplementary Information 5

This file contains part one of the Supplementary Video 3. (MOV 9237 kb)

Supplementary Information 6

This file contains part two of the Supplementary Video 3. (MOV 2469 kb)

Supplementary Information 7

This file contains part one of the Supplementary Video 4. (MOV 13143 kb)

Supplementary Information 8

This file contains part two of the Supplementary Video 4. (MOV 8722 kb)

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Dial, K., Jackson, B. & Segre, P. A fundamental avian wing-stroke provides a new perspective on the evolution of flight. Nature 451, 985–989 (2008). https://doi.org/10.1038/nature06517

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