Birds morph their wings during a single wingbeat, across flight speeds and among flight modes. Such morphing may allow them to maximize aerodynamic performance, but this assumption remains largely untested. We tested the aerodynamic performance of swept and extended wing postures of 13 raptor species in three families (Accipitridae, Falconidae and Strigidae) using a propeller model to emulate mid-downstroke of flapping during take-off and a wind tunnel to emulate gliding. Based on previous research, we hypothesized that (1) during flapping, wing posture would not affect maximum ratios of vertical and horizontal force coefficients (CV:CH), and that (2) extended wings would have higher maximum CV:CH when gliding. Contrary to each hypothesis, during flapping, extended wings had, on average, 31% higher maximum CV:CH ratios and 23% higher CV than swept wings across all biologically relevant attack angles (α), and, during gliding, maximum CV:CH ratios were similar for the two postures. Swept wings had 11% higher CV than extended wings in gliding flight, suggesting flow conditions around these flexed raptor wings may be different from those in previous studies of swifts (Apodidae). Phylogenetic affiliation was a poor predictor of wing performance, due in part to high intrafamilial variation. Mass was only significantly correlated with extended wing performance during gliding. We conclude that wing shape has a greater effect on force per unit wing area during flapping at low advance ratio, such as take-off, than during gliding.
The authors declare no competing or financial interests.
Conceptualization, B.K.v.O. and B.W.T.; Methodology, B.K.v.O. and B.W.T.; Formal Analysis, B.K.v.O. and B.W.T.; Investigation, B.K.v.O., E.A.M. and B.W.T.; Writing - Original Draft, B.K.v.O. and B.W.T.; Writing - Review & Editing, B.K.v.O. and B.W.T.; Funding Acquisition, B.K.v.O., E.A.M. and B.W.T.; Supervision, B.K.v.O. and B.W.T.
This research was supported by National Science Foundation grants GRFP DGE-1313190 to B.K.v.O. and IOS-0923606, IOS-0919799 and CMMI 1234747 to B.W.T. Undergraduate research support to E.A.M. was provided by the Herchel Smith Harvard Research Award.
Supplementary information available online at http://jeb.biologists.org/lookup/doi/10.1242/jeb.136721.supplemental
- Received December 24, 2015.
- Accepted July 25, 2016.
- © 2016. Published by The Company of Biologists Ltd