Aerodynamic consequences of wing morphing during emulated take-off and gliding in birds.

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 (C:C), and that (2) extended wings would have higher maximum C:C when gliding. Contrary to each hypothesis, during flapping, extended wings had, on average, 31% higher maximum C:C ratios and 23% higher C than swept wings across all biologically relevant attack angles (α), and, during gliding, maximum C:C ratios were similar for the two postures. Swept wings had 11% higher C 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.