Inside & Out Flight: aerodynamics and skeletal function during slow flight in birds
Slow flight is extremely energetically costly per unit time, yet highly important for takeoff and survival in birds. However, at slow speeds it is presently thought that most birds do not produce beneficial aerodynamic forces during the entire wingbeat: instead, they fold or flex their wings during upstroke, prompting the long-standing prediction that the upstroke produces trivial forces. Here, I examined the kinematics, aerodynamics, and skeletal contribution to the upstroke in birds that use both major upstroke styles that birds exhibit. Diamond doves that keep their wings extended in a “wingtip-reversal” upstroke (at Re=50,000) produce a kinematic and aerodynamic signature similar to the clap-and-peel mechanism previously reported only in insects (Re=8,000). In contrast, zebra finch use a “flexed-wing” upstroke that is aerodynamically inactive. Integrating an XROMM (X-ray Reconstruction of Moving Morphology) study of pigeons and starlings, I demonstrate that the supination of the tip-reversal upstroke occurs via a slide of the carpometacarpus across the cuneiform of the wrist. Collectively, I reveal that the clap and fling mechanism utilized by many species is a wing motion that is aerodynamically beneficial and largely due to an interaction of the skeletal elements.