Aerodynamics, kinematics, and ecology of slow flight in birds

The overarching goal of my dissertation is to elucidate the force production mechanisms of 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, skeletal drivers, and potential ecological influences of force production in flight. In chapter one, I establish that wings in upstroke posture are capable of producing beneficial aerodynamic forces. Chapter two illustrates 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 in chapter three, I demonstrate that both upstroke styles have similar skeletal kinematics (with a few notable exceptions), but the timing and extent of motion differs. Lastly, chapter four faces birds with an ecologically relevant task: transitioning from a compliant substrate reduces initial flight velocities, and birds do not appear to modulate force production to compensate. 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. These four chapters illuminate an energetically costly and ecologically relevant period of flight