Wake analysis of a robotic avian wing with moult gaps in flapping flight

Birds regularly moult their feathers when they are worn from use such as mechanical wear damage due to UV exposure or accidents. Moulting generates temporary, generally symmetric, wing gaps, which can reduce flight performance by altering aerodynamic forces, lift and drag. Birds can compensate for some of these effects through mass loss, enlarged flight muscles, and adaptive wing morphing. To circumvent compensatory modulation by the birds, we performed experiments with a biohybrid robotic wing in a state-of-the-art wind tunnel. The biohybrid robotic wing, resembling a jackdaw, enables precise control over flapping kinematics, pitching, and wing folding behaviours while closely approximating avian wing morphology.We replicated the moult gaps observed in a jackdaw previously studied in gliding flight by selectively removing feathers on the robotic wing. Stereo particle image velocimetry was employed to characterise the wake structure and quantify changes in vorticity production and distribution due to feather loss across a range of kinematics and flight speeds. By linking wake dynamics to kinematic parameters, we aim to elucidate how naturally occurring moult gaps and wing damage influence aerodynamic performance. Our findings provide insight into how birds mitigate the aerodynamic costs of moult through behavioural and physiological adaptations, helping to explain why moult often coincides with specific phases of the annual cycle. Understanding these compensatory strategies sheds light on how birds maintain flight efficiency despite temporary losses of the wing surface.Beyond avian flight, these findings could inform strategies for mitigating the effects of wing damage, particularly in micro aerial vehicles.