Tuning for feather loss: wing-tail kinematic adjustments of a moulting red-tailed hawk during perching flight.

Abstract

Birds periodically replace feathers through a process known as moult, often resulting in gaps in their wings or tail; however, many birds must retain locomotive capabilities even when moulting, such as perching. Although moult is known to affect flight performance, the compensatory strategies used by birds to adapt to moult-based changes remain underexplored. Using high-speed imaging, we extracted the wing and tail kinematics of a red-tailed hawk (Buteo jamaicensis), during moult and while fully feathered, as it performed a perching manoeuvre from three different heights. Our results show that the hawk adjusted its wing and tail kinematics to generate similar time-averaged force-to-weight ratios across trials for both feather conditions. However, during the first downstroke after take-off from all heights, the hawk used a significantly larger tail incidence angle and produced higher thrust during moult. Together, these observations suggest that moult only subtly alters time-averaged normalized aerodynamic forces, yet influences the timing of force production within the wingbeat cycle. This study contributes empirical evidence on how a moulting hawk can compensate for morphological variations and weight differences through kinematic changes. By identifying avian compensatory mechanisms, we can inform strategies for wildlife rehabilitation as well as damage tolerance for uncrewed aerial vehicles.