Red-tailed hawks can compensate for feather loss during molt by subtly changing their wing and tail movements, according to a new study by University of California, Davis, researchers in the College of Engineering and the Weill School of Veterinary Medicine.
The findings could inform wildlife rehabilitation efforts for birds of prey and inspire the design of more resilient uncrewed aerial vehicles, or UAVs, that can continue operating despite losing part of a wing or control surface.
The study was published June 17 in The Royal Society Publishing.
"We see during molt that the bird experiences a temporary reduction in aerodynamic capability due to the missing feathers but still maintains performance by adjusting the kinematics," said Alfonso Martínez-Carmena, lead author on the paper and a postdoctoral researcher in the Department of Mechanical and Aerospace Engineering. "For UAV design, this could translate into morphing structures or adaptive flight controllers that can compensate for changes in the aerodynamic configuration, such as damage or component degradation, rather than relying on a fixed optimal design."
During molt, which typically occurs at least once a year, birds shed old feathers to make room for new ones. The process leaves gaps in the wings and tail, which can make flight more difficult. Yet birds still need to fly to navigate their environment, hunt and perch.
Using four synchronized high-speed cameras, Martínez-Carmena tracked wing and tail movements of Jack, a resident red-tailed hawk at the California Raptor Center, a rehabilitation center for birds of prey at the Weill School of Veterinary Medicine. While Jack flew from a handler's arm to a fixed perch, the handler moved their arm between heights for each flight.
This maneuver, said Martínez-Carmena, involves intense coordination.
"It requires the bird to fly at low speed, with its wings operating at high angles of attack, a regime in which aerodynamic forces become nonlinear and difficult to predict," he said. "It necessitates continuous adjustment of the body, the wing and the tail kinematics to balance forces right up to touchdown."
The hawk was filmed performing this maneuver during natural molt and again after its feathers had fully regrown. The researchers found that during molt, several tail feathers were missing, reducing the hawk's tail area. The bird compensated for this by changing its tail angle immediately after takeoff, which may have contributed to the additional transient thrust observed by Martínez-Carmena.
The hawk also adjusted its wing motion, putting its wings closer together during part of each wingbeat, potentially helping reduce gaps caused by missing feathers. Overall, the hawk maintained flight performance despite having fewer feathers.
Researchers plan to extend the study to additional bird species and across the entire molting season to investigate how kinematic variations change depending on the location of feather gaps in the tail and wings.
This study is the first to emerge from the interdisciplinary partnership between Assistant Professor of Mechanical and Aerospace Engineering Christina Harvey and Michelle Hawkins , a professor of veterinary medicine and director of the California Raptor Center. Their partnership is now formalized as the Center for Animal Locomotion and Innovation .
