Bret W. Tobalske’s research while affiliated with University of Montana and other places

Hummingbirds routinely execute a variety of stunning aerobatic feats, which continue to challenge current notions of aerial agility and controlled stability in biological systems. Indeed, the control of these amazing manoeuvres is not well understood. Here, we examined how hummingbirds control a sequence of manoeuvres within milliseconds, and tested whether and when they use vision during this rapid process. We repeatedly elicited escape flights in calliope hummingbirds, removed visible light during each manoeuvre at various instants and quantified their flight kinematics and responses. We show that the escape manoeuvres were composed of rapidly controlled sequential modules including evasion, reorientation, nose-down dive, forward flight and nose-up to hover. The hummingbirds did not respond to the light removal during evasion and reorientation until a critical light-removal time; afterwards, they showed two categories of luminance-based responses that rapidly altered manoeuvring modules to terminate the escape. We also show that hummingbird manoeuvres were rate-commanded and required no active braking (i.e. their body angular velocities were proportional to the change of wing motion patterns, a trait that probably alleviates the computational demand on flight control). This work uncovers key traits of hummingbird agility, which can also inform and inspire designs for next-generation agile aerial systems.

When a hovering hummingbird performs a rapid escape manoeuvre in response to a perceived threat from the front side, its body may go through simultaneous pitch, yaw and roll rotations. In this study, we examined the inertial coupling of the three-axis body rotations and its effect on the flight mechanics of the manoeuvre using analyses of high-speed videos as well as high-fidelity computational modelling of the aerodynamics and inertial forces. We found that while a bird’s pitch-up was occurring, inertial coupling between yaw and roll helped slow down and terminate the pitch, thus serving as a passive control mechanism for the manoeuvre. Furthermore, an inertial coupling between pitch-up and roll can help accelerate yaw before the roll–yaw coupling. Different from the aerodynamic mechanisms that aircraft and animal flyers typically rely on for flight control, we hypothesize that inertial coupling is a built-in mechanism in the flight mechanics of hummingbirds that helps them achieve superb aerial agility.

Bird flight is often characterized by outstanding aerodynamic efficiency, agility and adaptivity in dynamic conditions. Feathers play an integral role in facilitating these aspects of performance, and the benefits feathers provide largely derive from their intricate and hierarchical structures. Although research has been attempted on developing membrane-type artificial feathers for bio-inspired aircraft and micro air vehicles (MAVs), fabricating anatomically accurate artificial feathers to fully exploit the advantages of feathers has not been achieved. Here, we present our 3D printed artificial feathers consisting of hierarchical vane structures with feature dimensions spanning from 10−2 to 102 mm, which have remarkable structural, mechanical and aerodynamic resemblance to natural feathers. The multi-step, multi-scale 3D printing process used in this work can provide scalability for the fabrication of artificial feathers tailored to the specific size requirements of aircraft wings. Moreover, we provide the printed feathers with embedded aerodynamic sensing ability through the integration of customized piezoresistive and piezoelectric transducers for strain and vibration measurements, respectively. Hence, the 3D printed feather transducers combine the aerodynamic advantages from the hierarchical feather structure design with additional aerodynamic sensing capabilities, which can be utilized in future biomechanical studies on birds and can contribute to advancements in high-performance adaptive MAVs.

Complex hydrodynamics abound in natural streams, yet the selective pressures these impose upon different size classes of fish are not well understood. Attached vortices are produced by relatively large objects that block freestream flow, which fish routinely utilize for flow refuging. To test how flow refuging and the potential harvesting of energy (as seen in Kármán gaiting) varies across size classes in rainbow trout (Oncorhynchus mykiss; fingerling, 8 cm; parr, 14 cm; adult, 21 cm; n=4 per size class), we used a water flume (4,100 L; freestream flow at 65 cm s−1) and created vortices using 45° wing dams of varying size (small=15 cm, medium=31 cm, large=48 cm). We monitored microhabitat selection and swimming kinematics of individual trout and measured the flow field in the wake of wing dams using time-resolved Particle Image Velocimetry (PIV). Trout of each size class preferentially swam in vortices rather than the freestream, but the capacity to flow refuge varied according to the ratio of vortex width to fish length (VW : FL). Consistent refuging behavior was exhibited when VW : FL> 1.5. All size classes exhibited increased wavelength and Strouhal number and decreased tail beat frequency within vortices compared with freestream, suggesting that swimming in vortices requires less power output. In 17% of the trials, fish preferentially swam in a manner that suggests energy harvesting from the shear layer. Our results can inform efforts toward riparian restoration and fishway design to improve salmonid conservation.

Avian takeoff requires peak pectoralis muscle power to generate sufficient aerodynamic force during the downstroke. Subsequently, the much smaller supracoracoideus recovers the wing during the upstroke. How the pectoralis work loop is tuned to power flight is unclear. We integrate wingbeat-resolved muscle, kinematic, and aerodynamic recordings in vivo with a new mathematical model to disentangle how the pectoralis muscle overcomes wing inertia and generates aerodynamic force during takeoff in doves. Doves reduce the angle of attack of their wing mid-downstroke to efficiently generate aerodynamic force, resulting in an aerodynamic power dip, that allows transferring excess pectoralis power into tensioning the supracoracoideus tendon to assist the upstroke—improving the pectoralis work loop efficiency simultaneously. Integrating extant bird data, our model shows how the pectoralis of birds with faster wingtip speed need to generate proportionally more power. Finally, birds with disproportionally larger wing inertia need to activate the pectoralis earlier to tune their downstroke.

Hummingbird flight is the epitome of extreme aerial agility and controlled stability, as hummingbirds routinely exercise a variety of stunning aerobatic feats. Yet, the control of these amazing maneuvers is not well understood. Here we examined how hummingbirds control a sequence of maneuvers within milliseconds and tested whether and when their vision is active during this rapid process. We elicited escape flight in calliope hummingbirds and removed visible light at various instants during the maneuvers and quantified their flight kinematics and responses. We show that the escape maneuvers were composed of rapidly-controlled sequential modules, including evasion, reorientation, nose-down dive, forward flight, and nose-up to hover. The hummingbirds did not respond to the light removal during evasion and reorientation until a critical light-removal time; afterward they showed two categories of luminance-based responses that rapidly altered maneuvering modules to terminate the escape. We also show that hummingbird maneuvers are rate-commanded and require no active braking, i.e., their body angular velocities were proportional to the change of wing motion patterns, a trait that likely alleviates the computational demand on flight control. Together, this work uncovers the key traits of hummingbird agility, which can also inform and inspire designs for next-generation agile aerial systems.