Backward flight in hummingbirds employs unique kinematic adjustments and entails low metabolic cost

Abstract

SUMMARYBackward flight is a frequently used transient flight behavior among members of the species-rich hummingbird family (Trochilidae) when retreating from flowers, and is known from a variety of other avian and hexapod taxa, but the biomechanics of this intriguing locomotor mode have not been described. We measured rates of oxygen uptake (V O2 ) and flight kinematics of Annaʼs hummingbirds, Calypte anna (Lesson), within a wind tunnel using mask respirometry and high-speed videography, respectively, during backward, forward and hovering flight. We unexpectedly found that V O2 in sustained backward flight is similar to that in forward flight at equivalent airspeed, and is about 20% lower than hovering V O2 . For a bird that was measured throughout a range of backward airspeeds up to a speed of 4.5ms -1 , the power curve resembled that of forward flight at equivalent airspeeds. Backward flight was facilitated by steep body angles coupled with substantial head flexion, and was also characterized by a higher wingbeat frequency, a flat stroke plane angle relative to horizontal, a high stroke plane angle relative to the longitudinal body axis, a high ratio of maximum:minimum wing positional angle, and a high upstroke:downstroke duration ratio. Because of the convergent evolution of hummingbird and some hexapod flight styles, flying insects may employ similar kinematics while engaged in backward flight, for example during station keeping or load lifting. We propose that backward flight behavior in retreat from flowers, together with other anatomical, physiological, morphological and behavioral adaptations, enables hummingbirds to maintain strictly aerial nectarivory. Supplementary material available online at

Into turbulent air: size-dependent effects of von Kármán vortex streets on hummingbird flight kinematics and energetics

“…The power curve for male Anna's Hummingbirds has been empirically determined using mask respirometry across the entire airspeed range for the species, from a fast 14 m s 21 down to hovering in still air [38], and also for backward flight [22]. As suggested by aerodynamic models [28] the power curve exhibited a minima at intermediate forward air speeds because of high induced drag during hovering and slow flight, and high parasite and profile drags during high-speed forward flight.…”

Hummingbirds control turning velocity using body orientation and turning radius using asymmetrical wingbeat kinematics

“…As a result, it is unknown how body-dependent force reorientation and body-independent wingbeat asymmetries are modulated to generate and control centripetal force. Hummingbirds have previously been used to study manoeuvrability, and the wingbeat kinematics of several behaviours have been described including take-off [12], hovering [13], forward flight [14], backward flight [15] and yaw turns [8]. Because hummingbirds can be trained to track a feeder following defined trajectories, they provide an opportunity to experimentally determine the relative contributions of wingbeat and body kinematics to radius and velocity control of arcing turns.…”

Implications of floral orientation for flight kinematics and metabolic expenditure of hover‐feeding hummingbirds

“…Consequently, aerodynamic consequences of syringe and mask size and shape are probably small, and we accordingly neglect their possible effects on wingbeat kinematics and metabolic rates. To further examine whether aerodynamic interaction with the respirometry mask caused leakage of respired gas from the mask, we repeated validation of the system as specified above while the respirometry system was installed within a wind tunnel (see details in Sapir & Dudley ) operated at an airflow of 12 m s −1 . Under these conditions, the measured due to argon dilution was 98·1% (± 4·8%) of the predicted value.…”