Backward 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 () and flight kinematics of Anna's 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 in sustained backward flight is similar to that in forward flight at equivalent airspeed, and is about 20% lower than hovering . 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.
This study was supported by the National Science Foundation, Division of Integrative Organismal Systems [grant number 0837866].
Supplementary material available online at http://jeb.biologists.org/cgi/content/full/215/20/3603/DC1
LIST OF SYMBOLS AND ABBREVIATIONS
- wingbeat frequency
- fractional concentration of O2 exiting the mask
- fractional concentration of O2 entering the mask
- advance ratio
- wing length
- upstroke to downstroke duration ratio
- mass flow of gas exiting the respirometry mask
- rate of oxygen consumption
- stroke plane angle in relation to longitudinal body axis
- stroke plane angle in relation to horizontal
- body angle in relation to horizontal
- head flexion in relation to the longitudinal body axis
- wingbeat amplitude
- ratio of maximum to minimum wing positional angle
- © 2012.