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Journal of Experimental Biology partnership with Dryad

Backward flight in hummingbirds employs unique kinematic adjustments and entails low metabolic cost
Nir Sapir, Robert Dudley

SUMMARY

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 (Embedded Image) 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 Embedded Image in sustained backward flight is similar to that in forward flight at equivalent airspeed, and is about 20% lower than hovering Embedded Image. 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.

FOOTNOTES

  • LIST OF SYMBOLS AND ABBREVIATIONS

    f
    wingbeat frequency
    FEO2
    fractional concentration of O2 exiting the mask
    FIO2
    fractional concentration of O2 entering the mask
    J
    advance ratio
    R
    wing length
    US:DS
    upstroke to downstroke duration ratio
    Embedded Image
    mass flow of gas exiting the respirometry mask
    Embedded Image
    rate of oxygen consumption
    βb
    stroke plane angle in relation to longitudinal body axis
    βh
    stroke plane angle in relation to horizontal
    χ
    body angle in relation to horizontal
    λb
    head flexion in relation to the longitudinal body axis
    Φ
    wingbeat amplitude
    Φmaxmin
    ratio of maximum to minimum wing positional angle
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