How the hummingbird wingbeat is tuned for efficient hovering

Abstract

Both hummingbirds and insects flap their wings to hover. Some insects, like fruit flies, improve efficiency by lifting their body weight equally over the upstroke and downstroke, while utilizing elastic recoil during stroke reversal. It is unclear whether hummingbirds converged on a similar elastic storage solution, because of asymmetries in their lift generation and specialized flight muscle apparatus. The muscles are activated a quarter of a stroke earlier than in larger birds, and contract superfast, which cannot be explained by previous stroke-averaged analyses. We measured the aerodynamic force and kinematics of Anna's hummingbirds to resolve wing torque and power within the wingbeat. Comparing these wingbeat-resolved aerodynamic weight support measurements with those of fruit flies, hawk moths and a generalist bird, the parrotlet, we found that hummingbirds have about the same low induced power losses as the two insects, lower than that of the generalist bird in slow hovering flight. Previous analyses emphasized how bird flight muscles have to overcome wing drag midstroke. We found that high wing inertia revises this for hummingbirds – the pectoralis has to coordinate upstroke to downstroke reversal while the supracoracoideus coordinates downstroke to upstroke reversal. Our mechanistic analysis aligns with all previous muscle recordings and shows how early activation helps furnish elastic recoil through stroke reversal to stay within the physiological limits of muscles. Our findings thus support Weis-Fogh's hypothesis that flies and hummingbirds have converged on a mechanically efficient wingbeat to meet the high energetic demands of hovering flight. These insights can help improve the efficiency of flapping robots.

Birds land reliably on complex surfaces by adapting their foot-surface interactions upon contact

“…The superfast claw movements are likely not governed by muscle contraction alone; muscle contractions in human hands are significantly slower (~50 ms) (Buchthal and Schmalbruch, 1970). Even the fastest vertebrate locomotory muscles recorded to date are still 5–10 times slower; the Etruscan shrew’s extensor digitorum longus contracts in 11 ms (Jürgens, 2002) and the superfast pectoralis muscle of a hummingbird contracts in 8 ms (Ingersoll and Lentink, 2018). Therefore, the speed of the parrotlet claw movements probably rely on the release of energy stored in an elastic tendon (McNeill Alexander, 2002) and the low inertia of the claw.…”

Birds repurpose the role of drag and lift to take off and land

“…However, increasing wing velocity would require increasing wing amplitude further, which is morphologically constrained, or increasing muscle contraction frequency, which is restricted to a narrow range for optimizing muscle efficacy 44 . Frequency is further constrained when inertial energy losses are to be mitigated by elastic recoil, which requires operating close to a resonant frequency of elastic storage 45,46 . Considering these constraints, the most parsimonious (remaining) solution is to increase C F , which can be done by generating a LEV.…”

The aerodynamic force platform as an ergometer

“…The vertical and horizontal aerodynamic forces of the dove were measured using a 2D AFP (Lentink, 2018;Lentink et al, 2015). Previous 1D versions of the AFP (Chin and Lentink, 2016;Ingersoll and Lentink, 2018) measured vertical forces by instrumenting the floor and ceiling of a flight chamber with carbon fiber composite panels. The new 2D AFP used in this study ( Fig.…”