The wrist joints of the hummingbird enable hovering by rotating back and forth.

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The hummingbird (Apodiformes) is able to drink the nectar of flowers while steadily hovering in mid air by flapping its wings over 80 times per second.   Hummingbirds are able to achieve this amazing feat by moving the air around their wings more efficiently than other birds. Birds are able to fly by flapping their wings up and down, which creates ‘lift’.  This is because the shape of the wing creates a lower pressure above and a higher pressure below, ‘lifting’ the object upwards. When birds flap their wings, the downstroke creates the lift, while the upstroke prepares the bird for the next downstroke. Insects, on the other hand, are able to use both their upstroke and downstroke to create lift because they are able to twist their wings at their flexible joints, which increases their aerodynamic efficiency. Most birds are unable to do this because their muscle and skeletal joints are configured to allow a repeatable back and forth motion. However, similar to the insect,  the hummingbird is able to rotate its wings back to front at the wrist joint to create lift during both the downward and upward stroke. The hummingbird is able to maintain its hover due to the constant lift force created.

The rotation in the wrist joint of the hummingbird can be compared to the human wrist controlling a kayak paddle.  When a kayaker moves forward, they rotate their wrists so that the paddle is tilted forward and down to push the water backwards.  If the kayaker wants to stay in one place, they will rotate the paddle to push the water forwards and continuously alternate the strokes. The physical mechanism in the hummingbird is the rotation at the wrist, similar to the kayakers ability to adjust the paddle and stay in one place.  

Watch this video to learn more about how hummingbirds hover.

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References

“Hummingbirds and insects have evolved for sustained hovering flight from vastly different ancestral directions, and their distinct phylogenies underlie the differences in their aerodynamic styles. In all other birds—and, presumably, hummingbird ancestors—the downstroke provides 100% of weight support during slow flight and hovering [hummingbirds today, produce 75% of their weight support during the downstroke and only 25% during the upstroke]. Given that many birds possess the mass-specific power (using anaerobic metabolism) to hover for short periods, the selective pressure on hummingbird ancestors was probably for increased efficiency (resulting in stiff wings with greatly simplified kinematics), and an upstroke muscle (the supracoracoideus) that makes the recovery stroke rapid, while contributing enough to the hovering power requirements to allow the downstroke muscle (the pectoralis) to operate within its aerobic limits. In other words, this pseudosymmetrical wingbeat cycle is good enough, and although hummingbirds do not exhibit the elegant aerodynamic symmetry of insects, natural selection rewards ‘good enough’ as richly as it does our aesthetic ideals.” (Warrick et al. 2005:1096)

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Hummingbirds and insects have evolved for sustained hovering flight from vastly different ancestral directions, and their distinct phylogenies underlie the differences in their aerodynamic styles. In all other birds—and, presumably, hummingbird ancestors—the downstroke provides 100% of weight support during slow flight and hovering [hummingbirds today, produce 75% of their weight support during the downstroke and only 25% during the upstroke].

Journal article
Aerodynamics of the hovering hummingbirdNatureJune 23, 2005
Warrick DR, Tobalske BW, Powers DR.

To understand the difference, Hedrick recommends trying to mimic a bird by flapping your arms. “You’re doing something not too different to what a seagull’s doing,” he says. To mimic a hummingbird, “hold your upper arm close to your body with your elbow on your hip, and flap your forearms back and forth”.

Web page
Hummingbird flight has a clever twistnature.comDecember 11, 2014
Ed Yong

In insects, active wing inversion must originate at the wing base because the wings have no distal joints. However, flying vertebrates have muscles and skeletal joints throughout their wings and may flex or rotate different segments according to aerodynamic demands. Thus, the source of wing inversion in the hummingbird flight stroke remains uncertain but is hypothesized to occur at the wrist.

Journal article
Morphological and kinematic basis of the hummingbird flight stroke: scaling of flight muscle transmission ratioPROCEEDINGS OF THE ROYAL SOCIETY B: BIOLOGICAL SCIENCESDecember 14, 2011
Hedrick TL, Tobalske BW, Ros IG, Warrick DR, Biewener AA.

The simulation captures the lift and power characteristics in a stroke cycle and also details of the flow field. Our result confirms and provides specific data for the lift asymmetry that was previously suggested based on the measurement of the circulation around the wing. Furthermore, we quantitatively analysed the sources of the lift asymmetry and pointed out the mechanisms that lead to the asymmetry. Summarizing the results, the downstroke produces 150% higher vertical force than the upstroke.

Journal article
Three-dimensional flow and lift characteristics of a hovering ruby-throated hummingbirdJournal of the Royal Society InterfaceSeptember 6, 2014
Song J, Luo H, Hedrick TL.

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Living System/s

Organism
Hummingbirds And SwiftsApodiformesOrder

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