I’m watching a male Ruby-throated Hummingbird on bee balm. He’s hovering, as if on a skyhook, taking on nectar. Then he backs out about 5 inches and freezes in space, his wings a blur. Then he vanishes up and away, as if shot from a canon. I’ve lost him. Now he’s back, pauses, and shoots up about 6 inches, pauses, and then flies head-first into another flower. I’m mesmerized.
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Of all avian attributes, none is as essential as flight. For most flying birds, flight is a variation on a common theme. But not for hummingbirds; they’ve gone their own way. Hummers have developed an astonishing flight style that makes other birds look ordinary.
Early in avian evolution, natural selection favored light and strong skeletal systems benefitting flight and massive breast muscles to serve as high-powered engines, also for flight. To fuel these engines, birds developed high metabolic rates that required high body temperatures and high-energy foods.
Flight requires movement of the wings. Because muscles work by shortening, they must be located between a structure and the direction in which it will be pulled. Downstroke was easy. The large breast muscle (pectoralis major) attaches to the breast bone and inserts underneath and near the head of the upper arm bone (humerus). When this muscle contracts, the wing is pulled down.
Upstroke was a problem for all flying birds, because there wasn’t enough room on top of the shoulder to attach a muscle large enough to lift the wing during flight. This engineering quandary was solved when birds cleverly utilized a second breast muscle (the supracoracoideus) that lies on the breast under the pectoralis. The tendon on the end of this muscle passes through an opening created by bones of the shoulder girdle and attaches to the top of the humerus, near the head. The opening in the shoulder acts like a pulley, so when this muscle contracts, it actually lifts the wing, even though it’s located below the rest of the bird.
Non-hummer flying birds have wings that bend at the elbow and wrist and beat in a vertical plane. Power is generated on the downstroke, and wing recovery occurs on the upstroke. The birds flap (and glide) through the air.
The elongated wings of hummingbirds are stiff and do not bend. The short flight feathers give the wing an oar-like appearance. In fact, the movement of hummingbirds’ stiff wings in flight is much like rowing through the air. Unfortunately, hummingbirds row so fast that it’s mostly a blur and hard to see, although if we’re close enough, we can hear the characteristic hum for which they are named.
The rate at which hummingbirds beat their wings is truly astounding. In general, smaller species beat their wings faster than larger ones, and the rates vary from about 25 to 80 beats per second. By contrast, most other flying birds beat their wings fewer than five times per second. The slow wing beat of some herons, for example, is a little more than two per second, while most songbirds are around five. Birds that have relatively small wings for their size, such as diving ducks and pheasants, have rapid wing beats but probably don’t exceed nine beats per second.
Another important difference between hummers and other flying birds is that hummingbird humeri (the upper arm bones) rotate in the shoulder joint. The rotation allows hummers to change the angle of their wings’ leading edges between downstroke and upstroke. During downstroke, the leading edge tilts down at about 45 degrees, making the underside of the wing push against the air. During upstroke, the leading edge tilts up at about 45 degrees, so the upper side of the wing pushes against the air. Consequently, the wings are able to generate power with both the downstroke and the upstroke, an avian trait unique to hummingbirds.
The breast muscle responsible for the upstroke is proportionally much larger in hummingbirds than in other birds and is about equal in size to the large breast muscle that creates the downstroke. This assures
near-equal power of the two strokes and is critical to the next step.
Once hummingbirds had achieved equally powered downstroke and upstroke, they were just a body contortion away from hovering. By tilting the body back and moving their wing beats from a vertical to horizontal plane, they resemble a helicopter and can hover with great precision. Moreover, by moving the plane of the wing beats from horizontal to slightly tilting back, they can fly backward and, with different tilting angles can fly virtually in any direction. And as everyone knows, the birds’ movements can be slow and smooth or fast and jerky. Hovering flight has been incorporated into their courtship and territorial displays and has increased their feeding efficiency.
The resourceful hummingbirds took to the air and rewrote the book on flight. They combine speed with endurance. And to the thrill of birdwatchers, they incorporate stunt and acrobatic flying as part of their amazing mastery of the air.
This article from Eldon Greij’s column “Amazing Birds” appeared in the July/August 2019 issue of BirdWatching.