Wings that “sing” are nothing new to ornithologists. The American Woodcock (Scolopax minor) circles upward in a breeding display. When the bird has climbed to 200-300 feet above the ground, it breaks into a kamikaze flight to the ground. The wing feathers twitter.
Club-winged Manakins (Machaeropterus deliciosus) use their secondary remiges (the flight feathers) to communicate in courtship displays. When they head for the avian version of a singles bar, known as a lek, they raise their wings over their backs, shaking them back-and-forth 100 times a second (twice the speed of hummingbird wing strokes), producing a high-pitched sound.
Other manakins make wing sounds in other ways. When it launches into its song-and-dance displays, the Red-capped Manakin (Pipra mentalis) slaps its flight feather against its sides.
Slow though they may be by comparison to manakins, the 35 species of “bee” hummingbirds have some talking feathers of their own. In a 2011 paper published in SCIENCE, Yale ornithologist Chris Clark and coauthors Damian Elias and Rick Prum found that the tonal sounds produced during dive displays emanate from the rapid spreading and closing of the tail feathers. An earlier study showed that clipping the tail feathers silenced the dive sound. Now, by putting the tail feathers of a number of hummingbird species in a wind tunnel and using a scanning laser doppler vibrometer, Clark and colleagues have actually measured the fluttering of the tail feathers. They determined that the feathers produce sounds in four different modes of vibration. Air flowing across the transverse edge of the vane (the flat parts of the feather that are attached to the central feather shaft which is known as a rachis) is one of the two most common modes of sound-producing vibration. This mode generally occurs in feathers of uniform width. For tapering feathers, a combination of torsional (twisting) and transverse airflow around the tip of the feather produces sound. In some species, the vibration is entirely torsional and in one species, the entire feather bends. And, it turns out, adjacent feathers oscillate in response to these vibrations and amplify the sound.
Some species have mastered three-part harmony. The Allen’s Hummingbird (Salasphorus sasin) produces two tones with its tail feathers and another with its wings.
Calling this sound-producing property “aeroelastic flutter,” the authors of the paper note that tonal flight sounds occur in many bird species that do not have modified feathers, probably because probably because all flight feathers are stiff, flat airfoils that are prone to aeroelastic flutter above certain speeds, regardless of the feather shape and structure. In hummingbirds, they propose, this inherent property became selected for through sexual selection. The tail feathers are sexually dimorphic (differ between males and females) and the feathers of each species differ from those of other species, producing a sound unique to each species. And so, a feature that might be catastrophic in an airplane – would you like to look at the window and see the wings vibrating and twisting? – turned out for many species of birds to be a characteristic that gave them the ability to communicate with the ladies.
This article summarizes information in this publication:
Clark, Christopher J., Damian O. Elias, and Richard O. Prum. 2011. Aeroelastic flutter produces hummingbird feather songs. Science 333:1430-1433. doi:10.1126/science.1205222
Photo: Volcano Hummingbird, captured from YouTube video, included in video courtesy of Anand Varma, www.varmaphoto.com (see video for description)