Three years ago, UC Berkeley students Chris Clark and Teresa Feo cracked the mystery of the dive chirp of the Anna’s hummingbird. The noise, produced at the nadir of a courting male’s vertical plummet, was thought by some ornithologists—included the late Luis Baptista of the California Academy of Sciences—to be vocal in origin, in part because its frequency was similar to that of the bird’s call.
However, Clark and Feo used high-speed videorecordings and wind-tunnel tests to establish that the dive chirp was actually caused by the passage of rushing air through the hummingbird’s tail feathers. They staked out female hummingbirds at the Albany Bulb to attract males and captured a 60-millisecond spreading of the displaying males’ tail feathers coincident with the chirp. Males with customized tails displayed but did not chirp. In the wind tunnel tests at the Hopkins Marine Station, the sound was reproduced when the inner vein of a male’s outer tail feather fluttered at a frequency of 3.3 to 4.7 kiloherz, four octaves above middle C. The feathers essentially act like the reed of a clarinet.
Clark went on to Yale, where he subjected more hummingbird species to flight recording and wind tunnel analysis with a scanning laser Doppler vibrometer. His new article, coauthored with Damian Elias and Richard Prum, has just been published in the prestigious journal Science (inaccessible to nonsubscribers, but the gist is on Youtube http://www.youtube.com/watch?v=MPaVjhUsdAw; thanks to Rusty Scalf for the link.) He and his colleagues studied 14 “bee” hummingbirds, including the Anna’s, the Allen’s, the black-chinned, and some tropical forms. All have aerial courtship displays culminating in a loud noise, and most of the noises result from tail feather vibration.
At a wind speed of 7 to 20 meters per second, the range of the hummers’ normal dive velocities, the tail feathers started to ripple rhythmically like the infamous Tacoma Narrows Bridge and emit piercing noises. Some effects involved multiple feathers: the middle tail feathers of a male Anna’s hummingbird amplify the vibration of the outer feathers. The Allen’s hummer has two sets of tail feathers that generate separate notes, and also makes a trilling sound with its wing feathers. “All the feathers I’ve tested…every single feather has made a sound,” says Clark. The species-specific sound depends on how the frequencies of individual feathers blend. Neighboring feathers can be 12 decibels louder than if they were vibrating separately.
Although the feature may be unusually widespread in the hummingbird family, they’re not unique in using feather-generated noises in courtship and other social interactions. The “winnowing” of snipe is a well-known phenomenon, again involving the aerolastic fluttering of tail feathers. The mechanism was only elucidated last year: a male snipe’s outer tail feather has a weakened hinge region in the rear vane that flutters like a flag to produce the noise when the bird dives. Differences in the structure and sound of the other tail feathers have been used to support classifying the common snipe of Eurasia and the Wilson’s snipe of North America as separate species.
Other birds, including northern lapwings, American woodcocks, and common nighthawks have noise-making wing feathers. A tropical songbird called the club-winged manakin produces a trilling sound by rubbing a curved secondary wing feather against an adjacent ridged feather, not unlike a zydeco musician playing a washboard.
For some years I was convinced that the alarming sounds made by a courting male great-tailed grackle, including the part that sounds like crackling twigs, were mechanical in origin. Not so: the whole performance is vocal.
Why are such sounds attractive to female hummingbirds, snipe, and others? Clark suggests two hypotheses. The volume of the dive chirp may be a proxy for fitness, a signal that the male has good genes. (And his genes are all that count: male hummers don’t take part in nest construction or parental care.) If females selectively mate with the loudest males, classic Darwinian sexual selection would drive the evolution of feathers better adapated to produce louder chirps. Alternatively, the dive noise could be what Richard Dawkins calls a boring by-product of some other trait, coupled to it by genetic linkage. We may have to wait until someone sequences the hummingbird genome to answer that one.