Home & Garden Columns
It may be cold outside, but it’s already spring to the Anna’s hummingbird, and courtship and nesting are well under way.
Last week two hummers, a male and a female, got into our living room, were trapped inside when a gust of wind blew the front door shut, and became entangled in the curtains. Matt the Cat spotted them (he doesn’t so much hunt things as point), and Ron, who moves faster than I do in these situations, retrieved them and released them on the front porch, apparently none the worse for wear.
I didn’t refer to this distracted twosome as a pair, because hummingbirds don’t form pairs. Mating is promiscuous, and males don’t involve themselves in the tedious business of nest construction and childcare.
You may have noticed the dive display of the male Anna’s. As described in the authoritative Birds of North America series: “The male sings 1–2 sets of buzzy notes while hovering 2–4 meters over the object of the display for 1–2 second, then climbs in a wavering fashion nearly vertically for 7–8 seconds to a height of 20–40 meters, plummets in a near-vertical dive for 2 seconds, ending the dive with a loud Dive Noise within 0.5–1 meter of object, finally returning in a continuous circular arc … to the beginning point over the object ... The object may be a female Anna’s Hummingbird, another hummingbird, another bird species, or occasionally a human; the sight of any perched hummingbird in its core area may initiate a Dive Display.”
The exact nature of the Dive Noise, or dive chirp, has been much debated among ornithologists. The late Luis Baptista, former curator of birds at the California Academy of Sciences, thought it was vocal, since the frequency of the chirp was similar to that of the hummer’s vocalization. (It was Baptista who established that Anna’s hummingbirds, like more conventional songbirds such as the white-crowned sparrow, have local song dialects.)
Recent work by UC Berkeley graduate student Christopher Clark and recent graduate Teresa Feo makes a compelling case that the noise is mechanical in origin, created by specialized tail feathers.
Their investigations involved a high-speed video camera and a wind tunnel.
Clark and Feo took the camera, with a 500 shot-per-second capability, to the Albany Bulb, where they alternately wired a stuffed female hummingbird to a bush or staked out a live female in a cage. Males responded to both variants. The camera captured a 60-millisecond spreading of the displaying male’s tail feathers at the bottom of the dive, coincident with the chirp.
Having observed that Anna’s hummingbirds had, as Clark puts it, “funny tail feathers with tapered or narrow tips,” the researchers then captured several male hummers and customized their tails—either removing the outer pair of tail feathers or trimming their inner vanes. Modified males still performed dive displays, but failed to produce the dive noise.
The final piece fell into place at Stanford’s Hopkins Marine Station in Pacific Grove, where Clark and Feo exposed outer tail feathers to wind speeds equivalent to a male’s dive speed of 50 miles per hour. The chirp was reproduced in the wind tunnel when the inner vane of the feather fluttered at a frequency of 3.3 to 4.7 kiloherz, four octaves above middle C. Tiny linking barbules kept the barbs of the inner vane stiff enough to vibrate like the reed in a clarinet.
The dive chirp is actually louder than the hummer’s vocalization. Clark and Feo suggest that this may be an evolutionary response to the constraints posed by the small size of the bird’s syrinx, or song box (the avian equivalent of the larynx). They suspect that close relatives of the Anna’s hummingbird, like the desert-dwelling Costa’s, may produce their chirps in a similar fashion.
Mechanical sound production in birds is unusual, but not unknown.
The “winnowing” noise of the Wilson’s snipe is apparently produced by its tail feathers, although no one has worked out the mechanism. Common nighthawks make a rude sound with their wing feathers, and I’m convinced that the bizarre rustling-grating-creaking sounds emitted by a displaying male great-tailed grackle can’t be entirely vocal. The club-winged manakin of Ecuador has specialized wing feathers that operate like a zydeco musician’s rubboard: a scraper feather hitting the ridged vane of another feather.
It’s good to have the riddle of the dive chirp resolved—and to be reminded that there’s still much to be discovered about even the most familiar of birds.