February, 2003

Nature’s World:  COMMON POORWILL

by Susan Tweit

     In January I traveled to Palm Desert, California, to teach a writing workshop. As I wandered the desert, I realized that I was not far from the site of a discovery more than fifty years ago that shattered our understanding of winter bird behavior. 

     On December 29, 1946, biologist Edmund C. Jaeger and his students were hiking in a narrow, rock-walled canyon when they spotted a common poorwill tucked into a shallow niche in the canyon wall, about two and a half feet above the canyon floor. The bird was “hidden” in plain sight, its speckled plumage almost exactly matching the pattern of the rock.

     Pressed tightly into its niche, the poorwill seemed asleep at first. But when Jaeger touched it, the bird didn’t respond.

       He picked the poorwill up: it was unusually light, and its feet and eyelids felt cold. As Jaeger replaced the bird, it slowly opened and closed one eye.

     It was alive and apparently hibernating—except that we knew then that birds couldn’t hibernate. Jaeger’s discovery caused considerable excitement among ornithologists.

     Despite their popular name of “common” poorwill, we actually know very little about these cryptic, insect-catching, ground-dwelling birds. Their mottled plumage camouflages poorwills so effectively that they are much more often heard than seen: their low, repetitive, “poor-will” calls are audible on spring and summer evenings.

     Common poorwills, along with nighthawks and whip-poor-wills, are in the goatsucker family, so named for the erring belief that these low-flying birds with the wide mouths stole milk from goats at night.

     In reality, a common poorwill hunts for flying insects at dusk and daybreak, and also on moonlit nights, watching for moths, chinch bugs, beetles, grasshoppers, and locusts from a perch on the ground, a low tree limb, or on a rock.

     A special structure inside its eyes improves a poorwill’s night vision by reflecting light back to the retina. When it spots prey, the poorwill takes to the air with its mouth open wide to scoop up its meal.

     Bristlelike feathers around its bill function like cat’s whiskers, amplifying the sensation of touch to help the bird zero in and capture its flying prey. Fluttering not far above the ground, a hunting poorwill looks like a giant moth or a long-winged bat.

     Common poorwills inhabit open, arid country throughout western North America. In Colorado, they are most abundant in summer in oak scrub, piñon pine and juniper woodlands, and ponderosa pine forests.

     The poorwill discovered by Edmund Jaeger and his students actually was hibernating, and furthermore, it returned to its niche in the rock wall each winter for three years. Jaeger and other researchers returned as well to study the bird.

     The hibernating poorwill “slept” from December through early March, apparently unaware of disturbances. Researchers banded the bird and measured its internal temperature every two weeks: its average temperature was 64.4 degrees, 42 degrees below normal poorwill body temperature.

     Its heartbeat was so slow that it was difficult to detect, and its breathing was negligible. But the bird’s metabolism was working, albeit extremely sluggishly, since the poorwill gradually lost weight over the course of each winter.

     Other bird species lower their metabolic thermostats for short times. Hummingbirds often drop into torpor overnight to reduce their food needs, white-throated swift nestlings “sleep” during inclement weather, and red-tailed hawks slow their metabolisms when food is scarce.

     Poorwills, however, not only drop their body temperature lower than any other bird known, they do it for much longer periods. Called hölchko, “the sleeping one,” by the Hopi, common poorwills regularly spend whole winters in a near-death state.

     Since Jaeger’s discovery, just a few other poorwills have been found while hibernating, and ornithologists assumed that these little-understood birds winter only in the relatively warm climates of the southern Southwest and northern Mexico.

     In January of 1975, however, a common poorwill was found in northern Colorado. Researchers think the bird “woke up” from hibernation during a warm spell, then, unable to find insect food, died when winter weather returned.

     Do common poorwills hibernate in Colorado? We don’t know. But I’m going to keep my eyes peeled for sleeping birds when I’m out hiking in rocky canyons in the hills.

© 2003 Susan J. Tweit

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  www.salidamillwork.com/sjtweit