Behold the common poorwill, the only bird on Earth known to hibernate! My first acquaintance with this fascinating creature happened in the late ‘80s. At the time, I was living in an old log cabin nestled into the stony folds of Montana’s Rocky Mountain Front. There, I heard an unmistakable signature call lilting out of dusk-wrapped foothills on warm summer nights, a plaintive two-note call that to some, sounds like “poor – will.”
Summer after summer, I heard their voices – sometimes I would glimpse one in flight. Much about poorwills remained a mystery to me, until one fateful evening. As I was driving up a dirt road, a small bird abruptly rose up in front of my pickup truck. The resulting collision was unavoidable. I stopped immediately, dismayed to find an inanimate lump of feathers caught in the grill. Gently, I extricated the bird and gazed at its limp form in wonder; cupped in one hand, its warm feathers felt gossamer-soft. My attention was immediately drawn to its head, quite large relative to the rest of the body. Two huge, liquid eyes characterized the bird’s visage. What looked like a ridiculously tiny beak barely poked out from a covering of feathers. The area around the beak (and the rest of the bird’s hidden mouth) was festooned with a fringe of curved, jet black, semi-rigid bristles.
Now it gets even weirder. I had read up on these birds, so I knew that they were grouped in a strange family known by several names: frogmouths, nightjars, or goatsuckers. (Yes, goatsuckers, but that is another story altogether.) Curious, (as any naturalist should be) I pried the beak apart. My jaw fell in synchrony with the yawning beak, for the beak did not merely open; the bird’s head nearly split in two! The hidden edges of the beak extended far back to form an absolute cave of a mouth, a great, gaping maw that could make any shark jealous. Why such a mega-mouth, and what was with those bizarre bristles?
Poorwills are highly specialized to feed on flying insects in low-light conditions. Having a wide gullet for a mouth makes it much easier to catch insects on the wing. Their heads are unusually large because plenty of room is needed to house the mouth and a pair of very large eye orbits. Big eyes gather more light, handy for spotting prey in the dark.
Some ornithologists speculate that the stiff bristles form a sort of tactile net around the mouth, effectively enlarging the bird’s open mouth. This increases a poorwill’s chances of success when chasing its evasive flying food. During the heat of summer, these desert-dwelling birds reveal another advantage enabled by their gigantic oral cavities. As they roost during hot days, poorwills open their mouths while fluttering the muscles in their throats, cooling themselves in a manner similar to a panting dog. The efficiency of such a cooling method is substantially amplified due to the large surface area of the bird’s mouth.
So what about poorwills and hibernation? Here, in the Sonoran and Chihuahuan Deserts, poorwills crawl into rock crevices where they go into a state of hibernation or extended torpor (these terms remain somewhat ambiguous) for months during the winter. Their respiratory rate and heartbeat drops substantially, while their body temperature plummets to nearly 40° F.! An astute scientist is credited with “discovering” that poorwills hibernate as he conducted research on them in 1948. However, indigenous peoples knew this long before 1948. For example, the Hopi Indian word for poorwill is (roughly) hölchoko, translated as “the sleeping one.” Thus, much like Columbus, the 1948 discovery simply revealed knowledge to a different culture, knowledge that was already known to another. (With respect to Christopher Columbus, it is impossible to “discover” a place where people already live. A combination of cultural condescension and arrogance can imprint our history books with less than the truth.)
There is much more about poorwills that makes them such captivating, beguiling birds. Their camouflage is utterly incredible. Poorwills roost on the ground where they literally merge with their surroundings. The cryptic patterning of their plumage renders them extremely difficult to spot. On more than one occasion, I have nearly stepped on them before they revealed themselves through movement. The genus name for poorwills is Phalaenoptilus, ancient Greek for “moth plumage.” (Our birds are a very unique species, for no other bird on the planet shares their genus epithet.) The scientific name Phalaenoptilus is fitting in two ways, for many moths are extremely well camouflaged, and moths make up a large part of a poorwill’s diet.
Like other members of their family, poorwills possess a special layer of cells at the back of their huge eyes known as the tapetum lucidum, rarely found in other birds. These cells allow them to see much better in low-light conditions. As for the glowing red eye in the photographs, that results from light reflected from the choroid, a thin layer of connective tissue surrounding the outer periphery of the eye. The choroid is nourished by a rich supply of capillary blood vessels, hence the red color. (The “red eye effect” sometimes seen in photographs of people is caused by similar choroid reflectivity.)
Weird enough? Not quite. On the Montana evening when I held the unfortunate poorwill in my hand, I had missed something: the bird’s feet. Poorwills possess pectinated claws (one per foot) with comb-like, serrated ridges that are used to preen plumage and to rearrange the rictal bristles. There is also the fact that these birds do not build nests – they simply lay their whitish, non-camouflaged eggs on the open ground. How they manage to keep their species alive via such seemingly risky egg-laying habits may seem enigmatic, but it works, for there are poorwills year after year. Here in the uplands and mesquite bosques of the San Pedro River Valley, their lonely calls are an intrinsic part of the desert’s twilight world.
Finally, a note on the poorwill images: they were photographed recently here in Cochise County, AZ, then generously supplied by a friend for use in this blog. Unless otherwise noted, all other photographs posted on this site are original copyrighted images by the author.