Rigor Vitae: Life Unyielding

Friday, June 05, 2015


On May 30, 2015 I was hiking in the Wasatch Mountains in Salt Lake County, Utah, just after dark at a elevation of about 7,000 feet, when I observed two Poorwills (Phalaenoptilus nuttallii) perched about half a meter apart on the ground. I watched the two birds in my headlamp beam through binoculars from a distance of about 10 meters. After some 30 seconds, bird B fluttered into the air briefly and landed, increasing its distance from bird A by about two meters. Ten or fifteen seconds later, bird A flew towards bird B, restoring the distance between the birds to roughly what it was before. About 45 seconds later, bird B flew away. some 10 seconds later, bird A also took off in the same direction. It wasn't until the next morning, when I looked at the photographs I took of the birds, that I was able to tell that both birds were males. In the photographs where both birds were perched close together, they were both clearly flaring their white throat patches and displaying them. Bird B also appeared to be spreading its tail, displaying the white patches there. Bird A's tail was obscured in these photos. I inferred this interaction to be a territorial rivalry, but the actual nature of the display is not clear. I was unable to discern any other behavior such as head-bobbing, but viewing conditions were far from ideal.  

I have not seen nor heard of Poorwills displaying in this manner. I sent this photograph to R. Mark Brigham, a Poorwill expert at the University of Regina, who told me he had never seen Poorwills do this either.
These birds are very interesting for a number of reasons, most famously for their manner of surviving winter's cold temperatures and lack of food resources. Hibernation is a common way for animals to deal with winter. Many birds, bats and even insects opt instead for seasonal migration, exploiting distant habitats during different seasons. A few, like the Monarch Butterfly (Danaus plexippus) and our friend the Poorwill use a combination of the two. Poorwills, relatives of Nightjars, breed in arid parts of western North America from southern Canada into northern Mexico; northern individuals seem to winter in the desert southwest. A number of bird species use daily torpor to minimize energy loss during cool nights or brief bad weather. Members of three related orders, the goatsuckers, hummingbirds and possibly the swifts, all show some abilities at metabolic adjustment, but none to the degree of the little Poorwill, which, in addition to its natural tendencies toward torpor, feeds heavily on beetles, rich in polyunsaturated fats, which remain liquid and metabolically 
available at low temperatures.I've watched Poorwills feed on darkling beetles on the ground, and have noticed that medium-sized ground beetles are usually common where there is a dense population of Poorwills.In the laboratory, these birds have been observed sustaining periods of torpor for over 80 days, and in the wild as long as 25 days. A shallow shelter, open to the southern sun is selected: a patch of cactus or rock niche to which the bird develops substantial fidelity. After sundown, the torpid Poorwill's body temperature begins to fall, until the ambient temperature reaches 5.5°C, an apparent optimum hibernating level which the bird tries to maintain. Solar radiation raises the body temperature daily, presumably allowing the option to forage during warm nights. I know of no human witnesses to a Poorwill rousing from torpor in the wild, but I imagine the bird backing out of his shelter to fully bask in the final evening rays, periodically flapping his wings to elevate his body temperature. It's not known how severe a winter these birds can survive, but a sufficient winter insect population, rather than temperature, is probably the limiting factor.

(top): Photo by cpbvk
(center): Poorwill field sketch; pencil 11" x 8.5"
(lower): REANIMATION: POORWILL (2012) acrylic 30" x 20"