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acrylic on illustration board 30" x 20"
is one of the more effective strategies temperate animals have
developed to survive winter's cold temperatures and lack of food
resources. 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 the Poorwill (Phalaenoptilus nuttallii) 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.
Like a number of other bird species the Poorwill uses 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. In the laboratory,
Poorwills 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. Rather than showing the kind of country where Poorwills are known to commonly winter, I tried in this painting to depict a habitat in the harshest extreme that I could imagine the bird toughing out. Also shown reviving are Glacier Lilies (Erythronium grandiflora), Convergent Ladybird Beetles (Hippodamia convergens), Western Boxelder Bug (Boisea rubrolineata), and snowfleas (Hypogastrura sp.), cold-adapted springtails that climb onto the snow's surface to feed on algae.