Hibernation is an effective strategy for survivng 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 nighthawks, 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 ability to make metabolic adjustments, 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 for as long as 25 days. They select a shallow shelter, open to the southern sun: a patch of cactus or a 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 degrees 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 the shelter to fully bask in the final evening rays, periodically flapping its wings to elevate its 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.
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