Torpor and Hibernation

Common Poorwill

Phalaenoptilus nuttallii

          A century ago, in a book described as a romantic view of bird life, the author claimed that although many birds migrate,” is it not possible that the corncrake, cuckoo, and swallow hibernate?” There were many reports of swallows diving into mud and corncrakes being found in mud walls. The cuckoo was said to ensconce itself and lie dormant in a hollow log full of feathers and if disturbed, cry out “cuckoo, cuckoo.” A cuckoo was reported to fly out of a log tossed into a burning hearth. Nothing as spectacular as mud hibernation occurs in the bird world, but there are at least 29 species of birds that demonstrate some sort of lowering of their metabolism and body temperature for periods of time. This state of decreased physiological activity is called torpor. Hibernation is simply a much deeper form of torpor and apparently only one bird species’ behavior approaches a form of hibernation, the Poorwill.

     The major reasons for entering torpor are limitations in the food supply and cold weather. Virtually all the birds that enter torpor are insectivorous, frugivorous, or nectar-feeders, such as hummingbirds, mousebirds, and members of the family caprimulgidae (poor-wills, nightjars, etc.) The mainly insectivorous female Puerto Rican Tody may exhibit torpor even with abundant food and tropical conditions, perhaps due to the stresses of breeding, and can lower her body temperature by 250F (140C) although she remains awake and alert.

            The normal daytime body temperature of hummingbirds is about 100-1040 F (38-400C). A study of 17 species of hummingbirds found that during nighttime torpor, their body temperatures would decrease to the ambient temperature but not below 64-680 F (18-200C), although another study reported a Rufous Hummingbird exhibiting torpor with a body temperature of 550 F (130 C).  In a laboratory study of Red-backed Mousebirds, researchers found that if the birds were deprived of food until they lost 35 percent of their body weight, they would enter torpor but did not allow their body temperature to drop below 640 F (180C); their normal body temperature was 1070 F (420C) during the day and 1000 F (380C) at night.

A couple of hours before dawn, with no apparent stimulus except its innate circadian rhythm, a hummingbird will begin to arise.. Respiratory and heart rates increase, the bird vibrates its wing muscles, shivers, and after 20 minutes to an hour or so the bird is ready to begin feeding. Torpor has arisen over evolutionary time as a sort of emergency measure for many birds facing low temperatures and/or temporary food shortages and for a few birds as a more usual way of saving energy and making it through the night. There is some thought that torpor might be used during migration to reduce the amount of foraging necessary enroute. There are still lots of unanswered questions.

            At least one bird goes into such a deep state of torpor it resembles hibernation. The Poorwill was called “holchko”, the sleeping one, by the Hopi Indians. Lewis and Clark may have found one of these birds in a state of torpor in 1804 although it appears that it may have been much later, in 1879 California that a sighting was confirmed. The Poorwill is a robin-sized insectivorous bird inhabiting the American southwest; all of its relatives- nightjars, potoos, and nighthawks, enter torpor, but the Poorwill is the only one to do so for extended periods. Torpor is used extensively by Poorwills when the ambient temperature is below 500F  (100C). They typically roost under a cactus or next to a rock, while facing a southerly direction. If they are warmed by sunlight they might awake and spend the day foraging but then return to torpor at night. Some Poorwills spend 10-20 days in a state of torpor, with no stirring at all. This behavior is very much like that of hibernating small mammals that awake occasionally to snack on their stored food cache. There has been speculation for many years that some swifts and swallows hibernate but most likely they are utilizing a moderate level of torpor.