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On the five mornings during the preceding eighty-five days, Jaeger had measured the bird’s internal temperature by inserting a thermo-probe into its cloaca. All his readings showed the bird’s internal temperature hovered near that of the air temperature, as is usually the case when an animal is dead. He could detect no heartbeat with an aid of a medical stethoscope and saw no breathing movements of the chest. No moisture collected on a cold mirror held in front of the bird’s nostrils. A flashlight shined a full minute into the bird’s right eye (which was almost completely open) failed to elicit any response whatsoever. Jaeger concluded: “I take it as evidence that the bird was in an exceedingly low state of metabolism, akin, if not actually identical with hibernation, as seen in mammals” (Jaeger 1949, p. 106).

To determine whether the bird returned every winter to the same spot, Jaeger banded it with a U.S. Fish and Wildlife Service aluminum cuff. To his great satisfaction, the bird returned to its hibernaculum on the granite cliff in the winter of 1948–1949. It survived a severe November storm of sleet and hail that left a layer of ice on the ground for a day afterward and air temperatures that remained near 0°C.

Given what the Hopi and Navajo knew already, we can’t say without qualification that Jaeger was making a discovery. But his reports surprised physiological ecologists perhaps as much as if he had verified the old fable that swallows hibernate in the mud. Jaeger’s two papers begat a flurry of lab studies: fifteen laboratory studies of the poorwill and related species have since appeared in the scientific literature. These reports extend, and perhaps require some reinterpretation (but not much) of, Jaeger’s original paper. They confirmed that the poorwill’s body temperature in torpor indeed becomes essentially identical with air temperature (Howell and Bartholomew 1959). The torpid birds can spontaneously arouse from body and air temperatures as low as 6°C, although when doing so the process can take several hours (Ligon 1970). At such low air temperatures, however, the physiological capability of arousing is seldom used (Howell and Bartholomew 1959), presumably because it costs the bird too much time and energy (and probably also because it brings them little since there are then no flying insects to catch).

The poorwill, although unable to arouse from temperatures near 0°C, can nevertheless survive such low body or air temperatures (Ligon 1970). Thus, there is no problem explaining the survival of the particular bird Jaeger observed in an ice storm at temperatures at or below 0°C. But I doubt that his birds spent eighty-five days in continuous torpor. Captive poorwills regularly enter torpor at night, but don’t remain in that state for more than four days at a time (Marshall 1955). Since Jaeger always measured body temperature of his bird between 10:20 A.M. and 11:30 A.M., it seems not unlikely to me that his poorwill could have warmed up to forage on some warm nights, to then return to its same perch and resume torpor by the next morning. Other caprimulgids, like the nighthawk, which is migratory, are much more reluctant to become torpid at night (Lasiewski and Dawson 1964), probably because they escape cold weather instead.

As would be revealed in the avalanche of studies following Jaeger, such patterns as those in the poorwill are only a slight modification on those later documented in innumerable other birds. The main feature that makes the poorwill remarkable is that it sometimes stays torpid for days at a time without rewarming, thereby showing that physiologically there is no distinction between hibernation and daily torpor.

THE SMALLER THE SIZE, the greater the physical tendency for rapid cooling and the greater the energetic advantage to staying cool. A turkey-sized animal is not likely to let itself become cold enough for torpor at night, but the smallest birds and mammals, weighing as little as about 3 grams, including hummingbirds, some bats, and some