Laura Erickson's For the Birds

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Hibernation

(Recording of a Poor-will)

Hanna-Barbera’s Yogi Bear and Walt Disney’s Humphrey to the contrary, bears don’t hibernate—at least not in the scientific sense of the word. True hibernators are relatively small warm-blooded animals that can allow their normally high metabolic rates and body temperatures to drop dramatically when the weather gets cold. In contrast, bears, with their huge size and sluggish metabolism, don’t need to hibernate. They spend the winter sleeping, but unlike a hibernator, their body temperature and metabolic rate don’t drop all that much. Chipmunks, skunks, and raccoons also spend much of the winter sleeping, but their body temperature stays fairly warm too, fueled by the layer of fat they put on in the fall. Among the very few true hibernating mammals of the Northland are the woodchuck, known on February second as the groundhog, the thirteen-lined ground squirrel, sometimes called a gopher, and the meadow jumping mouse. Bats are true hibernators that can also go into a similar state, called torpor, during cold days of other seasons when insects are inactive.

People used to believe that birds hibernated pretty much the same as small mammals. For ages people thought Barn Swallows burrowed into mud at the bottoms of ponds for the winter. During the 18th and 19th centuries, as travel and international communication allowed people to figure out where swallows go for the winter, the idea of avian hibernation became so completely discredited that ornithologists paid no notice to the first studies about dormancy in swifts and hummingbirds, published in 1917 and 1933.

Hummingbirds often go into torpor during their non-feeding hours, like bats. Swifts and birds related to Whip-poor-wills are also known to undergo torpor; and mousebirds or colies, a small order of birds found only in Africa, may possibly be another example. But these species remain in torpor normally for only a few hours at a time, and never for more than a day or two. Even after ornithologists accepted short-term torpor as a normal condition of a few species, they were certain that absolutely no birds actually hibernated.

Then on December 29, 1946, a professor named Edmund Jaeger went hiking through the Chuckwalla Mountains of southeastern California with two students and found a Poor-will, a relative of the Whip-poor-will. The bird was asleep in a small crevice in the face of the rock wall. Jaeger picked it up and noted that its feet and eyelids were cold to the touch. The bird’s respiration and heart rate were undetectable, but it was still alive. Jaeger shouted but couldn’t rouse it, so he returned the bird to its crevice and left. A day or two later he returned and banded the bird and took its temperature with a rectal thermometer—it was about 65 degrees, compared to a normal internal temperature of 106 degrees in an active Poor-will. Jaeger still could detect no heart beat or respiration, and during the winter the bird slowly and continuously lost weight, exactly like mammalian hibernators. The bird remained asleep for about 88 days, and took a week to get back to normal in spring. Jaeger found the same banded individual hibernating in the same crevice for the next several winters. Since then ornithologists have found a handful of other hibernating Poor-wills. Naturally they made a big fuss over Jaeger, though he was hardly the first one to notice this phenomenon–the Hopi Indians knew about it all along. Their word for the Poor-will is holchko, which means the “sleeping one.”

(Recording of a Poor-will)

This is Laura Erickson, and this program has been “For the Birds.”