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Not until 1948 did the scientific world have satisfactory evidence that
any bird hibernated. True, it was an established fact that sometimes in
cold weather some birds, notably swifts and hummingbirds, might become
torpid for a short time, but this was not hibernation.
The early literature, of more than a century ago, contained many
accounts, some claiming to be firsthand, of birds hibernating. Swallows
in particular were reported as seen to submerge in ponds in the autumn.
Numbers of them were said to have been found hanging to submerged
willow branches apparently sleeping the winter away. When ponds were
drained in winter, sometimes swallows were said to have been found
buried in the mud, revived, and upon occasion kept alive indoors
until the spring. Sometimes slime-covered swallows, evidently just
out of hibernation, were reported found in the spring. Swallows were
the most commonly recorded, but other species, too, were mentioned as
hibernating, such as the cuckoo that shed its feathers and crept into a
crevice to sleep away the winter.
Such accounts gradually disappeared from the literature. We can accept
none of them. The old records of underwater and also the featherless
hibernation of birds must be discarded. The occasional torpidity, in
cold weather, of swallows, swifts, and hummingbirds is another matter,
and appears to be of sporadic though not common occurrence.
FROGS MISTAKEN FOR BIRDS It is interesting to speculate as to how
the old "firsthand" accounts originated. They had certain basis of
fact. The first was that swallows were seen flying about in summer.
They disappeared in winter. Aristotle claimed they hibernated, in a
featherless condition, so there was nothing unusual in seeing them
that way. Observation was less critical, and it is probable that frogs
from the mud of ponds were mistaken for naked swallows, and perhaps
bats, which do hibernate, taken from caves or hollow trees, were also
mistaken for swallows.
AN AUTHENTIC RECORD In 1948, and again in 1949, Edmund C. Jaeger, of
California, published accounts of a poor-will he found hibernating.
This was the first acceptable evidence that such a thing occurs. In a
little cavity in the wall of a canyon in the Chuckawalla Mountains of
the Colorado desert in California, Jaeger found a poor-will in a state
of profound torpidity in December, 1946. He could pick out the bird in
his hand, examine it and put it back in the little cavity it occupied
without eliciting more than a slight movement of its eyelids. On a
later occasion handling it revived it somewhat.
The next winter Jaeger found a poor-will, perhaps the same bird,
hibernating in the same niche. Over a period of eighty days, from
November 26, 1947, to February 14, 1948, he visited it periodically,
examined it, and took its temperature. The body temperature was low,
64°-68° F., compared with more than 100° F. of an active bird; with
a medical stethoscope he could detect no heartbeat, and a cold metal
mirror held directly in front of its nostrils collected no moisture
from its breathing. The body processes were evidently very low. The
bird was banded for identification, and in the third winter the same
bird wearing the same band was found to have returned to hibernate
again in the same rock niche. But on subsequent visits it was
missing--perhaps having lived out its allotted span, perhaps the prey
of some predator.