The bear generally prepares his winter quarters in advance of the time needed. He may occasionally sleep in his den before taking possession of it for the winter. But this is exceptional. In two cases that I know of he lay outside the den, though near it; and a number of other times he kept away from the den until he entered it for the long sleep. After the den is completely ready, the grizzly continues his usual search for food. Generally this requires long excursions and he may wander miles from the den.
In climbing along the bottom of a deep, narrow ravine one November day, I saw on the slope above me what appeared to be a carload or more of freshly dumped earth. My first thought was that a prospector was at work driving a tunnel; but upon examination it proved to be a recently finished but not yet occupied hibernating-den. The entrance was about three feet in diameter. Just inside the den was a trifle larger. It extended, nearly level, about twelve feet into the mountain-side. At the back it was six feet across and four feet high.
The size of the den varies and is apparently determined by the character of the soil in which it is made and also by the inclination of the bear making it. Most other dens measured were smaller than this one.
The grizzly may use the same den for several winters or have a new one each year. He may dig the den himself or take an old one that some other bear has used, or he may make use of one shaped by Nature—a cave or a rock-slide. I knew of one grizzly hibernating in a prospector’s abandoned tunnel. Sometimes, like the black bear, he will dig a den on a steep mountain-side beneath the widely spreading roots of trees; sometimes beneath a large fallen log, close to the upturned roots which support it. In crossing the mountains one February I noticed a steamy vapor rising from a hole in the snow by the protruding roots of an overturned tree. I walked to the hole to investigate. The vapor was rank with the odor of a bear. Near my home on the slope of Long’s Peak I have known grizzlies to den up beneath the snow-crushed, matted tree-growths at the timber-line, at an altitude of about eleven thousand feet.
Twice I have known bears to hibernate in enormous nests that were made of the long fibres of cedar bark. It must have taken days to construct one of these nests, as more than forty cedar trees had been more or less disrobed to supply material for it. It resembled the nests of trash that razor-back hogs in the South construct, though much larger. The bear, after piling it up, worked his way in near the bottom, somewhat after the fashion of a boy crawling into a haycock. Over this hibernating-nest the snow spread its blanket and probably afforded all the protection needed.
Sometimes the entrance to a den is partly closed by the occupant. Once in, he reaches out and claws the lower part full of earth, or rakes in trash and leaves. In most instances nothing is done to close the entrance. The snows drift back into the den, pile upward, and at last close the entrance most effectually.
All the dens that I recall were upon northerly or easterly—the cooler—slopes. The snow as it fell would be likely to remain and close or blanket the entrance all winter long. Snow evidently enters into the grizzly’s winter plans.
Late one cold, snowless December I came upon a grizzly carrying spruce boughs into his den. Evidently he had used the den and found it cold. The den had a large opening; this he may have been intending to close. The rocky floor was already piled a foot deep with boughs. I have seen two other dens with floor-coverings; one of these was of pine twigs, and the other of coarse grass and kinnikinnick. But in most cases the bear sleeps upon the uncovered rocks or the naked earth.
Snow is a factor in determining when a bear begins his winter sleep. If he is fat and food is scarce, an early, heavy snow is pretty certain to cause him to turn in early. If no snow comes and food is still to be had, the bear is likely to delay his hibernation.
The individual inclination of the bear and his condition—whether fat or thin—are also factors which influence his time of retiring. I knew of two bears, apparently of similar condition, one of whom turned in three weeks earlier than the other. Two bears whom I noticed one winter ran about more than a month after all the other bears had disappeared. Both were thin—just why I should like to know. They also turned in shortly after they became rounded out. Generally bears of a locality turn in for winter at about the same time. Hibernating may begin early in November, but in most localities, and in most years, the time is likely to be a month later.