Page:Natural History of the Ground Squirrels of California.djvu/74

658 the kinds of plants eaten, from stomach examination, proved impracticable because the food is chewed so finely. Even young less than a third grown were feeding freely on green stuff. Six young taken on May 16 were found to weigh from 65.1 to 104.7 grams, averaging 82.4 grams as compared with 302 grams, the average weight of adults. Their stomachs were distended with finely cut food, and were found to weigh on an average 5.4 grams, or about one-fifteenth their total weight as against the one-to-twenty ratio in adults. It would seem that partly grown young eat more in proportion to their size than old squirrels—which was rather to be expected.

There seem to be two periods of maximum daily activity aboveground on clear days, about 9 a.m. and again in mid-afternoon. This squirrel seems to be preeminently a sunshine forager. One day when a thunderstorm came up in the afternoon the squirrels nearly all disappeared from aboveground coincidently with the gathering of the clouds.

The breeding season of the Oregon Ground Squirrel, as is to be expected, varies with altitude, or, rather, with life-zone. The young are born later in the Transition and Boreal zones, than in the Upper Sonoran. On May 15 scores of adults were seen on Bull Meadows, near Goose Nest Mountain, 5,000 feet altitude, in the Canadian zone, but not one youngster was seen; while on May 16 everywhere in Butte Valley around Macdoel, at 3,000 feet, in the Upper Sonoran zone, young were out in great numbers. All of these were of about the same size, one-fourth to one-third grown, showing the uniformity of time of birth throughout a region of uniform temperature conditions. On May 19, 1910, a collecting party from the California Museum of Vertebrate Zoology found small young, just out, on Sugar Hill, 5,000 feet altitude, Modoc County.

There is but one litter a year, and the number to a litter is supposed to vary from 4 to 15, averaging about 8. Exact statistics from which to determine these figures accurately are not available. A man who was irrigating an alfalfa field near Macdoel regularly day after day told us that he had drowned out many families of young and that the broods he had seen consisted of from 4 to 11, averaging, he thought, 8. Mr. W. C. Jacobsen, from his own extensive experience with this species, considers 4 and 12 to be extremes, and 8 the average. He knows, indirectly, of one case of 15.

The Oregon Ground Squirrels lie dormant in a dry nest beneath the surface of the ground for fully half the year, even at the lowest altitudes in the general territory inhabited by them. The bulk of the population goes into hibernation during July and does not come out until March. These statements are made upon the authority of Mr. W. C. Jacobsen, who is further of the opinion that the exact time of disappearance, which varies somewhat from year to year, is controlled by moisture and consequent supply of green food. The drier the year the earlier the squirrels go into winter quarters, this in spite of the hotter late-summer temperature at the lower altitudes. In 1915, the squirrels in Big Valley, Lassen County, had nearly all gone in by July 3; in Warm Springs Valley, Modoc County, they had gone in by July 10; but on the Warner Mountains, Modoc County, they were just going in on July 22 of the same year. In 1914, a year of more moisture and better feed, the time of