658 



THE MONTHLY BULLETIN. 



the kinds of plants eaten, from stomach examination, proved impractica- 

 ble 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 

 Avere 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 above- 

 ground 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 dis- 

 appeared 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 conse- 

 quent supply of green food. The drier the year the earlier the squirrels 

 go into winter quarters, this in spite of the hotter late-summer tempera- 

 ture 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 Moun- 

 tains, 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 



fi6 



