632 THE MONTHLY BULLETIN. 



scarce aboveground in January as compared with the younger animals, 

 those probably less than one year old. Further, the proportions of old 

 to young increases rapidly towards the last of that month; in other 

 words, as the breeding season approaches. It is probable that the full 

 old-adult population is not abroad aboveground until the last of Feb- 

 ruary, when the ratio of old adults to young of the previous year would 

 certainly not be nearly so little as 1 to 4, which is the mi Qimum possible 

 at the immediate close of the breeding season. 



In spite of the above lines of evidence, the real extent of this habit of 

 aestivation among our ground squirrels is not satisfactorily known. It 

 is exceedingly difficult to follow any individual squirrel under perfectly 

 normal conditions through all its various activities for any great length 

 of time. However, an important factor concerned in the work _ of 

 destroying these animals is suggested ; that is, the des irability of placing 

 emphasis upon the need of poisoning in the spring rather than in the 

 fall, when part of the breeding stock may be stowed away out of the 

 reach of poisoned grain. It is a question, too, whether or not a dormant 

 animal, in which respiration is extremely slow, would be fatally injured 

 by a fumigant before the latter would be dissipated. 



Human interest in the California Ground Squirrel nat arally concerns 

 itself most especially with the questions of total population, rate of 

 increase, and rate of re-invasion of territory previously cleaned of squir- 

 rels. As to the first question, we have found it difficult to find an 

 accurate basis for determining the squirrel population living on any 

 given imit of area, such as an acre or a square mile. Counts may be 

 taken of living squirrels that happen to be aboveground at any one time, 

 or of burrows which give evidence of current use. In the first ease 

 the count is never likely to cover all of the squirrels i n tlie area, because 

 the chances are overwhelmingly against all of the squirrels being above- 

 ground at one time. Season of the year, time of day, and state of 

 weather will affect profoundly this proportion of squirrels below ground 

 to those in sight. In the latter case some sort of estimate of ratio of 

 squirrels to burrows must have been arrived at. Season of the year 

 must again figure importantly in the estimate, because of the jump in 

 population following the breeding season, and progressive decrease 

 thereafter. 



On July 26, 1918, the authors took two censuses of ground squirrels 

 on a badly infested ranch about three miles northwes t of Walnut Creek, 

 Contra Costa County (see fig. 16). The first census was taken on a 

 south-facing hillside on an area 100 feet square, approximately one- 

 fourth of an acre. Three counts gave 19, 16 and 17 squirrels, respec- 

 tively, in sight at once. Twenty-five open burrows were counted in 

 this area. This, therefore, was at the rate of 76 squirrels and 100 

 burrows per acre, or 1| burrows to each squirrel. The breeding season 

 at that date was well passed. Allowing one adult to every four young 

 gave fifteen adults to every 100 burrows or between six and seven open 

 burrows to each adult squirrel after the breeding season. 



The second census was taken in a mowed field from which a crop of 

 barley hay had been recently harvested. The area taken was on a north 

 slope and measured 250 feet square, covering about 1^ acre. Three 

 counts were taken between noon and one o'clock. The number of squir- 

 rels seen out at once was 25, 26 and 25, respectively. Sixty burrows 



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