THE GROUND SQUIRRELS OF CALIFORNIA. 633 



were counted in this area, so that the infestation was at the rate of 20 

 squirrels and 50 holes per acre. This is at the rate of 2^ burrows to 

 each squirrel. 



On May 28, 1918, a single isolated colony was investigated at a point 

 twelve miles west of Fresno. This colony was in a plowed field which 

 had been planted to grain for several years past. Here, in an area 100 

 feet square 16 squirrels, eight of which were less than half grown, and 

 17 burrows were counted. The ratio here was close to one burrow to 

 each squirrel. This figure, again, applies to a period after the close 



1^ 



^^ 



. ^.^ 







-^^^M 





m^^ ^ - ^f~iss 





^^H^^^^^^^^^^^B^K^^ '■sA^^S^,'^^^-'' ^ 



s^lbi^..::-. mm 







Fig. 16. Hillside near Walnut Creek, Contra Costa County, badly infested with 

 "digger" squirrels. On August 15, 1918, squirrels were here present at the rate of 

 fifty per acre. From this breeding ground they were at this time invading the grain 

 fields on the opposite slope. 



of the breeding season, when the squirrel population had reached its 

 maximum. 



Counts taken before the breeding season naturally give different 

 results. At Berkeley on March 13, 1918, the junior author counted 47 

 squirrel burrows in a colony which occupied about one acre on a hillside. 

 By counting the squirrels which appeared aboveground in this area on 

 several successive days it was ascertained that there were about nine 

 adult squirrels inhabiting this acre of ground. This gave an average 

 of over five burrows to each squirrel. 



The above-cited instances are based on maximum infestation. Local 

 distribution is often very irregular, since squirrels may be abundant on 

 the southern exposure of a hill and yet be entirely absent on brushy 

 northern slopes only a hundred yards or so distant. Even on the plains 

 and in the valleys, although the distribution is much more uniform, 

 there is often marked unevenness in infestation irrespective of human 

 interference. Surgeon John D. Long (1912, p. 1596), in comparing 

 the cost of the various methods of destroying ground squirrels, based 

 his estimates of cost on an infestation of ten holes per acre. Presum- 

 ably, this was taken as representing an average infestation according to 



