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 2½ 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
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. Presumably, this was taken as representing an average infestation according to
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