Page:Natural History of the Ground Squirrels of California.djvu/49

Rh uncultivated land where they are least molested through human agency, and from this they spread out and invade nearby cultivated fields. The process is most conspicuously in evidence during late summer consequent upon the emigration of the young of the year, this being in compensation for the tendency to congestion of population brought on during the breeding season.

Reduction in the food supply locally causes the squirrels to spread out in search of new pastures. Such movements are usually less than a mile in extent, and of course come particularly to notice in the vicinity of grain fields and orchards to which the squirrels drift at the time the crops begin to ripen. Some idea of the rate with which ground squirrels reinfest cultivated fields which are adjacent to wild land may be had from the following instance. Mr. O. N. Garrison of Earlimart, Tulare County, stated in an interview that during the spring of 1918 thirty-six ground squirrels were drowned out on a five-acre field of alfalfa at the first irrigation and this in spite of the fact that the field had been free from squirrels at the end of the previous irrigation season in the fall of 1917.

From the earliest times of which we have record to the present day the California Ground Squirrels have given the impression of abundance. Changes in the status of the species within history have only concerned local occurrence. There is nothing to show that there has been any extension of the general range of the species, or any retraction in it either. As already set forth, the arrival of the white man and the institution of agriculture has undoubtedly had the effect locally of increasing the ground squirrel population. On the other hand, where man has been aroused by the seriousness of their depredations to the point of adopting and putting into force effective means of control the numbers of the squirrels have been conspicuously reduced. Thus at Earlimart on May 16, 1918, ground squirrel burrows were found to be abundant over a large acreage of "hog wallow" land. Live squirrels, however, were exceedingly scarce, only five being found on one tract of forty acres which had been thoroughly poisoned the previous season. A count taken on this tract showed that there was an average of fifty empty burrows to each squirrel present.

A very few localities have been reported in which the squirrels are, for the time being at least, things of the past; but the possibility of re-invasion presents itself, and this, as already shown, may be a very rapid process. It would seem that ground squirrels, like weeds or scale bugs, will have to be watched continually, and proper measures taken whenever necessary to prevent the reinfestation of land which is thought to have been freed.

The difficulties in arriving at a fair estimate of the damage done by the California Ground Squirrel, which is by far the most injurious species in the state, are many and various. We have tried to get at a satisfactory estimate (not a guess) in terms of dollars per annum, but have not succeeded. It may be of some interest, however, to give some other figures, indicative in partial degree of the loss that may be occasioned by this ground squirrel.

In order to ascertain the bearing of squirrels upon grazing interests we have found some basis for estimating squirrels in terms of livestock. We have weighed and examined the stomach contents of a series