Page:Natural History of the Ground Squirrels of California.djvu/127

Rh 7. The food preferences of ground squirrels are strongly in evidence and vary from species to species, and sometimes within the same species, from place to place and season to season. It is common testimony of those who have practical experience in poisoning ground squirrels that the Douglas is much more easily handled than the California; in other words, the former takes the strychnine-coated barley more readily. It is obvious that the success of any method of control by the use of poison must depend importantly on the nature of the bait employed. The fact that in some places the California Ground Squirrel has been found to pass up barley altogether for the seeds of bur clover suggests a likely way of improving poisoning methods locally.

8. In the "digger" category of ground squirrels there is evidence that a greater or less proportion of the population hibernates each winter. In the Douglas this feature of the annual activity of the animal is clearly evident, in that the majority, or at the higher altitudes all, of the individuals disappear for weeks or months together during the winter season. In the case of the California Ground Squirrel, however, numerous individuals are to be seen aboveground in the lower country in favorable weather at any time during the winter. But evidence at hand goes to show that these active individuals are chiefly young of the year and that most of the older scpirrels are then lying dormant belowground, in some extreme cases for as long a period as from August to February. During this interval, therefore, any method of poisoning, and probably also of gassing, will obviously be ineffective upon a portion of the population, and this portion which escapes will reappear at the beginning of the next breeding season to reinfest the area concerned.

9. Some obstacles to the success of control by the method of gassing arise through the unequal extent and irregular course of the burrows of the squirrels. It was found that although the volumetric content of the burrows of the California Ground Squirrel excavated averaged 5.2 cubic feet, in one case an extreme of 17.8 cubic feet was reached. This obtained in one of the "colonial" types of burrows in which several establishments supposed to have been originally separate had come to be intercommunicating. It was found that the usual dosage was ineffectual in this case. There is no definite way of distinguishing such "colonial" burrows, from surface appearances alone. Then, again, in some burrows there is an abrupt rise in the underground course of the burrow, which prevents the onward flow of a gas heavier than air, such as carbon bisulphid, and the squirrel is not overtaken. In either of the above circumstances we find a reason for the partial failure of extensive gassing campaigns with current methods.

10. Ground squirrels reproduce rapidly. In the California the average number of young in a litter is 7.2, with 4 and 11 as extremes. There is but one litter reared each year, and the young begin to appear aboveground about the first of May. The sexes are equally divided in a given population, and it is believed that each female breeds the first season of her life, that is, when she is slightly less than a year old, and that she has an "expectation" of rearing four more litters in case she lives to die of old age. Thus a population of 10 per acre in March may be expected to increase to 50 per acre by the last of May. Postponement of attention by the farmer is a losing proposition. A stitch in time actually saves nine.