so successful when applied to the Oregon Ground Squirrel, which animal apparently does not habitually gather and store seeds and grain to a large extent, but is active chiefly during the spring and early summer when green vegetation, upon which it depends for food, is to be had. Some sort of green baits might be expected to be more effective with this species.
It is very desirable at this time that we know more about the extent of æstivation and hibernation among our ground squirrels. We know little or nothing of the effect of gases upon squirrels in this condition; and it is obvious that such part of a squirrel population as is dormant at the time of a poison campaign will escape destruction and become a nucleus of reinfestation.
During the past spring and summer our work of excavating squirrel burrows after the occupants had been gassed, has shown that one frequent cause of failure in the ordinary waste-ball method of applying carbon bisulphid is due to the fact that sudden elevation in the course of the burrow, of as much as two feet in some cases, prevents the heavy gas from reaching the animal. This condition was found to occur much more frequently than is generally supposed.
The instances just cited all suggest that the present knowledge of our ground squirrels is far from complete, and they serve to emphasize the above contention that a more thorough knowledge of this subject is essential to securing the most intelligent and efficient methods of control of these our chief rodent pests. It is the purpose of the present paper to supply the information available from all sources in as much detail as it has proven feasible for the authors to secure it at this time. The facts and inferences are given just as they came, irrespective of whether or not they show obvious and immediate economic bearing. They are here available to everyone who is concerned with methods of control.
ACKNOWLEDGMENTS.
The present paper has been prepared at the suggestion of Mr. G. H. Hecke, California State Commissioner of Horticulture, and Mr. W. C. Jacobsen, Superintendent of Rodent Control under this commission. Both these men have rendered many valuable helps during the progress of our work and have co-operated to facilitate its final publication. Their emphasis all along has been upon the need of a summary of the facts relative to rodent natural history at this particular time, when efforts from every direction are being concentrated upon the problem of increased crop production.
Upon the facilities and auspices of the Museum of Vertebrate Zoology of the University of California the writers have been dependent for the opportunity of carrying through their undertaking; and behind this has been the continual financial and moral support of the founder and constant patron of the Museum, Miss Annie M. Alexander. The collections of specimens and the field records contained in this Museum, gathered during the past ten years, have been absolutely indispensable
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