The third type of burrow (see fig. 8) might well be called a "colonial burrow," as it is used by both sexes and also by the young after these leave the nest burrow and begin to forage for themselves. Colonial burrows are used largely as "safety zones." They afford convenient places for the squirrels to duck into when danger unexpectedly appears. These burrows are often from 100 to 200 feet in length and form a communicating system of underground runways connecting from six to twenty entrances or surface openings. The nests in the colonial burrows were old and had the appearance of having been used by many
Fig. 8. Plot (plan and elevation) of a "colonial" burrow-system of "digger" squirrel in sandy ground in irrigated section near Bakersfield; excavated by J. Dixon and H. G. White, May 3, 1918.
Various entrances at h; food store at a; "back-door" exit for emergency purposes at b; nest cavities as indicated.
Total length, 138 feet; average diameter, 4¾ inches; greatest depth reached, 4 feet; volumetric content, 174⁄5 cubic feet.
individuals at various times. These colonial burrows were not found to be in any case used as breeding burrows. It is possible that they may have consisted of one-time breeding burrows, now connected or linked together.
The relative extent of any one burrow system is thus dependent not only upon kind of ground—in other words, upon the difficulties encountered in digging—but also upon the estate of the individual or individuals directly concerned. Table I gives data in regard to the three types of burrows. In the seven burrows which were dug out and of which careful record was kept, the shortest occupied burrow was five feet long and the longest 138 feet. The average was 35.2 feet. The average diameter varied from 3½ to 5 inches, with a mean of 4.3 inches. The cubic air content was found to vary from 1.03 to 17.8 cubic feet, the average being 5.2 cubic feet.
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