THE GROUND SQUIRRELS OF CALIFORNIA. 



613 



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 bur- 

 rows were old and had the appearance of having been used by many 



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Fig. 8. Plot (plan and elevation) of a "colonial" burrow-system of "digger" squir- 

 rel 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, 41 inches; greatest depth reached, 4 feet; 

 volumetric content, 17| 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 encoun- 

 tered in digging — but also upon the estate of the individual or indi- 

 viduals 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 3J 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. 



21 



