It is quite likely that California Ground Squirrels construct new burrows from time to time, or, what is more probable, that each young individual as it approaches maturity leaves the parent burrow and digs a home for itself. In any event, in places there are many more burrows than individual squirrels present at any one time. Some of these tunnels, especially in the plains and foothill country, are joined together below ground to a greater or less degree and constitute the colonial burrows already described. When hurriedly seeking safety a squirrel will pitch down into the nearest one of a number of holes in the vicinity of the one about which it was first seen. The commonly uninhabited burrows may thus serve in extremity as temporary refuges.
The burrows of the squirrels are often inhabited by species of animals other than the rightful owners. Ground owls habitually make their homes in squirrel holes, probably deserted ones; and, to a less extent,
Fig. 9. Nest and male of "digger" squirrel as dug out after burrow was treated with carbon-bisulphid. The spherical shape of the nest-cavity and the structure of the nest itself is well shown.
the holes are frequented by California toads, Western gopher snakes and Pacific rattlesnakes. It is unlikely that the presence of the latter two animals is congenial to the squirrels, as both of these snakes are known to eat ground squirrels in numbers. Regularly communal occupants of squirrel burrows are scorpions, centipedes and mole crickets. Mole crickets were found to serve as reliable indicators of the efficiency of the gas when squirrels were fumigated in their burrows. If the gas had not killed the crickets it was found that the squirrels had not succumbed.
California Ground Squirrels are accustomed to furnish their underground quarters comfortably. Special nests are constructed and maintained in good order, where the individual may sleep or rest in warmth, free from contact with the damp earth. Each burrow occupied by a single squirrel was found to contain at least one well-made nest. In some eases there were two, one obviously older than the other. In the
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