Fig. 19. Plot of a used nesting burrow of the Oregon Ground Squirrel, as excavated by J. Grinnell and W. C. Jacobsen in a pasture near Macdoel, in Butte Valley, Siskiyou County, May 16, 1918.
Main opening of burrow-system at a, with shallow, bean-shaped mound nearly two square yards in extent opposite. Greatest depth of earth in mound, 100 millimeters. Only other opening, at s, flush with surface, and without any out-pushed earth. Mouth of burrow at a, 100 millimeters wide by 90 high. Mouth of opening at s, 75 by 60. Dimensions of burrow at f, 75 by 60 mm.; at c, 90 by 65; at d, 60 by 60; at o, 60 by 60.
Depth of burrow at b, beneath surface of ground, 300 mm.; at c, 350; at d, 200; at f, 300; at g, 520; at h, 590; at j, 750; at k, 1,000; at o, 660; at p, 520; at r, 550. Branches at i, e and t came nearly to surface of ground. Old nest cavity at k, 300 mm. in diameter, and floored with a damp mixed mass of old frazzled grasses, excrement, and earth. At m and l, chambers, 770 and 1,110 mm., respectively, beneath surface of ground, packed full of a mixture of excremental pellets and loose earth. Blind terminal at n, 850 mm. deep. New nest, in use, containing many live fleas, at g; cavity of this, 190 mm. wide by 170 high, and 700 mm. beneath surface of ground, nearly filled with a dry clean hollow mass of shredded juniper bark and weathered grasses. The excavating was begun at a, and the female and six third-grown young were overtaken at s and t, where they were about to escape.
Total length of this burrow system, 66 feet. Average diameter of burrow, 2½ inches. Volumetric content of entire system, 3½ cubic feet. Greatest depth reached, 45 inches, or nearly 4 feet.
short-cylindrical or elliptical in shape, and measure 6.5 millimeters in diameter by from 15 to 22 in length.
There were two nest cavities in the system unearthed, an old one, and the one in use. The latter contained a dry, hollow mass of frazzled juniper bark and weathered stems and blades of grass. The female parent and six third-grown young were overtaken farther along in the burrow system, near one terminus of it, but that the nest had but recently been vacated was shown by its feeling of warmth to the touch and the presence of numerous lively fleas. The female proved a tartar in defense of herself and young, biting effectively with her sharp incisor teeth and scratching with her strong claws. This was as a final recourse, however, as the first endeavor on the part of all the occupants, when the burrow was opened up, was to escape and run to the nearest shelter, such as offered by some neighboring burrow, or by a flat rock. As far as our observations went, there was no indication that the adult male lives in the same burrow with the female, or has, indeed, anything to do with the rearing of the young.
The burrow system in question was found to be 66 feet long, including the various windings and all of its branches. Its average diameter was about 2½ inches, and the volume, or cubical air content, 3½ cubic
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