The burrows of the Columbian ground squirrel have been described by [Shaw] (1919, 1924B, 1925, A, B, E, 1926) and [Bailey] (1918: 47). Two types of burrows are constructed, summer burrows and hibernation dens. Summer burrows are used year after year, perhaps by succeeding generations of squirrels. They vary considerably but usually are 3 or 4 inches in diameter and possess several entrances. The depth to which the burrow descends, as well as the number of forks and subsidiary burrows, depends to some extent on the whims of the individual squirrel and the length of time the burrow has been occupied. Usually the burrow reaches a depth of four feet and two or more branches are present. A turning-about chamber is present, not far from the entrance. Here a startled squirrel can turn about and watch the entrance and, if the alarm was false, the squirrel resumes its activities above ground. Nests are above the level of the main burrow system and thus are protected from flooding by rain or melting snow. Stored food and excrement are deposited in separate compartments. Escape entrances, concealed in grass and weeds, for use in case the burrow is invaded, are dug from beneath the surface of the ground and thus are unmarked by any telltale mound of earth.
Separate dens are used for aestivation. While the squirrel is in aestivation the entrances to the summer burrow are solidly plugged with tamped earth. Aestivation dens are short and may or may not communicate with the summer burrow. [Shaw] (1925B: 58) measured 50 such dens and found the aestivation cell to average 2 feet 6 inches beneath the surface with extreme depths of 6 inches and 4 feet 11 inches. Ordinarily a drainage burrow is dug beneath the level of the nest.
Fig. 85. Distribution of the Columbian and Beechey ground squirrels in Washington. A. Citellus columbianus columbianus. B. Citellus columbianus ruficaudus. C. Citellus beecheyi douglasii.
The time and length of aestivation is variable. Old males enter aestivation before the females and young, and emerge earlier in the spring. In the lowlands, as about Pullman, Whitman County, the squirrels begin to disappear in mid July. In the mountains of northeastern Washington they are active until late August. They emerge again in late February or early March. [Shaw] (1925B) mentions that food is occasionally stored in aestivation dens of male squirrels. This food is not eaten in the winter but in the spring, when the squirrels awaken.
The gestation period was determined by [Shaw] (1925C: 108) as 24 days. Two to five, rarely seven, young are born. The average litter is about 3.5. Mating occurs in late March and the young are born about the middle of April.
Most green vegetation occurring in its habitat is food for the Columbian ground squirrel. Bulbs, seeds, fruit, berries, grain, clover, alfalfa, and garden truck are eagerly eaten. The squirrels are especially fond of wheat and great damage results from their depredations. In northeastern Washington some wheat fields are almost entirely destroyed. [Shaw] (1925G) showed that one squirrel destroyed an average of more than 50 pounds of wheat in a season. When populations range as high as ten squirrels to the acre, little wheat is left.