small diameter, and not reaching to any considerable depth. Those in alluvial or sandy soil were found to be of large diameter, of greater extent, and to reach to much greater depths.
The most conspicuous signs of activity on the part of ground squirrels in any locality are the large mounds of earth that have accumulated in the course of excavating the burrows. This earth is commonly thrown out in a fan-shaped pile directly in front of, and to the sides of, the main entrance to the burrow (see fig. 4). These mounds of earth are often three or four feet in diameter and from six to ten inches above the general level. They vary greatly in size, but average larger in sandy soil than in clayey or rocky ground. The size of the mound is, however, no reliable index to the length or size of the burrow except in those cases where the burrow is of a straight or simple pattern. In
Fig. 4. Mound and burrow entrance of a "digger" squirrel, in sandy ground. Mounds of earth such as these are often three or four feet in diameter and rise from six to ten inches above the general level. The route taken to the feeding grounds being used at this particular time is indicated by the numerous tracks at the left-hand side of the entrance. Photographed at Tipton, Tulare County, May 23, 1918.
colonial or intercommunicating burrows the dirt is not always thrown out at those entrances which allow of the shortest possible "haul."
Most of the work of tunnel excavation is carried on during the spring months, as is shown by the mounds of fresh, soft earth accumulated at the mouths of the burrows in that season. In the lowlands, where there is a large crop of wild oats in the springtime, this newly excavated earth supports a ranker growth than the surrounding parts of the field, so that, as one of our party wrote in his field notes, "the plain looks like a cemetery overgrown with grass," with these taller stands of oats about the squirrel holes suggesting grave mounds.
To some extent the ground squirrels, like the pocket gophers, thus act on wild land as natural cultivators of the soil, and may thus serve a useful purpose. On the other hand, their burrows are frequently the cause of much destructive erosion on hillsides during heavy rainstorms. Numerous small landslides have been noted on steep hillsides
16
