At about 4300 feet in elevation I took a rest, perching on a stump atop a steep slope below which nothing remained of the clearcut forest other than rotting stumps, eroding dirt and brush piles. The glare and afternoon heat were bad but I'd spotted movement down below and I wanted to know what critter could possibly call such a raw, broken landscape home. Eventually he appeared, sprinting from beneath a log to grab something in the open, maybe a beetle, and quickly return to his hiding.
He looked a like a chickaree -- which looks like a small gray squirrel -- but I'd never seen a chickaree far from a standing tree.
The binoculars explained it. Here was something different, something gray with a broad, pale splotch on each side along the neck and above the shoulders. He was a California Ground Squirrel, CITELLUS BEECHEYI. You can see him, but with the starkly white patches I saw hardly showing at all, at http://www.death-valley.us/article22.html.
During the rest of the hike I spotted quite a few California Ground Squirrels, though never in the forest, just in open and semi-open areas. Though this preference for the ground set him apart from chickarees and tree squirrels, his outward similarity to those animals got me to wondering just what was the difference between them.
My field guide wasn't much help. In fact, it added marmots, woodchucks and prairie dogs to the mix, because those animals are members of the Squirrel Family, the Sciuridae, just like tree squirrels, chipmunks and these ground squirrels. Thumbing through the field guide's illustrations of the various Squirrel Family members I decided that just from outward appearances the various groupings more or less blend into one another. If all you know is the Eastern Chipmunk and the Eastern Gray Squirrel there's a world of difference between those two things, but consider all the other species in the family and it becomes hard to draw lines between major groupings, at least by outward appearances alone.
Ground squirrels and chipmunks do possess thin inner cheek pouches for carrying food or nest material, but tree squirrels, including the Chickaree, don't. This is a big anatomical difference but you can't see it from the outside. Also there are striking differences of behavior, but a picture doesn't show that, either. The ground-dwelling species even have the nice habit of rearing up on haunches and looking around, just like tree squirrels on a limb.
The Sierra Nevada Natural History handbook says that at one time California Ground Squirrels were "enormously abundant," even in the Central Valley below. However, these rodents dig tunnels that can break a cow's leg and they relish the irrigated crops grown down there, so the species has been exterminated over large tracts. Still, here and there in the Sierras they are represented "scatteringly well," as the handbook says.