TreeDog,

They are California Ground squirrels. The scientific name for them is Spermophilus beecheyi. They dig holes in the ground where they live. I once saw them in an oak tree that had a big hole in it where a branch had broken off, but normally they don't climb trees. In really dry weather, they will strip the bark off of Buckeye trees to get moisture.

More pictures of ground squirrels here: http://www.varmintal.net/avarm.htm