Apparently the squirrels we get around here are fox squirrels (Sciurus niger). Coincidentally, one of the photos on the Wikipedia entry for fox squirrels was taken at Lake Merritt.
And a reader of the local newspaper's animal/wildlife column sent the columnist a photo of the nest of baby fox squirrels that he'd found in a sheltered rain gutter on the side of his house. Luckily it's the dry season now, so the babies should be grown and gone by the time the gutter needs to be cleaned out to avoid flooding. (WUZZA WUZZA BABY SQUIRRELS!)