I'd agree that it's most likely ground squirrels and this has been a banner year for them.
Don't be fooled by the name as ground squirrels are not the same as bushy tailed CA grey squirrel.
Ground squirrels are rodents and it takes a lot of work to get rid of them once established.
Feeding: California ground squirrels are omnivorous. They eat seeds, nuts, fruits, bulbs, fungi, and stems and leaves of grasses and forbs. They also eat some insects, bird eggs, and carrion. Forage on the ground, and in low shrubs and trees. Also dig up plants. Store food for inactive periods.
Cover: Cover is provided by burrows excavated in friable soils, often near rocky areas or under trees or logs. Burrow system may be elaborate, with 6-20 entrances. Tunnel lengths average 11 m (35 ft), and range from 0.9-42 m (3-138 ft).
Reproduction: Build cup-shaped nests of dried vegetation in the burrow.
Water: Apparently very little water required.
Pattern: Suitable areas for California ground squirrels are herbaceous sites, and openings in most brush and forest habitats. Friable soils are required, and rocks, scattered trees, logs, and other ground cover enhance the habitat.
Reproduction: Females are polyestrous. Timing of breeding season varies greatly throughout California. Mating occurs from January through July, with peak activity in March through June. Following a gestation period of about 30 days, an average of 6-7 young are born (range 3-15). Litter size is correlated inversely with latitude (Chapman and Lind 1973). Females produce 1 litter per yr. Young are weaned at about 55 days, and are full-grown in 7-8 mo.
Niche: California ground squirrels are important prey for many carnivores. Population numbers usually increase in heavily grazed areas, as forb production and visibility increase. Can damage crops. Can harbor plague, which may decimate numbers.
http://nrm.dfg.ca.gov/FileHandler.ashx?DocumentVersionID=17592
