Check with your local Fish and Game warden to identify any potential endangered or protected species on the property. Your local Agricultural Commissioner may be able to assist in this identification as well.  If using rodenticides, check the Department of Pesticide Endangered Species Project website that aids in identifying endangered species that may be harmed by the particular rodent control method. The project hosts a data base that give information on restrictions of particular rodenticides in your specific area.

Inadvertently harming any endangered or protected species while controlling for ground squirrels can be grounds for penalties and/or jail. Ignorance is not an excuse!

Some species of concern in California include:

Burrowing Owls
Range: Found throughout most of California.
Habitat: Open,dry grassland and desert
Feeding: Insects, small mammals, birds, small reptiles, carrion

Blunt-Nosed Leopard Lizard
Range: Higher elevation(<100ft above sea level) counties in western central valley of California
Habitat:  Dry grassland, desert
Feeding:  Insects, small lizards
Informational slideshow

San Joaquin Kit Fox
Range:  Mostly western central valley
Habitat: Dry desert 
Feeding:  Ground squirrels, kangaroo rats, desert cottontails, mice, insects, carrion, ground-nesting birds
Informational slideshow 

Kangaroo Rats (Giant, Tipton, Fresno, Stephens')
Range:  Throughout California depending on species
Habitat:  Most areas where soils are present
Feeding:  Mainly granivorous, grass seeds, grains
Informational slideshow

Other
Swainson's Hawk
San Bernardino Kangaroo Rat