I now have a good idea of what is in my immediate area: skunk, raccoon, opossum, red shouldered hawks. On the peripher, there are red tailed hawks, kites, and Falcons. On the grasslands, I've seen coyotes. But so far, a covered run and a coop wrapped in hardware cloth has served me well.Down there you are looking at just about everything except mountain lions and probably not too many bears, unless you are close to some foothills and oaks. So that means Raccoon, red fox, gray fox, opossum, mink, weasel, Norway rat, black rat, skunk, bobcat, coyote, badger, great horned owl, ravens, crows, red-tailed hawk, red-shouldered hawk, probably not golden eagle but it's not out of the question, feral cats are only a threat to babies on pasture, and feral dogs can occur anywhere but usually closer to urban areas. You shouldn't have to worry about the barn owls, they specialize on rodents so they are good to have around.
California is a state of predators. We are allowed to hunt "meat animals" like deer, quail, ducks, rabbits, etc., but not the things that eat those things. So we have them in abundance AND (as a special bonus) they don't get shot at or trapped so they have little reason to fear humans. Best advice is to have a secure coop for them at night with no cracks or openings that a teeny tiny weasel can get into, and build a run that is totally enclosed with hardware cloth (but chicken wire will do ok) and bury a skirt 6" deep around the perimeter. If you're really paranoid add a hot wire.
That's worst case scenario. We are in the process of moving from a place where our ducks don't even have a coop and just graze the lawn all day and night, to a place against the redwoods in a clearing with a lot of raptor perches and we've been told a very high mortality rate. So your level of concern depends on your specific location, but you might have to lose a couple birds in the process of figuring out what that level is.
My little flock stays in the run during the hours the hawk activity is at its peak. I let them out in the later hours of the day when I've noticed there is little predator activity. My neighbors who have chickens say the biggest problems here are dogs and homeless people. My yard is safe from both, unless our homeless shelter started passing out blowtorches or chainsaws.