I don’t think it’s a garter.
Frankly I have no idea what kind of snake that is. I consider myself more familiar than the average person with the snakes of this country but I am stumped. There are only a few species with that striping that are from California, including ground snakes and California king snakes, but that looks like neither.
However, the only venomous snakes of California are all rattlesnakes, which this is definitely not, so I would say let it be. Certainly not a threat to chickens or humans.