An update:
Last week I called around, found that the county won’t do anything about wild animals. Their response is, call a pest control business.
There are no sources for live traps locally, but you can buy them online. However, I talked to a local wildlife rescue organization, and they actually recommend against trying to relocate ground squirrels. Apparently they would be killed in territorial conflicts; also unless individual squirrels don’t do well on their own so you’d have to move the entire colony at one time. It’s more humane just to exterminate them, if it’s done right.
So I’ve started setting out rat traps, but I had to take special measures. First, to reduce the risk to birds, the traps are under cardboard boxes which have an opening cut in one side.
Second, because squirrels are large and tricky, it took a while to figure out the best bait, and the best trap configuration. Often they’d ignore the bait or trip the trap without getting caught.
I found that the most effective method was to use two traps side by side. The first trap, nearest the opening, is set but unbaited. The second trap, farther from opening, is baited with a peanut still in the shell. (For some reason the squirrels seem to prefer them in the shell.)
To get to the bait, the squirrel has to step over the trigger of the first trap. Then, if he somehow manages to avoid getting caught by the baited trap, he’s almost sure to set off the unbaited one in his haste.
So far I’ve gotten clean kills on two of the little pests this way.