There's a couple vans fitted out as mobile ORs. Any cat found on the street without a collar is picked up and neutered. When they do it, they cut a notch in one of the ears so they don't pick up the same one again. Then they put them back on the street. (I don't know the details of recovery and release timing.) They can schedule more/fewer days in operation per year to keep a balance between cats and rodents.