I think they go live in the garage wood pile and go out and dine at the bird feeder.
Bird feed was part of my problem. We keep our cockatiels in the basement. I vacuum it up every day now, plus I replaced all the insulation and sealed it in with plastic. They were coming in through the garage through the wall into the basement using the studs as a highway into the house walls where they could infest everything up to the attic. Had to replace all the plastic ductwork in the attic they ruined. Mice are destructive little monsters.