You are bold to leave a bag of feed out. Aside from the likely consequence of a rodent accessing it and spoiling it before you find the damage (rats and mice will defecate and urinate throughout the bag, so even if they don't cart it off, they will make it unuseable.) Rats can chew through plastic. Mice will also do so if given more time. Also, leaving your bags of feed out makes them more prone to absorb moisture, making them prone to mold. If you've seen one rat, you most likely have an infestation. Usually, they are very well hidden, and you may never see them, only see their damage: as in... your feed is not lasting as long as it used to. Are you leaving feed out at night? During the day, your freeloading pests can include chipmunks, squirrels, birds, and even ground hogs.