Answer: You might loosen the soil in the boxes and see if they are after some sort of insect, in which case you might sift the soil and remove the insects. You could also place some wire mesh over the soil until you are ready to plant the boxes. If they are digging throughout the season, you might try stringing some heavy nylon fishing line above or around the boxes to confuse their depth perception. You might also try hanging a scare eye or imitation owl or similar "frightening" thing nearby to discourage them from coming near.
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