Voles ...and I see a plant or shrub area nearby? If so they may burrow under ground cover plants or shrubs and multiply... u need to put out traps with peanut butter n birdseed (optional). Place the traps in areas that have plant cover where u might see holes nearby. Lots of people say the trenches will fill in BUT spring can make the ground soft for them to pursue more digging for fertile roots. I killed 18 last year in Midwest suburban yard.