Answer: If the soil in your yard has a healthy population of earthworms, you may find 1-inch high piles of small, granular pellets of soil. These castings were passed through the body of earthworms the night before and were brought to the surface as tunnels were cleared. They are more common in spring and fall when soil moisture and temperatures are conducive to earthworm activity. There is usually no hole in the top.
There are also many insects that spend the winter in the soil, during which time they transform from a larva into an adult. In the spring and early summer, especially after a rain, you may see nickel-size holes caused by their emergence. These holes may be surrounded by small mounds of loose soil and fecal pellets. June beetles and cicadas are examples of insects who do this.
I expect the mounds will stop appearing soon and your grass will return to normal.
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