Common causes of small holes in your lawn are birds, insects, solitary bees or voles. The size of the holes, as well as the presence of piled dirt, can help you determine which creature caused the holes.
If the holes are throughout your lawn and are clean with no piled dirt around them, the cause could be birds, which use their beaks to pierce small holes in the lawn in search of food. Nickel-sized holes found in the spring or early summer could be caused by insects wakening from hibernation and emerging from your lawn. Cicadas and June beetles are two examples of insects that hibernate in the ground and typically emerge after a rainfall.
Solitary bees are known to create homes in the ground as adults and even lay their eggs there. The holes are typically 1/4 to 1/2 inch wide, with as much as 2 inches of piled soil near the hole. These holes are usually located where little to no vegetation exists.
Voles are small rodents that dig tunnels underground. The openings to these tunnels are typically near plants such as rose bushes, hostas or hibiscus plants. As voles do not hibernate, these holes, which can be 1 to 1 1/2 inches wide, can appear any time of year.