If the holes are tapered (wide at the top) it could be birds ie green woodpeckers searching for ants, crow,jays, or magpies retrieving burried food, starlings probing for leather jackets (crane fly grubs) or Squirels digging up acorns or horse chestnuts that they burried last autumn.
I agree - squirrels or large birds. I have a squirrel that buries nuts in the grass and a magpie who follows him around digging them back up! Funny how the squirrel never seems to take any notice :o)