If you walk around in the woods much you know there are holes everywhere. There are holes in the ground, holes in trees and holes in rotten logs. There are holes in leaves and holes in tree bark.
Holes that have a sharp, well defined edge or that are perfectly round are most likely made by insects, especially beetles of various kinds.
Rougher edged holes like these are most often made by birds, primarily woodpeckers though other birds will peck away at wood and bark as well. Often the holes start out as insect holes and are made larger by the birds poking at them as they look for food.
And then there are the bigger holes like these that make an old tree sort of an apartment house for birds, mice and squirrels....bigger holes may house raccoons and owls. When in doubt, keep your eye on some of these holes to see who might use them. Often that will differ from summer to winter but most likely they are offering shelter to at least one of not more birds or animals.