This is a bee house for native bees. It's not really a house, but a place for them to lay their eggs. I've had one in my garden for many years. They don't last for more than a few seasons, because woodpeckers love to peck on the wood. Sometimes they peck to get to the bee larvae and sometimes they just seem to peck on the roof and sides for fun. (If I was a more conscientious bee house owner, I would insert straws into the holes and take them into the refrigerator for safe keeping over the winter).
This is one of two bee houses my father-in-law made for my birthday this year. He has patiently waited for evidence of bee activity. I suppose the holes may have been used by another insect and I welcome any help with identification - if it is possible by the type of plug made.
Learn more about native bees here.