Answer: If the holes are around the circumference of the tree and are all about the same size, you're dealing with a woodpecker or sap sucker. Beetles do bore holes, but they're random rather than in rows. There's not much you can do to keep birds from making holes, other than to wrap the trunks with hardware cloth so they can't get to it. I'd just leave the tree alone. The damage isn't as bad as it appears.
Best wishes with your landscape!
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