I'd walked by this White Pine many a time over the past year and sometimes stopped to ponder the perfectly round holes. Too perfect for a bird, almost like a power drill, the answer came from articles in Val Littlewood's Pencil and Leaf blog. Thanks Val! :-) Do check out her wonderful and growing series of bees!
Don't the Carpenters perform a fine job? Notice how they work through each annual ring, through the harder and softer parts of each year's growth.
Just to give an idea of actual size...
Here's a bit of frass left over after construction. What didn't come through that well was a whitish material that I suspect was blocking off some inner chambers. (It shows up rather fuzzy here.)