The house design was inspired by some links I came across:
Nests for Native Bees - Invertebrate Conservation Fact Sheet
USDA Natural Resources Conservation Service - Backyard Wildlife Habitat Tips
It simply involves drilling some holes of variable diameter and depth into wood. Since we have lots of dead birch stumps lying around, I thought I'd give it a try.
I was surprised to recently learn that most native bees do not form hives but live a primarily solitary life.
I will be keeping an eye on these homes to see what eventually emerges, though I may easily miss that event. Not sure if they are actually bee nests, or some other insect who prefers this convenient shelter in the dead paper birch stump.