Don’t forget to make sure that your mason bees can find a good source of mud while they are laying their eggs in your tubes. I discovered a year ago that if you provide a reliable source within a few metres of the bee houses, they will use less energy to go to gather mud and will therefore be more efficient.
See this post I did at that time. Another observation has led me to believe that they prefer to collect mud in horizontal holes in the side of a trench. This trench is kept wet throughout the nest-building period, and I dig holes into the walls of the trench. It is probably an adaptation to prevent predation, as they would be easy targets on an open patch of wet soil. They also prefer “clayish ” mud, as any good mason knows that their mud needs to be sticky… Sandy doesn’t do it!
This year I gathered some freshly exposed clay and added it to my mud trench.