So this is a safe time of the year to talk about this I think. It's a time to prepare, and weather you're ordering nesting blocks or building your own. these are a great way to add some pollination to your yard.
Territorial Seeds and Knox Cellar are both places selling nesting blocks and tube cans. I'm sure there are others too but these are the two I've bought from.
Basically these are bees that take advantage of tubes, meaning one entrance hole. The holes are the result of beetle grubs boring out of dead wood. These are active in the spring and summer but they don't live that long. They're also specific to the size hole they like.
It's said that Mason Bees and other Natives are better pollinators than Honey Bees. I find this questionable at best. Sure they use Buzz Pollination (where they use their wings to force the flower to produce more pollen and nectar) but they're only alive for 4 to 6 weeks. That might be good for plants blooming at that time of year but good luck getting a stock of Mason Bees active all year. Most Ground Nesting bees hold similar lifestyles. But even then, 4 to 6 weeks of the year the adults die off and they won't be back until next year.
Mason bee homes are easy to make. Just drill into lengths of wood. Stay around the drill bit size 5/16 and you should attract a variety of species.
The best part about these bees is they are timid! I've never been stung by one ever, even when doing macro shots of them up close.