I have kept mason bees for ten years. I started with one unit and now have eight. Mason bees are passive, so ideal if children or pets frequent the garden.
The bees start to leave the unit at this time of year. The males first, they stay in close proximity waiting for the females.
After the female has been served the male will spend the rest of his days, sunning himself on foliage and feeding amongst the flowers and blossom. The female will spend her whole life laying grubs in the units, filling them with pollen, and sealing each with damp soil. At the end of her life she is tattered and worn, and more often than not senile.
If there is such a thing as re-incarnation I do not want to come back as a female mason bee :)
Have a lovely weekend............BEE HAPPY :)