I have been sitting in the rare sunshine in the garden watching the busy life of insects. I am disproportionately pleased that my bee houses have attracted several visitors who are fighting over residence. Four of the cells are now occupied to my knowledge.
The bees are, I think, red mason bees. They crawl into the holes, lay an egg, female eggs first, and then fly back and forth building up a mass of pollen for the grubs to eat. Then they seal the cell with mud and lay another egg. The males hatch first and await the emergence of the females.
I didn't know if it would work or if my attempts at conservation would be largely ignored but to my delight, no sooner were the bee houses out last summer than creatures were investigating. I also had heard the size of the holes was quite important so I did a variety of sizes. The smaller ones seem to be occupied by what I surmise to be Harebell wasps who seal the holes with a silken thread made of what seems to be chewed-up wood.
It might only be a small thing: Drilling some holes in some offcuts of wood, tacking on a small roof and hanging them up in the garden, but I confess, it has caused me significant pleasure watching nature adopt my handiwork.