Mason Bees will nest in the strangest of places. Remember my blog on nests found in a folded umbrella? This is a similar situation. The nest was placed inside a wooden bee home. Bees proceeded to fill the nesting tunnels. When all nesting tunnels were filled the bees found nesting space behind the nesting block. This space was perfect. The cocoons fell out of the cavities when we removed the block from the wooden bee home. In other words, bees will find nesting space in the most unlikely spaces. Their drive is to survive and produce offspring no matter what kind of nesting space they may find. 
Nest space- in the most unlikely places
on in General
Jean
It seems we have a population explosion each year and fill 2 large houses. Concurrently, the “excess” girls have moved across the yard to make their nests in the dirt in between boulders on the rockery. We cannot harvest the cocoons from the rockery, but each year we have more and more bees emerging.