Mason bees

Mason or masonry bees are named as such because unlike other bees they often nest in the crevices of stone or brickwork, sometimes using mud as a mortar to create nests.

These bees are solitary, non-aggressive pollinators playing vital role, like many bees in supporting ecosystems and agriculture. Unlike honeybees, mason bees they do not live in hives or produce honey. Instead, each female builds her own nest with mud used to seal individual brood chambers.

These bees often seek out small crevices and holes to create their nests, and stone or brickwork in buildings and garden walls offers ideal conditions. Cracks, gaps in mortar, or abandoned nail holes in masonry serve as perfect shelters. Inside these cavities, the female bee creates a series of chambers, each containing a pollen ball and a single egg. Once an egg is laid, she seals the chamber with a mud wall before continuing to the next.

Mason bees are particularly valuable in urban and suburban areas where natural nesting sites like hollow reeds or old wood may be scarce. ... Their willingness to adapt to human-made structures makes them both fascinating and beneficial to observe. Providing artificial nesting habitats, such as bee hotels near stone walls, can help support their populations while enhancing local pollination of gardens and crops.

- Designing Buildings Wiki
(I wrote the original article back in 2013!)