Osmia bicolor [emerge] as early as February in their native range of South England and Wales. As solitary bees, there are no queens and workers; females build their nests alone. Males emerge, mate, and then die.
They repurpose empty snail shells, belonging to a small group of bees known as “helicophiles” (snail-lovers).
Once she’s gotten her shell positioned, the female bee provisions her nest with a mix of nectar and pollen that she chews up and makes into a ball. One egg is laid on the pollen ball, and then mom-bee seals the egg up with plant material she chews into a paste and daubs into a wall.
Next, she fills the rest of the shell with grains of sand and tiny pebbles, for a tiny predator barricade. [S]he covers her shell nursery with a pyramid of dried grass and plant material. She sews the sticks and mosses together with her saliva to keep the mound in place.