The nest boxes we use (also known as bee hotels) are designed for wild, solitary bees that like to nest above the ground (called above-ground cavity nesting solitary bees). Many types of wild bees are also below-ground nesting solitary bees. In fact, 95% of bees are solitary, and do not live in hives like honey bees do.
In the wild, these above-ground cavity nesting bees choose to nest in old, empty plant stems, or holes in wooden structures, like the leaf cutting bee below.
Solitary bees will emerge in the spring as adults, and quickly begin to forage for pollen and nectar as food. Once they are mated, the females will begin to lay their eggs in their nests, and those eggs will develop into pupa and spend the next winter as such until hatching in the spring.
Different types of above-ground cavity nesting bees will use different materials to make their nests, and this often gives them their name! For example, mason bees will use mud to line their nests, and leaf-cutting bees will use leaves.
Above-ground cavity nesting bees
Mason bee (Osmia sp.) 
Carpenter bee (Ceratina sp.) 
Resin bee (Hoplitis sp.) 
Leaf-cutting bee (Megachile sp.)
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