Inside a mason bee nest
A great photo by Mike N. of Vancouver, BC. It shows 3 out of 4 occupied nesting tunnels. The three occupied nesting tunnels are full of overwintering solitary summer bees. The upper nesting tunnel was not used by any insect. The middle two nesting tunnels are filled with cocoons of a bee species that uses masticated leaf material for the chamber walls and partitions. You can see the greenish material on the sides of the cells. The pale yellow and orange pellets over the surface of the cocoons are fecal pellets produced by the developing and feeding larval stage. The different colours show you that this bee foraged for pollen from two different plant species.
In the 3rd row from the top look at the contents. The 5th and 7th compartment (counting from the left) do not contain a cocoon. What it does contain is a load of pollen that the female bee has deposited in the cell. The pollen is still present because the new bee in its larval or grub stage has died, leaving the pollen. This can happen under cold weather conditions. Bee larvae basically starve since it is too cold to eat.
The 4th and lowest nesting tunnel contains resin bees. These are solitary bees that use resin or tree sap to make their nest partitions and nest walls. The resin is rock hard and is great protection against any predators who might want to feed on them. The life stage is actually the pupal stage. This is the way it overwinters. In the summer months each pupa grows into an adult bee, and when the resin softens, adults emerge and begin the cycle over again.
When you find these occupants in your nests, leave them the way they are in the nesting tunnel. These occupants are beneficial to your garden. They will pollinate your plants during the summer months. Close the nest and set it out where offspring will emerge when the temperature is right.