Great question! These bees are rather shy, so you will need to be very observant and patient if you want to catch a bee in the act of nest-building. You are more likely to see the cardboard tubes being filled with the bees’ chosen nesting substrate. This is most commonly leaves, soil, or tree sap. You can see a few examples of this in an old blog post ici. The bees build their nests starting at the back, so if you see a cardboard tube capped like this, that means the bee has likely filled the whole tube with eggs and pollen!