A "bug hotel" on the wall in our estate edible garden....the holes stopped up with mud contain the eggs of solitary mason bees, who lay their eggs in little mud compartments in a hole in wood (for example) with some pollen/nectar for food -- female eggs at the back, male eggs at the front. The males hatch first in the spring and hang around for the females to hatch so they can mate. Then the males die, and the females look for another hole to lay eggs...and it all starts again.