Those very small, and perfectly formed clay pots are made from an insect called, appropriately, a potter or a mason wasp. There are about 3000 species of potter wasps, which means that though I know these are made by potter wasps, I do not know the species!
The lifecycle of potter wasps is fascinating in that they construct these tiny pots, mixing saliva with clay that they collect. They fashion the clay into pots, lay one egg in each, and go find an insect, usually a spider, that they paralyze, and then put into the pot. They then seal up the opening and leave them. When the tiny larva hatches out, it has a meal already provided for it!
Mother Nature never ceases to amaze me! So many cool adaptations that make animals perfectly suited to their environments.