These very neat little clay nests are made by potter wasps (I don't know which species), a group of wasps of various species, known for creating clay 'pots' in which they lay their eggs. Each one is about the size of a marble. The one on the top right has been taken over by a spider, who is barely visible hiding behind the 'pot' top left. I find these are amazing little creations, and just think, a very small wasp makes these symmetrical clay 'pots' by carrying minute amounts of clay one mouthful at a time. The work involved is remarkable.