When I saw a maroon and yellow wasp working on an incomplete mud nest, my immediate assumption was that she was building it. In the first photo in this series, one cell is about 3/4 complete and there is a dark area at the edge of the opening which is obviously wet mud.
Mud-daubers (Eumeninae, also known as Potter wasps) collect little balls of mud and stick them in place to make nests in which they lay a single egg (here and here are other species doing it). They place provisions, i.e. paralysed spiders or caterpillars, in it and then seal it and move on. Inside, the egg hatches, grows as a grub-like larva and pupates in a cocoon which it makes within the nest. It emerges from the cocoon and the nest as an adult wasp, and the life cycle begins again.
Here she is again, working on the same area. No real progress is evident.
And here is the nest a while later. By this time it was clear to me that she was not building the nest but taking it away. Why? It was some time before I had an answer, once again from the helpful folk on my Flickr groups.
terraincognita96 said, “It is quite common for Eumeninae wasps to demolish vacated mud nests of the previous season. They bring along a mouthful of water, regurgitate it on the old nest, form a mud ball and take it to their actual building site. The barrel shaped object left behind could be a cocoon of a previous inhabitant?”
So it proved when I removed it to photograph it and cut it open:
That looks like an exit hole at the top, and the lower shot shows a thin-walled cell with a little organic debris remaining inside.
In retrospect, the cocoon visible inside the incomplete nest should have been enough to tell me immediately that this wasn’t normal nest-building, but I had never heard of wasps recycling building materials before. There’s (still) always something new to learn!