I put out a block of fir in late June, with several 5mm holes drilled ~6" deep in the end grain. It filled up over several weeks during mid-summer. The nests seem to be constructed of a paste of green plant material with a final cap of mud. One day this female spent a good hour clamped onto the butt of another individual, as shown here. I interpret this as competition for the nest cavity, as opposed to mate guarding or something else. Any insights are appreciated!