When wasps go to bed

In July 2014, I observed some bees snoozing upside down on a salvia stem. So interesting! Well, one evening last week at my mother's house, I sat down on a porch swing and immediately observed what I thought were some upset wasps, flying above my head around the chains that held up the swing. I got off the swing, thinking I'd jostled their nest. I stood nearby and watched the wasps. The more I watched, the more I realized that there was no nest. Then it dawned on me–the wasps were jockeying for a sleeping position on the chains! Just like the bees had done. A few hours later, I went back outside after dark and checked the chains. Sure enough, the wasps were snoozing there together. So I was right! These are a species of Chalybion, a blue mud-dauber wasp. I found a paper on the behavior: "Nocturnal clustering of the solitary wasp, Chalybion californicum." Isn't nature so cool?

UPDATE: The experts at Bugguide.net have identified these wasps as Aztec mud-dauber wasps (Chalybion zimmermanni aztecum).