I found this mud-dauber wasp nest last week, and I was able to watch the mother wasp working on it.
When I first saw the nest, the bottom tube was open, and as I was studying it, the wasp came up with a glob of mud, and sealed the opening. I didn't have my camera on me at the moment, but I rushed to get it and took a picture of the fresh plug.
Here she's added a bit more mud.
Here's the missus herself starting another chamber. Mud daubers aren't particularly aggressive, and this one was fine with me getting close and photographing her as she worked. According to the insect field guide I have, the mother mud dauber stocks her brood chambers with captured and stung spiders for the wasp larvae to feed on. While wasp adults feed on nectar, wasp larvae are generally carnivorous, and must be fed some sort of meat. Social wasps (such as yellowjackets and hornets), bring pieces of insect or other flesh back to the nest for the larvae. Solitary wasps generally capture whole insects (and spiders, in some cases) to take back to their nest and stash with the eggs. Others are parasites, and lay their eggs directly on the host, which will hatch in a larvae that will consume it from the inside, while it's still alive and functional. Anything for baby...