Perhaps one of the best lessons one can take from a Sunday afternoon walk is to learn to be content with the wonders readily at hand. Though I wasn't far into my intended rambles, and was aiming for miles before I slept, looking back, I didn't have a good reason beyond that to leave those bees at all, and I am rather sorry I went on, though the rest of the walk was lovely. They were beautiful. ("A really big sort of carpenter bee," I guessed. "But aren't those solitary?") It would have been a worthy afternoon's work, either to have managed a good picture of one of them, at least to have given more time to figuring out exactly what they were doing, or merely to have enjoyed watching their loud, determined flying rather indefinitely. (People go whale-watching as a treat; why not bee-watching?) As it was, the only one I managed to capture on film was the one in the lower right corner of the photo below, and I left, none the wiser.
I had a session with Google when I got home, and found several interesting things. First, you will be relieved to see, much better pictures of similar creatures, courtesy of BugGuide.net, here, here, and here. They are beautiful, aren't they? Secondly, as you may have gathered, from reading under the pictures I've linked to, they are indeed carpenter bees, Valley Carpenter Bees, or Xylocopa varipuncta, to be exact. . .and they are indeed solitary. But the examples I saw were males, hilltopping, congregating in hopes of attracting females. An interesting aside--the females (of which I saw no examples on this occasion) are quite distinct from the males.
It seemed such an unusual encounter that I was quite astounded, a day or two later to come across another two of the creatures, this time having claimed the airspace over a Citrus Heights parking-lot garden, about a yard square, as their own. Which, if you ask me, makes a pretty good case for taking bee-watching as you can get it; when is the last time you saw a whale on your way to the grocery store?