So what is this?
Well, it’s black, and it’s really small (about 4mm long.) Around here, that immediately suggests one of two groups: Ceratina, a small carpenter bee, and Lasioglossum, a sweat bee. They can be hard to tell apart, but I’m pretty sure this is Ceratina (probably male). Why? It was polite enough to show off its tongue. Ceratina have longer tongues than Lasioglossum, but even better, "Lasioglossum" means "hairy tongue", and it's not.
Anatomical trivia: what looks like the "tongue" is actually composed of two parts, the prementum (attached to the head) and the glossa (the business end.) When people talk about "long tongued" and "short tongued" bees, they're usually talking about the glossa. If this bee were Lasioglossum, the glossa would have a distinctive branched structure at the end.
The bee on the left isn't showing her tongue (or all of her wing cells, which would also be useful,) but the nesting behavior -- boring a tubular nest into the end of a cracked, pithy stem -- is absolutely classic Ceratina. They're not called "small carpenter bees" for nothing. (This shot also absolutely rules out any of the Megachilidae, which would carry their pollen under the abdomen instead of over the legs, and any of the Andrenidae, which nest in the ground.)
I mentioned before that I spent some time studying carpenter bees in California. Those carpenter bees (Xylocopa) are huge things, bigger than bumblebees. The females make a distinctive noise when they fly, a harsh buzz that’s easily audible from several meters away, if you know what to listen for. They’re not aggressive at all, but unless you know that, they’re probably the most intimidating bees in the desert Southwest.
I had absolutely no idea that their closest relatives were these tiny little things in the Pacific Northwest, but here they are. The really surprising thing is that there are no middle-sized carpenter bees: they’re either absolutely huge, or barely bigger than a fruit fly. There’s a story there, but I don’t (yet) know what it is.