Thanks Downy, Gary and Gary. This is a snag, an old dead tree right by the waters edge. In fact, it's so dead, I saw it swaying in the high winds yesterday while the live trees next to it were solid, so I fear it's not going to last too many more years. But it has holes like the one seen above, on all sides at the top ten feet or so.
I didn't see any other swallows actually use the other holes, but there were several others around, and at times landing on that broken top. Maybe I caught these in the first days of nesting and the other haven't started yet. I'll just have to keep an eye on it over the next few weeks. I wouldn't mind seeing a Flycatcher using one of the holes. 
Karen, look around for a dead tree near water with woodpecker holes in it. I think that is a popular nesting site for these guys. I'll be looking at some others I know about, now that I've seen this. 