Yeah, hi there. I've been m.i.a., and I apologize, but I have at least managed to i.d. the little bird in our backyard: it's a House Wren! I had been relying on the Merlin app to give a good example of its song, which it doesn't, but online searching helped me find it.
The House Wren is aggressively territorial, and it seemed to find the Merlin app very accurate; when I played its song, the bird came swooping around looking for the intruder. So, mystery solved.