When I was down in the basement this morning, I heard something moving around in the empty boxes I have stacked by the sliding glass door. (I haven't yet come up with a reasonable way to put up a curtain or something that isn't a huge project, but boxes sort of work.) I could tap on the boxes and whatever it was would rustle around some more. (This was disconcerting, by the way.) I finally decided it was fluttering, and guessed it might be a bat, since I know bats have gotten in the apartments before.
So I closed up the door upstairs, opened the outside door, and started peeling back layers of boxes until a bird finally slid out from between a few and flew out the door as fast as it could. I have no idea how the bird got inside. It does not speak to the apartment being well-insulated.