Thanks, all! The picture of the boxelder matched these precisely, in fact, and when I looked it up on google (very cursorily) I read that they do feed on maple seeds also. I suspect the area where I moved that dusty miller from was an especially great place for them to hide, congregate, raise their babies, build a city, all that.
I also wonder if they feed, or can feed, on mimosa seeds because hoo lawdy we have tons of those, and the squirrels and the few birds who like them can't keep up.
