Today, I finally got to observe them up close, and they turned out to be Yellow-Rumped Warblers. They are hanging, sometimes upside down, pecking off some kind of bugs -- probably aphids -- from the underside of the remaining leaves (the plum trees have already dropped about 50% of their leaves).
This is especially welcome since I haven't seen these birds since early in the spring when they were occupied in the same activity.
