I found them! One night, just as we reached the Tiber for the beginning of a walk, we heard the noise. I’d heard it the day before too, but hadn’t been able to see anything. This time, I looked up and saw this:
Listen out for the bonus commentary from MJ.
Then, reminded of the value of reading my camera manual (which I’ve never done so I couldn’t get very good shots), I captured a few more images from the Ponto Sisto.