O body swayed to music, O brightening glance, How can we know the dancer from the dance?
—W. B. Yeats, “Among School Children”
It was near sunset many years ago when I saw hundreds of small birds hovering in mid-air. They moved in unison, veering in one direction then another, rising then suddenly swooping. From a distance the flock took on the aspect of a single creature lumbering through the evening sky. I had never seen anything like it. What fascinated me was that each bird responded instantly to what appeared to be the random movement of the whole flock. How do they do it? I wondered. How do they communicate the next move? Is there a lead bird or some signal beyond my seeing or hearing?
That aerial dance, ...