Okay boys and girls, this'll be fun. One of the weird-but-cool things about being a writer or editor is that you learn an unbelievable lot of otherwise-useless stuff, trivial little notes of all kinds of things. This has always been one of my favorites.
I noticed about 20 years ago when I was sitting out on the deck with a couple friends that people duck when birds fly over. A BIG bird flew over where we were sitting, and none of us saw it (roof) but all three of us caught the shadow of the bird on the lawn next to us in our peripheral vision. And all three of us ducked! First it amazed us, then it cracked us up. I don't know of any way to test this (remote-control hawk perhaps?) but if you ever see it, you'll grin now.
Apparently it's something in our longterm genetic makeup, something vaguely remembered from when humans were prey and birds were predators. Or perhaps it's just a genetic aversion to pigeon poop on one's head.
But it's fascinating, I think, that a bird SHADOW nearby will make people duck.