They were very LARGE bird tracks.
After some research, I've concluded that they were from a heron or an egret. Here's a photo of both kinds of bird, taken a few days earlier at a different location.
I'd never realized what large feet they have.
I suppose that, like the birds, some of us leave unexpectedly large tracks, for good or for ill (and I'm not just talking about a couple of my relatives who wear size 15 shoes). Which people have left behind large, wonderful tracks in your life?