Mary Holland, author of Naturally Curious. She suggested mourning dove droppings, and I think she is right.
Under a winter's sun, frozen ponds and lakes boom and yelp and screech as ice expands and contracts. I remember one of the first times I heard a frozen lake boom in the Boundary Waters of northern Minnesota. We thought the lake would open up and swallow us. Mostly though, frozen wetlands are still and silent and beautiful, especially under a winter's clear blue sky. The sun casts long shadows and the wind creates patterns in the ice and snow. Plants are frozen in place. All arranged as if in a series of still-life paintings.
We wandered among a colony of great blue heron nests, which sit idle, waiting for the return of the large, graceful birds after Spring thaw. Their nests seem perched precariously atop the dead trees, especially as seen up close in winter. Many of the standing dead trees in this wetland have lost their top-most branches, limiting their appeal to herons. In time, this colony will need to search out a new wetland that has suitable tree-tops.