iblist.com user synopsis
On a low-tech, agrarian future Earth, slow moving metallic "birds" randomly pop out of nowhere, slowly float through the air, and, after a while, disappear. Except sometimes they blow up, vaporizing everything nearby, and leaving behind a giant circle of glass. If this goes on long enough, all the remaining land on Earth will eventually be glass.

First published in The Magazine of Fantasy & Science Fiction. Reprinted in The Year's Best Science Fiction: First Annual Collection.