It's ash. Not necessarily ash from a burning fire, but often from a bird that has come through a burned area.
The first time I saw this was several years ago when a group of white-crowned sparrows arrived during fall migration and we had just had a wildfire that burned miles of hillside north of us. The white-crowns seemed to line up at the bird bath and when they were done the water was almost soapy.
This past spring a large swath of the Santa Monica Mountains at the Camarillo Grade was severely burned. The area is northwest of us, up the coast. When I saw the state of the water I had changed the evening before, I knew that a bird had arrived who was flying down the coast. The amount of water splashed around suggested something larger than a white-crowned sparrow.
Then we heard an unfamiliar call, an American kestrel. This is a small bird of prey, much smaller than our resident Cooper's hawk or red-tailed hawk. We spotted the female kestrel sitting in the top of a dead tree calling. It's been years since we've seen kestrels in our neighborhood. It could be that this bird was returning to territory that was burned out. It will be interesting to see if she stays or moves on.
Rinsing out the ash only took a moment or two. It was nice to know we helped a traveler freshen up. That is what being an oasis of habitat is all about.
