I love things like this, a good nature mystery. Animals do some mighty weird things sometimes, things that are completely out of the ordinary for them. And then there are the simple things that just take a second to step back and analyze. But that can be boring!

We don't have bears in our area yet, but for the time being, that will be something to look forward to. We do, however, have lots of shorebirds and wading birds, and I wonder if one of these might have something to do with your mystery algae on top of the birdhouse. You say you are fairly close to water. Is is possible that a heron or egret or maybe a duck flew out of the water and had some algae in the old undercarriage and landed on your pole for a second to clean off its feet? I am not terribly familiar with very many kinds of algae, but I do know that the stuff, including the kinds you don't really want, can be easily spread from pond to pond by water birds.

Or maybe it's something totally bizarre and unexpected--and much more exciting!
Hopefully we shall see!

Thanks, I posted this over in the Blue Bird forum too and the concensus is a large bird. My pond is just a small 500 gal preform with 2 50gal ponds above it for waterfalls. I had just cleand some of the alge out of my pond and had not gotten it to the compost pile yet. I have never seen a shore bird in my yard, but we are within a 5 minute walk to lakes and rivers on 3 sides of the property. They also sugested an Owl. I have plenty of frogs.I just want to be sure I don't have somethig that can get to my nesting birds.
Lisa