It is Black mold. Linda Hammid had a Marans die of it last year.I've never heard of it either Amy. But my birds are barely panting these days, so you shouldn't need the misters anymore. I don't have misters in my flock and (knock on wood) have yet to lose a bird to heat. (though you are hotter than me in the summer)
We can catch it too. I would wear a mask when cleaning. It comes from wood getting wet from the misters or other moisture sources like a leaky roof and etc.
ron
