My lower perch usually gets poop on it, from the birds on the upper perch. The problem is that the poop is HARD. In fact the dry poop is so hard that I use #50 sandpaper to remove it.
Well today, I found blood on the perches.

It was from my young juv zebra. A spot on the bottom of one toe was raw. I think his foot was scraped raw on the hard rough poop. On Fri I moved him from his foster parents to a male cage. And today I had to traumatize him again, by moving him into a hospital cage, w a smooth cardboard floor. He is now in the master-bath, away from the other birds, so he is not excited by the calling of the other birds. Hope it will only be a few days for his foot to heal.
I rearranged the perches to eliminate the perch crossing over a lower perch. Tomorrow I'm going to check on 2 of my other cages where I might have a similar problem, but with curved branches that I used as perches.