If you add too many leaves and then get a good amount of rain, it can mat. Even with the birds scratching they might not get down to the lower layers if it's thick enough. I can build up maybe 4-6" of leaves in the run through fall, so I do need to turn it over once in a while to loosen up the layers. It needs to aerate to break down well.I'm curious. Has anyone ever seen this with leaves? I've been using leaves for 5-6 years and never seen it. The chickens do too much scratching around for it to be a problem.