Get a package of sheets of rough gravelly sand paper and use soft wide blue painter's tape to secure them in a couple places on the plastic so they occasionally get some roughage on their beak wiping or a large 12x12 inch paver stone block inside the pen might also work - that's if the birds never get to go outside for dust-baths and scratching around in the grass and weeds. The poop that gets on the paver stone can easily be hosed down outside, dried in the sun to freshen, and placed back inside their pen. No one ever said keeping house chickens was easy but it's just a different kind of pet maintenance from having dogs or cats.
