I've had a few topple off a shelf, usually when i open a door and there they go! I've also caught them in midair, no idea I could move that fast and accurately. I do keep a thick layer of straw and shavings on the floor because accidents happen. It seems to me the last few days of the hatch the hen stays put in the nest...so do the chicks. I move mine because some hens get extra eggs laid in there, and i also want them in a snake proof brooder when I'm not there...the hen can come and go at will once she,s on her shelf. If i had to move them it would be either during the first part of the brood or after fully hatching. I've put four inch ledges around each shelf. I've also had chicks hit the floor with a thud, fortunately OK...but still...it can be a concern. If i had room enough I'd let them all brood on the floor as nature intended.