It turns out that they can grip pretty well if they REALLY want to. I stand by the "3 inch round fence rail or 2x4 on the flat" but:
One of my 7 young birds (nearly full size at 15 weeks) was found sleeping in the run (barn alley) on a "ladder" with 1x3 rungs, 1" (true) side up after the auto door closed. I took her (Eos) into the coop and put her on the roost. I checked the next night and there she was again. Picked her up and put her on the roost. Then I picked up the 2 White Rocks (one at a time) that were roosting side by side, one against the wall, on one of the nest box dividers. That thing is a piece of 1/2" plywood! I had suspected something like that was happening because I had found poop in the nests and my layers have NEVER pooped in the nests. I figured someone was perching there occasionally (not a lot of poop) but when I found the two of them, it was WAY dark and they weren't likely to move to a roost then. I think the 3 of them were trying to avoid the drama of the nine 3 Y/O hens pushing their weight around. Interestingly, I had two parallel 12' roosts at 4' high and an 8' one at 2'. The birds chose to be higher (4') on the skinny piece of wood rather than go down to the 2' high rail.
My dad and I had made a 4' long, 16" deep support for the broody buster so I could take it off it's precarious position on the end of the roosts, thus making more room for the chickens. I took the buster off and put 2x2s on the sides of the 16" 2x4s so the girls now have yet more space. And I screwed a conveniently sized and already in the barn piece of particle board (no *I* didn't buy it, stuff is left from prior owners) at a steep angle over the nest boxes so they can't roost there any more. That night Eos REALLY wanted to go back up on the ladder. I had to get her off 3 times before she went in the coop. Haven't found her out on the ladder again, though I don't look every night. But I have seen her up on the roosts before the door closes so I ASSUME she is back to sleeping on the roosts every night.
I believe it. From my limited experience chickens do what they want. They aren't worried about what they aren't supposed to be able to do or should normally do. I got a silkie and everyone told me not to expect many eggs, but my little Jane lays nearly every day and is the coop boss too among a bunch of heavy breeds.
Edited by heatherlaw - 10/6/15 at 8:03pm