Chickens, like other birds, have special tendons and ligaments, so that when they curl their feet around a roost and fall asleep, the legs lock into place and they don't fall off. I've had a hen die of a broken neck when she fell asleep and slipped off of a shelf in the coop (since removed!) but have never had a problem with the round roosts.
Mmm. By that reasoning, the narrow end of the 2x4 might be BETTER, then? B/c the wide side is more like a shelf, but the Chix can curl their feet around the narrow end (as in pic above)?