Thanks to all of you for your replies. After considering them all, yesterday afternoon, I cut a small pressed wood rectangle just large enough to cover the area on the poop board where the chickens were erroneously roosting. See the picture below.
For at least last night, it worked exactly...
I have 11 chickens that are from 10 to 18 weeks old. (See the picture.)
They are good about going into the coop at dusk, and they all are able to climb up to the roost pole, which is about 8-10 inches above the poop board. Usually, six of them get onto the roost pole, but five insist on...