Has anyone done a study on what chickens like in the way of perches?
About 4 years ago, before I got my chickens, I did a lot of YouTube research on having chickens, coop building, etc... One of the videos I watched was about roosting bars in the coop.
According to the guy in the video, a state university's poultry department conducted research into the topic of best practices for roosting bars and came up with the 2X4's laid flat. Reason being, chickens do not roost with their feet wrapped around and locked on a branch like wild birds sitting on a tree branch. Chickens preferred to roost on a wider surface where their feet could rest flat underneath their bodies, tucked up into their feathers all nice and warm. Hence, he stated the university's recommendation was to use a 2X4 flat as a roosting bar, which is what he was doing in the video, and which I did when I built my coop.
FWIW, I live in northern Minnesota and we can get temps down -35F to -40F in the dead of winter. My chickens will fluff themselves up to almost twice their size by trapping air inside their feathers to keep them warm. On a flat 2X4, they are able to lay down on their feet at night and keep their feet warm. As long as there is not an up draft under the roosting bar, they should be able to keep themselves warm even in very cold temps.
If you live in warmer climates, I don't think you would have to worry about feet freezing, but still, a flat 2X4 would be comfortable for your chickens.