Never come across this before but we have a group of cockerels in a run together waiting for us to have the time to slaughter them, and today I noticed one of them was sitting on the floor a lot, then he was being mounted by one of his half brothers. This happened 3 or 4 times that I saw.
I'm sure he his a he as they were separated from the hens at around 12-14 weeks when the differences were quite clear, and though he is perhaps slightly smaller than some of his brothers, he is clearly bigger than his sisters.