Very odd. It must be how they were raised. I put a perch in my chick brooder and all my very young chicks have been hopping up and sitting on it.You can see here. I put them all up after they had taken themselves to the coop for the night (but still a bit light out). One will jump down, then another, then finally the sussex. And alkl will just cram into the corner. 10 days now! You can see the first BO to jump down seems to know shes meant to be up, trying to get on the 6 inch bar, but doesnt even seem to be able to do that! it's like they don't know they can even hop! But that said, why are they actively jumping down? The only way I get them to stay up all night is if I put them up when it's proper dark. Then they settle and stay up.