I assume that the adults realized that they could get to the chicks, or the chicks were trying to roost with them.
I had been waiting until this cold snap ended in a day or two before removing the wall and heat lamp. The chicks' pen is in the direct path of the pop door and wind. Otherwise, I would have removed the wall and lamp several days ago.
I might add a roost in the duck pen for the chicks. Then again, they need to be on the roost with the other chickens.