I have a feather question.

This last batch of baby birds seem different. Their feathers are full on their wings, chest and belly however, they are bald on the back of the head and on their backs. I see the dad picking at them alot but I don't know if he is the cause or if they are lacking something in their diet or something. Anyone have any ideas? I was going to take the dad out of the cage but he seems to be the one that is feeding them the most. Not sure if the mom with step up or not. I am quite sure that I don't need another decision to make. Do you know what I mean? I hope you are having a good night whatever you are up to. Lisa