Hi Charles. Many adult passerines put so much effort into bringing up their young they don't look after their own feathers at this time of year. It's possible either parasites caused the feathers to fall out or the head feathers were knocked out whilst going in and out of a small nest hole many times a day to feed the young. There may be other reasons too which i'd be interested to hear more about.
Thanks. Henry.

Great Tit in Garden today with a compleatly featherless head.
Surely its to early for the moulting season.