Bald Blue Tit!?

I have a bird feeder just outside my bedroom window, I haven't seen a sparrow feeding on it for absolutely ages but always see loads of Blue tits, coal tits etc. Just recently I've noticed a blue tit with no feathers on it's head visiting the feeder, he seems fine otherwise and has a full feathered body a part from the head, he looks kinda funny :) I wondered if anyone knew why he hasn't any feathers on his head? blue tit stress maybe? :)

Thanks,

Clare