Old Australorp Hen Balding?

[Chickenlove64]

I noticed this quiet a while ago and only decided to post it now. This is my (I would say little, but she's huge!) hen Dory. She's pretty old and she's an Australorp.

I was wondering why her head was balding? We don't know how old she is because we bought her from some lady who didn't tell us. The rest of her feathers seem to be fine, all except the ones on her belly which she picked off to keep the eggs warm. There's probably already a thread about this but I'm too lazy to find it XD