Hi Waterman it looks like it has a case of feather mites that is causing the loss of feather on its head.
If it is fluffed up and lethargic it could also have Trichomonosis have a read here www.bto.org/.../trichomonosis
You may have to watch it around food and water as it can pass the disease on, if it was me I would be shooing it away when I saw it just in case.
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