Most likely a bad case of feather mites - parasites which feed on the feather tissues. They are kept under control through preening, but birds cannot preen their own head plumage, only scratch it, so precision feather care is more difficult in that area. When feather mites really take hold the result can be a (temporarily) bald-headed bird. The mites have to move on at this point, and the feathers should regrow quite quickly.
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Looks to be something wrong with it's right foot too ( or is that a trick of the light)?
2013 photos & vids here
eff37 on Flickr
I noticed that too, but I think it's just the photo.
Unicum arbustum haud alit duos erithacos
(One bush does not shelter two Robins)
Zenodotus (3rd Century B.C.)