Scruffy birds

Among the birds visiting our feeder during lockdown  there's a robin who has a bare (featherless) neck and missing feathers in its red breast.  It's been like this for weeks but now seems to be losing feathers down its back. We also have a very scruffy looking great tit.  Feathers missing patchily all over.  Do you think they've been in fights or is it a disease?

apologies if this post is in wrong thread.