No interactions present.
This type of plumage abnormality is not uncommon and could be down to abnormal moult, mites, territorial dispute and so on and so on. There tends to be a lot of argumeents but little certainty. There was some discussion here..
If you scroll down here, there is something on bald birds, too...
My Flickr photos...
Thank you !
Not only does the BTO have a lot of information but you can also report incidence of sickness and poor health in your garden birds at http://blx1.bto.org/gwh/login/login.jsp
Feel free to look at my ongoing (hopefully improving) collection of pictures on Flickr:
Lat/Lng: 56.4, -3.5
OS grid ref: NO1226