Deficiencies do not always affect all the birds...at least not in the same way. Sometimes you are provifing everythinh they need, but for some reason one bird may not be conuming everything, may not consume adequate amounts or may not be able to assimilate it in the correct way. For example....you can give calcium supplements to everyone at the same rateand they may all cunsume the same amount, but if one is a loner and spends all of it't time in the coop with no exposure to the sun, it may not be getting enought D3 to allow it to process and make use of the calcium.My silkie is getting worse he cant hold his own head anymore. if it was a deficency wouldnt it effect all the silkies? Someone on the silkie thread said maybe brain damagr. What are the symptoms of brain damagr and what do i doi hate seeing him suffer![]()
That said, I tend to agree with some of the others that it is sounding more and more like some sort of injury. You may have to consider culling.

There is an post here on another forum on head injuries in chickens with links to a Youtube and an article. Many of the symptoms described are early on. Your bird may have progressed beyonf the initial symptoms here. Steroifs is one of the things I am seeing here as treatment, but you might have to get that from a vet.
http://dlhunicorn.conforums.com/index.cgi?board=casestudiesemergencies&action=display&num=1160927052