For anyone happening upon this thread with a similar situation... a follow-up.
I had to cull her yesterday (day 2) and cried all afternoon. Although an active/happy chick for most of the day, she would occasionally go into these fits of extreme pain and screaming. Frantically scratching at both sides of her head and neck. I began to think this wasn’t a topical (skin) problem... but an extreme pain inside that area and scratching was her only way of trying to get rid of it. She would flip out... it was intense. So I mercifully culled her.
On to why... I can’t know but have theories. One is a genetic abnormality... another is the possibility of a botched Marek’s injection. They’re so tiny and their vaccinator is under a time crunch so it has to happen on rare occasions, right? She came to me with a swollen bald spot on the top of her right shoulder... so I have only to guess, as nothing is risk free. For me, the vaccination was worth the risk.
The hatchery sent two extra chicks so I’m still ahead one chick... but I couldn’t help but apologize to her... this wasn’t on her terms... but onward, I guess.