last year we had a silver laced wyandotte chick that appeared healthy when we got her, but when she was a day or two old she suddenly started having seizures where she would flail about cheaping madly and loudly. It was scary and I was sure she was going to die. She got worse and worse, but, would go a full day, or most of it, without having a seizure at all. At those times we thought she was past whatever it was, just to have her go through what we called spells. After a few days she slowly stopped having them. I can't remember exactly how long it took, but, from then on she had a head twitch that she couldn't control. She would just twitch about every minute or so. It didn't seem to bother her, and she grew up just fine, became a great little egg layer and was very friendly. No seizures either.
I don't know what it was ,but, it was scary.