I have a chicken that's doing that right now. She acts completely normal when eating or drinking or scratching, then she will just flip her head to the sky and do circles backwards, usually losing her balance and sitting down. She doesn't seem to be in pain or anything. The other 9 chickens are fine. They were moved to their winter coop (no outside access) about 2 months ago as it's too cold to go outside (-30). She has been removed from the flock into a seperate pen, but it's still inside the same room, just for her safety from the rooster. I've been giving them all an electrolyte from the local Ag store which has vitamin b12 for 2 days now. I am unable to get to town for 3 more days to get the childrens drops. If this is a vitamin deficiency, why does their feed not carry it, and how does only one bird get it? How long does it take for the vitamin drops to "cure" her?