There are several things that can cause neurological issues. Which is what you are describing. It can be genetic, tumors, infection, ingested some sort of toxin, or nutrient deficient. All of these could explain why only 1 is having problems. I don’t think it’s a toxin or infection as they are pretty lethargic with these. And from what you said, it still tries to act normal despite the tremors or seizures. If it’s genetic, there isn’t a lot you can do about it. But I may have an idea on that if it comes to it. That leaves tumors or nutrient deficient. How are the eyes. Pupils the same size? Blinking normal?
Nutri drench is a concentrated poultry liquid vitamin. If it is a vitamin deficiency, it is easy to treat. I’ve seen it in my flock. If you have a feed store near you, we can find you something to give them. If it needs extra vitamins, you will know within a few days to a week if it’s the problem. And is a common issue seen here on BYC.