Chicks hatching with nervous type twitching/tremors in the neck are most likely AE, Avian Encephalomyelitis. It is a virus passed into the egg from the parent birds. And sometimes even the booster shot they get at the hatchery will give them a touch of it.
It causes tremors to the point of falling over. Sitting on hocks, not eating or drinking. If the chick has yellow, foamy poo, than this is sure sign. If it does not kill the chick, it will pass after about 5 days. The chick will be immune to it forever, however can have lingering affects the rest of it's days.
I have had 2 chicks with this issue from different broods, all surviving, however it is a brutal thing to watch the chick go thru. Sometimes you have to pick the chick up and dip it's beak in the water to keep it hydrated and hand feed it what ever it will eat to keep it going til the virus passes. There is little else you can do. Keep the chick warm and hydrated, keep your fingers crossed.
As for the hens, it is possible that they also have AE, but generally the adults have immunity enough to ward off AE. Mereks is more likely for adults and loss of twitching, tremors and other issues. Here is a link to the most common poultry diseases. I refer to it all the time and can give you some idea what you are dealing with.
http://edis.ifas.ufl.edu/ps044
Good luck with your baby.
