Chicken Vitamins?

[Kanthaka]

Does anybody supplement their chicken's diet with vitamins? My chickens' molt is almost over and the weather is getting colder. They have also been lethargic after a recent close call with a hawk that actually swept down on me while I was carrying them to safety. I'm sure it stressed them out...it certainly stressed ME! They are healthy birds but I wonder if vitamins would help them through the coming winter.
There are many types and brands of chicken vitamins available. Is one better than another?
Just curious,
Karen from The Erie, PA Poultry Underground