I'm not using the CBD oil. Unless it's in a carrier oil, it's not easy to use. I confess I don't know how strong CBD oil is that they derive from hemp, which is usually where CBD oil comes from that is sold legally. And I wouldn't know the dose which would depend on how diluted the oil is in its carrier oil.
I use the plant leaves for my chickens, a hand full each day for the seventeen hens.
Ah, yes, you did write as much in your first post.
I have little doubt CBD given to chickens may have a calming effect. It would be handy to know the dose strength and a reputable supplier should state the strength on the bottle somewhere. From that one could work out the effective dosages.
I do have some reservations.
Chickens are of a nervous dispositon for a reason, it helps them to survive, panic first think later seems to have served them well for centuries before we locked them up and stole their eggs on an industrial scale. I don't want chickens that I care for wandering around the field quoting parts of a Cheech and Chong movie.
I've understand there is a negligible amount of THC in CBD products but none the less, CBD does have the reputation of being a mood altering drug; we wouldn't be writing about it here otherwise.
Where I live (sheltered housing for the over 55s) I know a few people who take CBT oil as a stress or pain management drug, hoping it a) works, and b) has far fewer side effects than the drugs their doctor advises them to take. Other forms of cannabis are pretty popular for similar reasons.
Suppose it does work for feather picking and it works because the bird has become more docile. I can just see an awful lot of people dosing up their aggressive cockerels and unfiendly hens so they can cuddle them.

This is not something I feel we should be encouraging.
In many cases are already livng in prisons, some more luxurius than others, drugging them up so they are more docile seems just one further piece of abuse by humans with little thought for the welfare of the chicken.
On the plus side, I can see a use for it when treating the sick for example and even perhaps something that might make dying a little easier.
For free ranging chickens and unsupervised ranging the chicken needs to be fully switched on for want of a better phrase and dosing them with CBD may just decrease their survival chances.