OP, how well trained are your dogs? Do they reliably obey the "leave it" command every time?
If your dogs are not well trained and used to training, then you'll have to start at the beginning BEFORE you let them anywhere near the chickens. Teach sit, and down, and make sure you get a good down-stay and leave it before you do anything else. This may take a year (and professional help if you don't know much about dog training).
If your dogs are already well trained, start with the down-stay and let some chickens (potentially sacrificial birds, so don't use your favorites) eat some scratch and walk around them, praising the dogs the whole time they stay in the down-stay. If they go for the chickens, a sharp "leave it" should make them desist. Offer a treat for a successful "leave it." Train them to leave the chickens alone the same way you train them not to eat your children's toys.
Once they handle the chickens walking around them during a down-stay, allow them to be around a bird or two while closely supervised, with you ready to tell them "leave it" at the first sign of their interest in the chickens. Remember the treats!
You may not be able to leave them out unsupervised with the birds for a while, until you feel pretty comfortable that they know that birds are not for eating. Then watch through some place like a garage door window so the dogs think they are alone, and see if they go for a chicken. Be vigilant and ready to rush through that door hollering "leave it" if they go for the birds. There's lots of research that shows that dogs know when humans can see them, and change their behaviors accordingly. You want your dogs to think you can see them even if they can't see you!
It is possible to train dogs to leave chickens alone. My pitbull/mastiff mix wanted to chase chickens for about a year, but we have trained her that chickens are not for chasing using the above method. She now pretty much ignores chickens, and yesterday I saw a clutch of 12 week old birds walk all around her while she was lying in the yard and she didn't even flinch. She still chases rabbits out of the garden (we encourage this) and chases the cat for fun (they're good friends, the chasing is a game) but she leaves the chickens alone.