@Newmamabear Good point! I also hold the chickens and show the dogs that They are friend. It's also nice to know that a dog who has gone that far could be rehabilitated!
And also you clarified the difference between interest and fixated. When fixated you can tell the difference in the body language of the dog. Ears forward, eyes focused, tail erect, mouth closed probably not ready. When the dog is relaxed, it's mouth should be slightly open, tounge maybe hanging out, no whites of eyes showing, tail hanging, probably more relaxed. Also a dog puts it's ears back when it is being submissive. If you are familiar with your dogs normal behaviors you can probably easily tell. If you were unfamiliar with these indicators, just pay attention a bit and you will start to see the differences.
I never chase the chickens in front of the dogs either. And when they (the dog and the chicken)are calm enough, I allow the dog to sniff the back end of the chicken that I am holding since that's where they really wanna smell.
Another thought I had is that keeping the training sessions short is important, maybe 15 minutes. It helps to avoid frustration. If you feel your tension level rising or the dogs, it's time to end it. Always try to end on a positive note. Dogs truly want to please their humans. Persistence and practice is key.
One thing, belly up is a very vulnerable position for a dog. When some behaviors persist like aggression towards another animal friend, if I know I am in control of the situation... I will grab the scruff and push the neck to the ground where I then make the dog go belly up, a sign of submission, until the dog quits fighting that position. Once calm, I let them up.
We haven't had any incidents with chickens SO FAR. But when we have chicks I can tell it is sooo much harder for all of my dogs to see those fluffy little squeakers zipping around. So I don't let the small chicks out of the run until they get a certain size. I am positive that given the opportunity we would have an issue. Even if the dog isn't intent on killing and only wants to play. Unfortunately dogs playing includes running, chasing, biting, and shaking which ends very badly for small animals. And my dogs are able to discern what they are and aren't allowed to hunt. Everything else that shows up on my property is fair game. And we have lots.... gophers, feral cats, other birds, opossum, moles, even (sadly) another dog once. They don't seem to care about the snakes.
I know all dogs and people are individuals. And what works for some will not work for others. I know my opinion is not significant to you, but no one can fault you if you try your best and the dog doesn't come around. Hopefully you will be able to make adjustment that suit you all and live safe and happy together. Nothing but good vibes coming your way!