It might be. He’s just a guardian and he’s always scared of everything so it was much of a surprise I really thought a hawk or opossum ate the first one, but I saw him killing it. I couldn’t throw her away I buried her and put big rocks on top. I was very attached to dear Debbie. She laid her last egg and got killed right after I still have her eggI'm so sorry for your losses! It's always favorites, too.
Your dog and chickens need to be separated by good fencing, a safe coop, and run. Then, retraining will take time, while there can't be any time that the dog and chickens are together unsupervised. It won't be that easy, and this dog may or may not ever be safe unsupervised with your birds.
Back to on leash training, then long leash, then electric collar as a very long leash.
Does this dog have actual work? Being bored, especially when as smart and work oriented as your dog likely is, will inspire poor behaviors too. And a tired dog is a good dog!