Last night one of my dogs killed my EE roo 'mellow'. He died protecting his 3 girls, my dog hurt my RIR named 'Red bird' she is now afraid to move and when she does she limps. I have been keeping a close eye on red. 'Mellow' would have been 5 months old on the 13th of this month, my 'red bird' turned 4 month old today. R.I.P. 'Mellow' you were a pretty boy
It is just a dogs natural instinct. My border collie used to try and bite the chickens (but luckily never ate one). I started to leash my dog and sit by the chickens with her. Everytime she would lay down, sit or look away from the chickens I would give her a treat. If she tried to bark or nip at them I would say "Ah Ah" in a short sharp voice.
As you can see from this picture she is now very good with the chickens.
Zoey (my border collie) is very good with the chickens however, if a wild bird comes in the yard she will go after it.
I wish you luck with your dog and I highly recommend trying to train him. It is fun to train your dog and is also very rewarding. Good luck.