Why does my Australian Cattle Dog try and and catch my Chickens

I have an 8 month old ACD and the first time I noticed him attacking my chickens was when he dropped one of my chicks at my feet ( lucky the chick was unharmed). Anways whenever I turn my back I seem to notice him chasing and trying to catch my chickens. Today he grabbed hold of my roosters neck and actually pulled out a few feathers. I got this dog to help me protext my chickens but I dont know how I can train him to STOP attacking them! When ever I find him doing this I grab his scruff, sit him down and say NO!! But this obv is not working because he doesnt stop. I know about the breed and how they are prey animals but there has to be a way for me to make him stop what he is doing before he actually kills one of my chickens. So can someone please tell me what I am doing wrong so that he understands that the chickens are not prey and they are apart of out "pack". Thanks so much
Emma

A cattle dog is more of a around up sheep kind of dog(and cattle) I would suggest that maybe he is not getting enough exercise and that is why he is playing around with the chickens and one day he might think he is playing with the chickens and harm one of them.