Training, training and more training. That's where you need to start. At 5 months, they may already be a little too old for easy training, but there is still hope. I don't buy the "once they've tasted blood they'll always be killers" myth. I think it's more, "Once they've discovered a fun self-propelled squeaky toy they'll continue to want to play with it unless taught not to." You will also need to separate the dogs from the chickens until you're 100% sure that the chickens will be safe. There are those who will say that dogs and chickens never mix, and there are those like me who trust our dogs completely with our birds. Every situation is different. I've never owned an Aussie, but my neighbor used to breed them. In my opinion, they're a little too aggressive. At least hers were. Once again, consistent training will be very helpful here. As in, EVERY SINGLE DAY, and don't let them get by "just this once" with any bad behavior toward your chickens.