Hi Lougle
The clicker training should work, but you will need to go back to a point where he isn't reactive and build it up from there - you probably also need to do some work on his relationship/responsiveness to you, so that he will focus on you regardless of distractions, then you can build up to getting him to completely ignore the chickens - you can use the clicker for that too.
I've been watching a fab clicker trainer on youtube called kikopup and she has some videos that would help you build up your control via clicker training. She has a website as well, with all the free videos on it. Annoyingly I watched one last night about getting your dog's attention, but can't seem to find it now. I'll go and have a search through and post the link when I find it.
Always start clicker training with the fewest possible distractions and make what you are asking easy enough that you are setting him up for success.
Can you see the chicken run from the house? If so, you need to wind back the clicker/positive reinforcement to rewarding him for looking out the window/door without reacting and build it very slowly from there.
Once he is reliably able to look outside in a totally calm manner, perhaps the next step would be a baby gate across the exit, so he can see them but not get to them - again, building up until he's totally calm every time he looks out.
Next, on-lead, just one step outside of the door - more reinforcement of calmly looking in the direction of the chickens with no response - or even better looking at you, instead of the chickens. Then it's a case of keeping going, literally a step closer at a time. If at any point he starts getting really reactive and unable to focus on the training, go back a step and reinforce that until he's 100% again before moving on.
In the meantime, keep all toilet trips on the lead. Take your treats and clicker with you and click/treat every single time he looks away from the chickens/towards you - even for a second.
You are going to need top grade treats, so either extra tasty liver cake or lovely smelly greasy sausage or hot dogs.
It's not going to be easy, but with lots of short-sharp, positive sessions it is doable. If it were me, I would persist with the training and also get him out and about socialising him and positively rewarding meeting as many different types of birds as possible in the meantime, but if you aren't in a position to do that at the moment, then I would second what others have said about screening off the chicken run from view - although really that's only a temporary fix and you'd still have a problem if, say, one of the chickens got out - or he got wise to where they were.