Our Sarplaninac LGD was raised around goats and chickens from a pup. He, too, was a little older when we got him...6mo.
When we got him home, he was terribly upset as his whole world literally disappeared overnight. He was in a strange place with strange people and strange animals.. He frankly just wanted to be left alone.
The goats were upset, too, as they'd never had a dog in with them for any length of time. The dominant doe of our herd pushed him relentlessly, trying to make sure he understood that she was the alpha (and the omega) in that pasture... He snapped at her face about 50 times trying to make her go away, but she ignored him.. The last time he snapped, she jerked her head back, circled uphill, and went up on her hind legs to butt him in the side.....he knew what that was and basically said "NO MORE."
He lunged -- goat still on two legs in mid-ram -- and grabbed her by the ear. She jerked her ear out of his mouth, vocalized, and ran like hell...he chased her for probably 15-20'..
Bear in mind that my wife and I are going "...oh crap..." the whole time thinking...this isn't going to work.
We went and checked the goat and she didn't have a mark on her. Just some slobber....and boy was she ticked! He didn't even break skin, though. After the chase, he just went and laid down.....and the goats left him the hell alone!
Now...had it been chickens circling him...chickens who didn't know dogs...would he have eventually snapped a few of their heads off simply because he was upset and wanted to be left alone?
It's entirely possible. If he grabbed a chicken like he grabbed the goat's ear, it wouldn't survive..
Remember -- most LGDs have a good deal of fear aggression built-in.. A 5mo pup that's just been removed from everything its ever known is probably going to be primed with fear. It won't look like fear in most dogs, though -- the dog will most likely just keep its head and tail low and look fairly calm..
Anyway, that's my guess at what happened. I don't recall if there were any survivors, but if there were, you can rest assured that it wasn't prey drive... If it were prey drive, you'd have been wiped out completely.
Sorry this happened.