My hen (Jack) and her chicks have been living in the pig pen for a few weeks. We lost a chick yesterday of hers, and I put it down to "It happens sometimes". Looking back, that was idiotic. A few hours ago, I went out to check my hen and her chicks and they were fine. I went out a few minutes ago and her polish x chick was sitting in a corner, alone and not moving. I picked her up and she just kind of closed her eyes and sat still. I instantly connected the two sick chicks with the pig pen, and collected the rest of the chicks and locked them in the nursery. I took the sick chick (Cheeky) inside, then went back to the pig pen to ensure there weren't any more chooks in there. In another corner, sat my Silkie hen (Pinkie).
My theory is that their little bodies can't handle something, a toxin maybe from scratching in the pig plop?, and this is their reaction. I syringed some apple cider vinigar mixed with water into Cheeky's mouth. Their poo looks fine, and this is very very sudden. What can I do????