I've been raising chickens for over a decade and never seen anything like this. I had 10 3-week old chicks, who'd always been among the healthiest and most vigorous I'd ever had. Yesterday I filled their food and changed their water at around 5:00. As I normally do, I added a splash of organic apple cider vinegar to the water. However, instead of using the new bottle I'd been using all along, I grabbed an old bottle with only a 1/2 inch or so left, the cloudy "mother" at the bottom.
The chicks were COMPLETELY fine when I left. I'm well aware of how chickens look when they're ill: tail down, ruffled dull feathers, etc. These girls were all perfectly fine.
I came back and hour and a half later to double-check the temperature in their enclosure, since it was supposed to get very cold last night. ALL BUT ONE WERE PRONE ON THE FLOOR. Many were belly up, dead. It was horrible, heartbreaking. One lone chick seemed fine, and three were still breathing. I brought the four into the house and tried to get them to drink. Two were too far gone and died in my hands, convulsing. The one healthy one seemed fine, and the one I'd gotten to drink (who was lying down breathing heavily and very listless) did make it through the night and is a bit better today.
The ONLY thing that was different yesterday was the different apple cider vinegar. The cage was the same, everything else the same. I cannot imagine what else could cause such a sudden death than some type of poison, so I'm assuming there was something about that vinegar. My theory is that the one unaffected chick didn't drink any.
If anyone has any other ideas, I'd love to know, but for now I'm assuming that's what it was and putting it out there as a warning...