Yesterday we let the chickens in the big coup out. We had about 40 chickens. We give them fresh water everyday as the weather is so cold that it will freeze over night. It's been between 20-50 degrees F for almost 2 months. We've been buying the same brand of feed since we got them in April 2020 we bought them all as day old chicks. We got up yesterday let them out fed and watered them then hooked up the heat lamps in the coup for the first time of the year with the doors open. They We're acting perfectly normal. We opened a new bag of feed for them that we had purchased a couple days earlier. Our hens have not laid any eggs that we could find in about a month. We put that down to much shorter daylight hours. They've been acting normal though. About an hour after we got the chicken chores done i noticed one of the wellsummer was dead outside of the coup. The wellsummers have had issues since we got them so i was not surprised. Then we found another wellsummer dead in the coup doorway. Then a golden comet unable to get up so we put her down. Within 3 hours we had 12 dead chickens. We unplugged the heat lamps and got rid of the feed and picked up a different brand of feed from a different store. Today we lost 3 more. We let them all out they were all fine and active and eating and drinking and running around weekend we let them out. We've lost wellsummers australorps golden comets sapphire gems and silver laced winedots. No sores in their mouths nothing of note except milky eyes. It's quite sudden. They are fine and Then they can't get up and soon after they spasm and die.