Hi, first time backyard chicken owner here. I live in eastern washington state and have (had) three pullets around 15 wks old. This morning I came out to find one of my chickens died in the night - in the hen house no less. Yesterday she seemed fine, nothing out of the ordinary. Active, eating, and drinking. It has been cold and rainy the past 4-5 days so I didn't think much of her being hunched most of the time (they all were a little but still looking for food and moving about the coop). I checked her body and couldn't find any obvious signs of death - no puncture marks or anything like that, no swollen eyes or crop. I checked the other two, and they seem fine besides being particularly lethargic and hunched as well. They came running for breakfast as usual, and both ate but then went back inside the house and huddled together. I thought maybe they are too cold so I went and got a heat lamp. However, I have done some further reading and think that can't be the case. It is only ~40F, so not that cold, and they are Buckeyes, which are a particularly cold-hardy breed. https://www.backyardchickens.com/a/winter-coop-temperatures made it sound like they would be fine well into negative temperatures. Their house is small - 3ft X 4ft X 3ft, with plenty of bedding. Not much warmer than the outside as the small 1ft X 1ft door stays open all day, but a little warmer none the less. So, even though they don't have diarrhea or bloody poop, I thought I might treat for coccidiosis. I know they are very susceptible to it, and the lethargic behavior matches. Other than that I can't think of anything. Their eyes, beaks, and crops seem fine. Any suggestions?